After an underlying layer, made of a single crystal metal material, has been formed on a semiconductor layer, part or all of the underlying layer is changed into a metal oxide layer by supplying oxygen thereto from above the underlying layer. Then, a ferroelectric or high-dielectric-constant film is further formed on the metal oxide layer. Since the film made of a metal material is formed on the semiconductor layer, a silicon dioxide film or the like is not formed easily. Thus, a dielectric film, which includes an underlying layer with a high dielectric constant and has a large capacitance per unit area, can be obtained. Various defects such as interface states in the semiconductor layer can also be reduced advantageously if these process steps are performed after a thermal oxide film has been formed on the semiconductor layer.

The present invention relates to a method and apparatus for forming an excellently crystalline dielectric film with a high dielectric constant, like a CeO2 film, or ferroelectric film out of metal Ce and oxygen on an Si substrate.
In recent years, the number of C-MOS devices that can be integrated together on a single Si substrate has increased significantly because those devices have been tremendously downsized. To catch up with this trend, reduction in thickness of a gate insulating film, which is part of a MOSFET, is also in high demand. A thinner gate insulating film is needed because of the following reasons.
First of all, although the operating voltage has been reduced day after day to conserve power as much as possible, the quantity of charge required for the operation of a device remains almost the same and has not been reduced so much. Since the relationship of Q=CV (where Q is the quantity of charge, C is static electricity and V is voltage) should be met, the static electricity C that can be retained in a gate insulating film must be increased to reduce the voltage V with the quantity of charge Q kept substantially constant. The static electricity C is given by C=(∈rxc2x7S)/d, where ∈r is relative dielectric constant, S is the area of a capacitor and d is a space between electrodes. Accordingly, the static electricity C can be increased if the thickness d of a gate insulating film, which is currently made of SiO2 in many cases, is reduced. For that purpose, the gate insulating film has been thinned to a thickness between 10 and 15 nm or less than 10 nm.
However, if a gate insulating film is thinned that much, then various inconveniences might be concerned about lately; the breakdown strength of the gate insulating film might decrease or the leakage current might increase.
In view of these potential disadvantages, alternative gate insulating film materials, which have a relative dielectric constant ∈ r higher than that of SiO2 and yet exhibit pretty good electrical properties comparable to those of SiO2, have been searched for. That is to say, if the relative dielectric constant ∈ r is higher, then the static electricity C can be kept high even when the thickness d is increased to a certain degree. Accordingly, the required charge quantity Q is attainable even with a reduced operating voltage. Taking these points into account, methods for forming, on an Si substrate, an insulating film made of a novel insulating material with high dielectric constant and breakdown strength and low interface level and leakage current have been researched and developed to attain characteristics comparable to those of the SiO2 gate insulating film currently used.
Efforts have also been made to form an insulating film of non-SiO2 insulator material on an Si substrate by a different type of demand. For example, an example disclosed in Japan Journal of Applied Physics 35, 4987, (1996) (which will be herein called a xe2x80x9cfirst documentxe2x80x9d for convenience sake) report-research for implementing a transistor with memory function by providing a thin film with ferroelectric for the gate of field effect transistor. According to the technique disclosed in this document, a thin film of PbZr1-xTixO3 (PZT) with ferroelectric, i.e., a PZT film, is formed as an exemplary thin film of that type. However, since it is difficult to form the PZT film directly on an Si substrate, an insulating film of CeO2 for example, is interposed as a buffer layer between the PZT film and the Si substrate.
Methods for forming a novel insulator film on an Si substrate to attain those characteristics, including high dielectric constant and breakdown strength and low interface level and leakage current, as in the gate insulating film mentioned above, have also been researched such that a ferroelectric or other dielectric film (e.g., superconductor film) can be formed on the Si substrate.
According to any of these suggested techniques, a CeO2 film is one of very attractive insulator materials for a buffer layer. This is because the lattice constant of CeO2 is closer to that of Si than any other known material and a lattice mismatch between CeO2 and Si is only xe2x88x920.37% (i.e., aceO2=5.411 xc3x85 and asi=5.431 xc3x85). In addition, since the crystal structure of CeO2 is like that of fluorite, CeO2 can form a continuous crystal lattice with the Si substrate having a diamond structure. The coordination number for all the atoms is four in Si, whereas the coordination number for oxygen atoms is four and that for Ce atoms is eight in CeO2. However, since both Si and CeO2 crystals belong to a cubic system, which is represented as a face-centered cubic lattice as a matter of principle, Si and CeO2 crystals can be stacked one upon the other by epitaxial growth (because mole ratio of oxygen to Ce is 2:1). Thus, it is possible to form a thin film with excellent crystallinity on the Si substrate, and it is easier to stack a ferroelectric or superconductor film with high crystallinity thereon. Furthermore, since the relative dielectric constant of CeO2 is as high as around 26, it is very likely that CeO2 will be used as a novel gate insulating film material in place of SiO2.
Various techniques of forming CeO2 on an Si substrate have been proposed in numerous other documents as well as the first document. Following is typical examples of them.
According to an example disclosed in Japan Journal of Applied Physics, 1765, (1993) (which will be herein called a xe2x80x9csecond documentxe2x80x9d for convenience sake), CeO2 is evaporated from a pellet-like CeO2 sintered compact by irradiating the compact with an electron beam (EB) in a molecular beam epitaxy (MBE) system including an EB evaporation unit, thereby forming an excellently crystalline CeO2 thin film on an Si substrate. In this case, decline in crystallinity of the CeO2 thin film due to the oxygen deficiency is prevented by supplying oxygen gas while CeO2 is being evaporated. In the first document, the CeO2 film is also formed by the same method.
An example disclosed in Japan Journal of Applied Physics 270, 1994 (which will be herein called a xe2x80x9cthird documentxe2x80x9d for convenience sake) uses a thin film forming technique different from that of the first and second documents. In the third document, a reactive sputterer including a target of metal Ce is used and Ce atoms are sputtered out of the target with oxygen gas supplied thereto and reacted with oxygen on the Si substrate, thereby forming an excellently crystalline CeO2 thin film on the Si substrate.
An example disclosed in Applied Physics Letters 2027, (1991) (which will be herein called a xe2x80x9cfourth documentxe2x80x9d for convenience sake) forms a CeO2 film by a different technique from any of the techniques mentioned above. In the fourth document, an MBE system, into which ArF excimer laser radiation can be introduced externally, is used, a pellet-like CeO2 sintered compact placed inside is irradiated with the laser radiation to evaporate CeO2 therefrom and oxygen gas is introduced at the same time. In this manner, an excellently crystalline CeO2 thin film is formed on the Si substrate.
These methods of forming a crystalline CeO2 thin film as disclosed in the documents cited above, however, have the following shortcomings.
It should be noted that a family of crystallographic planes including (100), (010), (001) and so forth will be collectively referred to as a (001) plane in the following description, although such a family of planes should be labeled {001}. The same statement will be applicable to a (011) or (111) plane. Similarly, (001), (011) or (111) substrate or film will mean a substrate or film with a (001), (011) or (111) plane as its principal surface.
First, in accordance with the example disclosed in the first and second documents, oxygen and Ce are supplied at the same time by evaporating CeO2 from a pellet-like CeO2 sintered compact being heated. That is to say, since Ce and oxygen reach the surface of the Si substrate at a time, SiO2, as well as CeO2, is formed thereon. Should SiO2 be formed, the sharpness of crystallinity decreases at the interfacial structure and the planarity of the surface also deteriorates since SiO2 generally has an amorphous structure. Also, if the structure with the SiO2 film is operated as a device, a voltage applied will be concentrated on the SiO2 film with the lower dielectric constant in spite of the presence of the CeO2 film with the higher dielectric constant. As a result, it is difficult to store charge to a quantity large enough to ensure the intended function of a gate insulating film. In addition, even when such a CeO2 film mingled with SiO2 is used as a buffer layer for a ferroelectric or superconductor film, it is also difficult to apply a required voltage to the ferroelectric or superconductor layer.
Following the example disclosed in the second document, a CeO2(111) film can be formed on an Si(111) substrate. But only a CeO2(011) film, not a CeO2(001) film, can be formed on an Si(001) substrate. That is to say, even though the lattice constants of CeO2 and Si are close to each other, the plane orientations thereof are different from each other. Thus, the effects of suppressing lattice strain and preventing the generation of defects cannot be expected at all. Furthermore, although a CeO2(011) film is formed, the film actually has a polycrystalline structure, in which two types of crystals coexist on the principal surface of the Si substrate so as to be symmetrical around an axis and form an angle of 90 degrees between them. Accordingly, it is difficult to obtain a smooth and uniform single crystal thin film.
Japan Journal of Applied Physics 31, L1736, (1992) (which will be herein called a xe2x80x9cfifth documentxe2x80x9d for convenience sake) explains the reason of this phenomenon. Specifically, it is believed that higher stability is attained where the (001) plane of Si crystals is continuous with the (011) plane of CeO2 crystals rather than where the (001) planes of these two types of crystals are continuous with each other. This is probably because dangling bonds appearing on the 2xc3x971 streaks on the (001) plane of Si crystals that are formed in high vacuum are located close to oxygen atoms within the (011) plane of CeO2 crystals.
The fourth document shows that a CeO2(111) film with excellent crystallinity can be formed on an Si(111) substrate. In accordance with this example, an oscillation of diffraction pattern intensity (RHEED oscillation) is observed by a reflection high-energy electron diffraction (RHEED) analysis during the crystal growth. The generation of the RHEED oscillation indicates that the crystals are growing two-dimensionally, or layer by layer, while keeping high smoothness at the surface. Even when the cross section thereof is observed by TEM, the existence of large defects is hardly observable. The formation of SiO2 is not found in the interface between Si and CeO2, either. However, this document does not report on successfully forming a (001) plane of CeO2 crystals on a (001) plane of Si crystals, either.
Japan Journal of Applied Physics 29, L1199, (1990) (which will be herein referred to as a xe2x80x9csixth documentxe2x80x9d for convenience sake) makes a disclosure about this. Specifically, since Ce and oxygen are also supplied at a time even by using such a system, a CeO2(011) film is formed unintentionally on an Si(001) substrate.
Even in the example disclosed in the third document, a (111) plane of CeO2 with excellent crystallinity is formed on a (111) plane of an Si substrate as in the second and fourth documents. In accordance with the method disclosed in the third document, metal Ce is used as a source material. Thus, the method succeeds in supplying only Ce onto the interface of the Si substrate and suppressing the formation of SiO2. However, a layer consisting, of metal Ce alone, which is needed in obtaining the excellently crystalline CeO2 film, is as thick as 5 nm. Accordingly, if the film is used as a gate insulating film for a transistor, then the operation of the transistor device is seriously affected by the existence of the thick metal layer. In addition, the third document does not report on successfully forming a CeO2(001) film on an Si(001) substrate, either. The reason thereof cannot be found in the document. But we think it would be difficult to form CeO2 crystals with the same plane orientation by relaying information about the crystal structure of the Si substrate if the metal Ce layer with the thickness of about 5 nm exists between them.
An object of the invention to work out a technique of forming an excellently crystalline metal oxide layer, such as a CeO2 film, on an Si substrate, which has been impossible according to any of the documents cited, and thereby develop a technique of forming a ferroelectric film with good orientations on the crystalline metal oxide layer.
First, it will be described what analyses we carried out to arrive at the inventive idea on the novel method for forming a metal oxide layer like a CeO2 film.
The fifth document discloses the reasons why only a CeO2(011) film, not a CeO2(001) film, can be formed on an Si(001) substrate. Those reasons will be described in detail below.
FIG. 19 illustrates how CeO2 crystals grow epitaxially on a (001) plane of an Si substrate and corresponds to FIG. 2 of the fifth document. In FIG. 19, the larger open circles represent Ce atoms 1, the medium dotted circles represent Si atoms 2 and the smaller open circles represent oxygen atoms 3. On the surface of the Si substrate, a (001) plane of Si crystals with a unit cell 4 has appeared. One side of the unit cell 4 is parallel to the [100] direction, while another side of the unit cell 4 is parallel to the [010] direction. In other words, supposing the paper of FIG. 19 is a plane parallel to the (001) crystallographic plane of Si crystals, the x- and y-axes of Si crystals exist within the paper and the z-axis of Si crystals is the direction coming out of the paper. On the other hand, two types of CeO2 crystals, which are represented by two unit cells 5 and 6 shown in FIG. 19, are created at an equal percentage to match the (001) plane of Si crystals. One side of each of these unit cells 5 and 6 is parallel to the [100] direction (i.e., the x-axis direction), while another side thereof is parallel to the [011] direction (i.e., the direction tilted from the y-axis by 45 degrees). The x-axes of the unit cells 5 and 6 cross each other at right angles, and the [011] direction of the unit cell 5 and the [011] direction of the unit cell 6 also cross each other at right angles. In other words, the unit cell 6 is obtained by rotating the unit cell 5 by 90 degrees around an axis in the direction coming out of the paper of FIG. 19. The y- and z-axes of each of the unit cells 5 and 6 are tilted from the paper of FIG. 19 by 45 degrees.
As shown in FIG. 19, the O atoms 3 are located over and between columns of Si atoms 2 in the Si crystals in the (011) plane of CeO2 crystals. When viewed one-dimensionally along the [100] direction of the lattice structure of Si crystals, i.e., when viewed in the direction coming out of the paper, these O atoms 3 are located very close to the dangling bonds appearing on the 2xc3x971 restructured structure on the uppermost surface of the Si substrate. Accordingly, Ce and O (oxygen), which are supplied at a time onto the Si substrate, are likely to form the (011) plane of CeO2 crystals rather than a (001) plane of the CeO2 crystals. And when the (011) plane of CeO2 crystals is formed on the Si substrate, two types of crystal structures, i.e., the unit cells 5 and 6, appear at an equal percentage to be symmetrical to each other around an axis as shown in FIG. 19. Thus, when CeO2 crystals are grown epitaxially on the Si substrate, two types of crystals with two different orientations coexist and form respective domains. As a result, a CeO2 film, which is regarded as polycrystalline as a whole, is formed.
FIG. 20 is a microgram obtained by observing the coexistence of two domains in a CeO2 film by high-resolution transmission electron microscopy (HRTM) as disclosed in the fifth document.. As shown in FIG. 20, a domain CrA with an x-axis in the [100] direction parallel to the horizontal direction of FIG. 20 and a domain CrB with an x-axis in the [100] direction parallel to the vertical direction of FIG. 20 coexist. The sizes of these domains CrA and CrB are in the range from 10 to 50 nm.
FIG. 18 is a schematic cross-sectional view illustrating a state where two types of CeO2 crystal domains exist in a CeO2(011) film 9 formed on an Si(100) substrate 8.
Next, a process through which CeO2(111) crystals are formed on an Si(111) substrate will be considered. The structures of Si and CeO2 crystals in such a case are disclosed in the sixth document. FIGS. 21(a) through 21(c) correspond to FIG. 4 of the sixth document and illustrate orientations of CeO2 crystals, which grow epitaxially on (001), (111) and (110) planes of an Si substrate. FIG. 21(a) illustrates that two types of CeO2 crystal domains, whose film plane is a (011) plane, are formed on the (001) plane of the Si substrate as in the fifth document. On the other hand, FIG. 21(b) illustrates that CeO2(011) and (111) films can be formed on an Si(011) substrate.
As can be seen from FIG. 21(b), the CeO2(111) film is likely to grow on the Si(111) substrate and there is little lattice mismatch therebetween. In this case, the CeO2 crystals have such a structure as including a layer consisting of Ce alone and a layer consisting of oxygen alone that are alternately stacked one upon the other vertically to the surface of the substrate, strictly speaking. However, since these two layers are located very close to each other, two types of atoms can be regarded as substantially existing in a common plane. Thus, the energy required for eliminating one of these two types of atoms, i.e., Ce and O atoms, and thereby forming a layer consisting of only one remaining type of atoms is not high for any layer of either type of atoms. That is to say, even if Ce and O atoms are supplied at a time onto the Si substrate, the CeO2(111) film can still be formed. And we can say that no other CeO2 film with a different plane orientation will be formed.
However, since the (111) plane is the densest plane of crystals making up a diamond structure, the greatest number of Si dangling bonds exist on the (111) plane. Accordingly, if O and Ce atoms are supplied at a time onto that surface of the Si substrate, then not only CeO2 crystals but also an SiO2 layer will be formed on the surface of the Si substrate. That is to say, crystallinity might decline and dielectric constant might decrease.
In the first, third, fourth and sixth documents, the crystallinity of a CeO2 thin film obtained is analyzed with X-rays. Among these XRD analysis curves, that of the fourth document represents a diffraction peak with the smallest full width of half maximum (FWHM), which is still as large as 3500 arc sec., though. The FWHMs shown in the other documents are much greater than that. This is a very bad value considering that the lattice mismatch ratio between Si and CeO2 crystals is only xe2x88x920.37%. For example, the FWHM of ZnSe, which has a lattice mismatch ratio of 0.26% with respect to GaAs (where aGaAs=5.6533 and aZnSe=5.668), is 300 arc sec. or less (as for a 2xcex8-axis-fixed, xcfx89-axis-scanning rocking curve). The FWHM values shown in the respective documents are obtained by performing scanning on both axes of xcfx89xe2x88x922xcex8 (xcex8xe2x88x922xcex8). Thus, supposing half of each of these values is equivalent to the value obtained by xcfx89-axis scanning, the largest FWHM is still about 6 times as large as the FWHM of ZnSe. That is to say, the CeO2 film according to these documents seems to be smooth and less defective at a local level observable by TEM or the like. In the CeO2 film, however, significant lattice disorder and irregularity like defects are found within a range corresponding to a spot diameter of a beam of X-rays. Accordingly, compared to crystals used for a compound semiconductor, the crystallinity thereof seems to be much inferior. As can be seen, such poor crystallinity of the CeO2 film seems to be one of the reasons why the SiO2 layer is formed unintentionally between the Si substrate and the CeO2 film. Specifically, if the lattice is out of order in the CeO2 layer already formed, then oxygen (O) atoms are easily transmitted through those disordered parts. As a result, a greater number of oxygen atoms are supplied onto the surface of the Si substrate. In addition, a great number of dangling bonds also exist on parts of the surface of the Si substrate just under the disordered parts of the CeO2 crystal lattice, and therefore oxygen is easily bonded to those parts, thus promoting the formation of the SiO2 layer.
In view of these findings, the present inventors arrived at the idea that if monatomic Ce and O layers are stacked one upon the other on an Si(001) substrate with Ce and oxygen alternately supplied onto the Si(001) substrate and with Ce, La and oxygen supplied in controlled amounts, then a CeO2(011) film with double domains will not be formed but a single crystal CeO2(001) film can be formed. That is to say, we paid special attention to the fact that when crystals of a CeO2(001) film with a fluorite-type crystal structure grow, a layer where only Ce atoms exist and a layer where only oxygen atoms exist will appear alternately and repeatedly at regular intervals in the crystal lattice.
Taking these results into consideration, we also acquired the idea that if Ce and oxygen are alternately supplied after a thin film having a structure continuous with the diamond cubic structure of Si (e.g., an oxide with a fluorite-type crystal structure) has been formed on the Si(001) substrate, then a CeO2(011) film with double domains will not be formed but a single crystal CeO2(001) film can be formed.
Furthermore, we got the idea that even a metal oxide layer containing a metal other than Ce can be formed to show excellent crystallinity by taking measures for suppressing reaction between Si and O and that a ferroelectric layer with good orientations should be formed on the metal oxide layer.
Hereinafter, the present invention, which is a natural result of these findings, will be described.
A first inventive dielectric film forming method includes the steps of: (a) preparing a substrate including a crystalline semiconductor layer; (b) forming an underlying layer, which consists essentially of a metal material, on the crystalline semiconductor layer; (c) forming a metal oxide layer by oxidizing at least part of the underlying layer with oxygen supplied thereto from above the underlying layer; and (d) forming a ferroelectric layer on the metal oxide layer.
According to this method, an excellently crystalline underlying layer is formed while preventing an oxide film from being formed due to oxidation of a semiconductor material. Thus, a ferroelectric layer with good orientations is formed on the metal oxide layer, which part or all of the underlying layer. Consequently, a dielectric film qualified for a ferroelectric device can be obtained.
The step (c) may be performed in parallel with the step (d) by using oxygen supplied in the step (d) to form the ferroelectric layer.
The underlying layer formed in the step (b) preferably consists essentially of at least one metal material selected from the group consisting of Mg, Zr, Y, Ce, La and Bi.
The method may further include the step of forming a thermal oxide film by thermally oxidizing a surface of the crystalline semiconductor layer after the step (a) and before the step (b). In the step (b), the underlying layer consisting essentially of the metal material may be formed on the thermal oxide film. In this manner, a thermal oxide film showing high affinity for the semiconductor layer can be used and yet the thermal oxide film, underlying layer and ferroelectric layer can be formed with almost no interdiffusion caused among them. Consequently, a dielectric film qualified for a ferroelectric memory device can be formed easily.
A second inventive dielectric film forming method includes the steps of: (a) preparing a substrate including a crystalline semiconductor layer; (b) forming an underlying layer, which consists essentially of a metal material, on the crystalline semiconductor layer, where affinity of the metal material for oxygen is higher than affinity of a semiconductor material of the semiconductor layer for oxygen; (c) forming a Ce layer on the underlying layer; and (d) forming at least a CeO2 layer by supplying oxygen from above the Ce layer.
According to this method, an excellently crystalline CeO2 layer can be formed while preventing an oxide film from being formed out of the semiconductor material. Thus, even if the dielectric film is relatively thick, the dielectric film has large capacitance per unit area, small leakage current and high breakdown strength.
In the step (d), at least part of the underlying layer is preferably oxidized.
The steps (b) through (d) are preferably performed continuously within an ultrahigh vacuum epitaxial growth system.
The steps (b) and (c) are preferably performed by an MBE process using an EB evaporation system.
The underlying layer formed in the step (b) preferably consists essentially of at least one metal material selected from the group consisting of Mg, Zr, Y, Ce, La and Bi.
A third inventive dielectric film forming method includes the steps of: (a) preparing an Si substrate; and (b) forming a CeO2 film on the Si substrate by an epitaxy process using metal Ce as a source material.
According to this method, Ce can be supplied independently onto the surface of an Si substrate before oxygen is supplied thereto. Thus, it is possible to suppress the formation of an SiO2 layer on the surface of the Si substrate. For example, by evaporating and supplying metal Ce using a K-cell or a valved cracking cell, a Ce layer can be deposited at a deposition rate of 5 nm/min. or less. As a result, its thickness is controllable with a precision corresponding to the thickness of several atomic layers (5 xc3x85) or less, and therefore, a CeO2 film with a desired plane orientation can be formed.
If the principal surface of the Si substrate is a (001) plane, then a surface of the CeO2 film can also be a (001) plane. Thus, it is possible to prevent the generation of double domains, which are observed when a CeO2 film with a (011) film surface is formed. As a result, an excellently crystalline CeO2 film can be formed.
In the step (b), at least one atomic layer of Ce may be formed on the Si substrate. Then, it is possible to prevent an SiO2 layer from being formed on the Si substrate with more certainty.
In the step (b), Ce alone and oxygen alone may be alternately and repeatedly supplied such that monatomic O layers and monatomic Ce layers are formed alternately on the at least one Ce atomic layer located on the Si substrate. In this manner, it is possible to prevent a CeO2 film with a (011) surface from being formed.
A first inventive dielectric film includes: an underlying layer formed on a crystalline semiconductor layer and made of a metal material, the affinity of the metal material for oxygen being higher than affinity of a semiconductor material of the semiconductor layer for oxygen; and a crystalline CeO2 film formed on the underlying layer.
In this structure, an oxide film of a material for a semiconductor layer, e.g., a silicon dioxide film, is less likely to be formed on the semiconductor layer. Thus, a dielectric film with a large capacitance per unit area can be formed to include an underlying layer with a high dielectric constant and a crystalline CeO2 layer. Consequently, the leakage current can be reduced and the breakdown strength can be increased by thickening the dielectric film sufficiently.
In the first dielectric film, at least part of the underlying layer has preferably been oxidized.
A second inventive dielectric film includes: an underlying layer formed on a crystalline semiconductor layer and made of a compound oxide containing a metal element and a semiconductor material for the semiconductor layer; and a crystalline CeO2 layer formed on the underlying layer.
In this manner, a dielectric film, which includes: an underlying layer showing high affinity for the semiconductor layer, low distortion and low interface levels; and a CeO2 layer with a high dielectric constant, can be obtained.
A third inventive dielectric film includes: an underlying layer formed on a crystalline semiconductor layer and made of a crystalline metal oxide that substantially lattice-matches crystals of the semiconductor on the principal surface of the semiconductor layer; and a crystalline CeO2 layer formed on the underlying layer.
In this manner, a dielectric film including an excellently crystalline CeO2 layer, which has reflected information about the crystal structure of a semiconductor layer by way of an underlying layer, can be obtained.
A fourth inventive dielectric film includes: an underlying layer formed on a crystalline semiconductor layer and made of at least one metal material selected from the group consisting of Mg, Zr, Y, Ce, La and Bi; and a ferroelectric layer formed on the underlying layer.
In this manner, an underlying layer including almost no oxide film of the semiconductor material, e.g., silicon dioxide film, and a ferroelectric layer with good orientations can be obtained. Thus, it is possible to effectively impart a voltage applied to the entire dielectric film to the ferroelectric layer. Consequently, the ferroelectric layer attains sufficiently large residual polarization.
In the fourth dielectric film, at least part of the underlying layer has preferably been oxidized.
A fifth inventive dielectric film includes: an oxide layer formed on a crystalline semiconductor layer and made of an oxide of a semiconductor material for the semiconductor layer; an underlying layer formed on the oxide layer and made of an oxide of a metal material; and a ferroelectric layer formed on the underlying layer.
A layer made of an oxide of a semiconductor material for a semiconductor layer shows low interface levels and very high affinity for the semiconductor layer. By taking advantage of these properties, a very reliable dielectric film including a ferroelectric layer with good orientations can be obtained.

Dielectric film and method for forming the same

Matsushita Electric Industrial Co., Ltd.


The legs of a swivel caster assembly horn are mounted to a pair of horizontally-spaced beam plates which are pivoted to the legs ahead of the swivel axis. The caster wheel is mounted on an axle behind the swivel axis. A pair of urethane springs is mounted to the horn legs ahead of the pivot axis. A bolt across the front ends of the beams receives collars, each collar having a threaded transverse aperture which is screwed onto threads on the lower ends of bolts whose upper ends have heads with flanges which bear downward on a spring cap plate bearing downward on the tops of the springs, whereby the load on the caster horn is applied by lever action to the springs which are pre-loaded by the collars screwed onto the bolts.

1. Field of the Invention
This invention relates generally to caster wheels for support of cabinets, carts, dollies and the like.
2. Description of the Prior Art
Casters have been used for a great many years to support furniture and other items in a way whereby they can be conveniently moved. In the use of casters, it has been found desirable in some instances, to use springs in the casters to support the loads in a somewhat resilient manner. This can make it easier to move the item supported by the caster and to accommodate irregularities in the surface on which the item is supported or along which it is moved. In addition, the use of springs enables less jostling of the item supported on the casters.
There are situations in which a caster which employs metal springs can be caused to oscillate undesirably. An example is where a set of casters supports a cart or cabinet or furniture that is being transported on or in a vehicle. To address one or more of these problems, various efforts have been made and some have resulted in issuance of patents employing various types of vibration damping systems. Examples are as follows:
______________________________________ U.S. Pat. No. Inventor Date Issued ______________________________________ 2,442,831 Suttles 6/08/48 4,485,521 Welsch et al. 12/04/84 4,559,669 Bonzer et al. 12/24/85 4,575,896 Nakao et al. 3/18/86 4,685,174 Hager 8/11/87 77 23790 (French) Hervieu 8/02/77 ______________________________________
These efforts notwithstanding, there has remained the need for a caster assembly having the spring and shock absorbing features, well adapted to high volume production, easily adjusted to provide the desired pre-loading for capacity, and wherein the springs can be easily replaced if and when needed. The present invention is directed to meeting these needs.

Lever action caster with shock absorbing spring


A flexible display panel includes a non display part and at least a first screen, a second screen and a third screen. The first screen and the second screen are oppositely located. The third screen is connected between the first screen and the second screen. The first screen, the second screen and the third screen jointly form a display region. The non display part is connected to one side of the display region and is provided with a drive chip that drives the first screen, the second screen and the third screen to show images.

With the constant improvement of the display technology, the application of the flexible display screen has already become wider and wider. The flexible display screen is a display screen utilizing flexible plastic, and at present is realized with an Organic Light-Emitting Diode (OLED). In comparison with the regular flat panel screen, the elasticity of the flexible display screen is better, and the screen is hard to be broken, and the appearance is novel for easily attracting consumers. How to reasonably design the structure of the flexible display screen and to be applied on the display apparatuses, such as the cell phone, the tablet is the research point of the display technology in the future.
In prior art, the flexible display screen for the display apparatuses, such as the cell phone, the tablet is merely that a bending with a small radius of curvature at the edge of the display screen, and the structure of the flexible display screen is simple, and the image display region is small and multifunction of image display cannot be realized.

Flexible display panel and flexible display apparatus

Wuhan China Star Optoelectronics Technology Co., Ltd


The invention provides a method of progressive engagement of all-wheel drive for a mobile vehicle. A set of vehicle parameters is sensed, and an all-wheel drive lock value is determined based on the sensed set of vehicle parameters. Vehicle subsystem control activation is determined, and the all-wheel drive lock value is adjusted based on the vehicle subsystem control activation determination. An all-wheel drive controller is engaged based on the adjusted all-wheel drive lock value.

This invention relates generally to the field of vehicular control, and in particular, to a method and system for progressive engagement of an all-wheel drive for a mobile vehicle.
All-wheel drive (AWD) systems use a center differential that allows the level of locking between the front and rear axles to be controlled. When the differential is totally unlocked, the vehicle stability in a turn may be maximized since the wheel speeds can be controlled by a Vehicle Stability Enhancement (VSE) subsystem without affecting the other axle. When the differential is totally locked, the traction of the vehicle is maximized since traction forces are supplied to all four wheels at a similar level. The optimal amount of locking should maintain a balance between the stability of the vehicle, the amount of all-wheel drive traction available, and driver comfort.
In order to achieve satisfactory vehicular performance under all road conditions, a control algorithm should provide variable engagement of AWD systems. When AWD systems are engaged without any estimates of surface capability, driver command, and vehicle state, instability of the vehicle may result or driver comfort may be negatively affected. Accordingly, it would be beneficial to have a system and method for using data from existing VSE subsystems such as yaw rate, lateral acceleration, and sensed wheel speed to provide these estimates, thereby engaging the AWD in a progressive manner to maintain stability and traction and to enhance the comfort of the vehicle user.
Furthermore, it would be beneficial to consider and incorporate the status of other vehicle subsystems, such as an anti-lock braking subsystem and a traction control subsystem, to produce smooth, progressive engagement and disengagement of an all-wheel drive system under varying road conditions and driver demands.
The object of this invention, therefore, is to provide a method and a system for progressive engagement of all-wheel drive, and to overcome the deficiencies and obstacles described above.
One aspect of the current invention provides a method of progressive engagement of all-wheel drive for a mobile vehicle. The method may include the steps of sensing a set of vehicle parameters, determining an all-wheel drive lock value based on the sensed set of vehicle parameters, determining vehicle subsystem control activation, adjusting the all-wheel drive lock value based on the vehicle subsystem control activation determination, and engaging an all-wheel drive controller based on the adjusted all-wheel drive lock value.
The sensed set of vehicle parameters may include lateral acceleration, steering wheel angle, yaw rate, and wheel speed. The all-wheel drive lock value may be based on a ratio of a sensed lateral acceleration and a surface capability value. The surface capability value may be computed from the sensed set of vehicle parameters.
The vehicle subsystem control activation determination may include determining whether a vehicle anti-lock brake subsystem is active, and adjusting the all-wheel drive lock value to an unlocked condition when the vehicle anti-lock brake subsystem is active. The vehicle subsystem control activation determination may include determining whether a vehicle traction control subsystem is active, and adjusting the all-wheel drive lock value based on a traction control subsystem gain value when the vehicle traction control subsystem is active. The traction control subsystem gain value may be a function of the surface capability value.
The vehicle subsystem control activation determination may include determining whether a vehicle stability enhancement subsystem is active, and adjusting the all-wheel drive lock value based on a stability enhancement subsystem gain value when the vehicle stability enhancement subsystem is active. The stability enhancement subsystem gain value may be a function of the surface capability value.
Adjustments to the all-wheel drive lock value may not be made when it is determined that the anti-lock brake subsystem is not active, the traction control subsystem is not active, and the vehicle stability enhancement subsystem is not active.
Another aspect of the current invention is a computer usable medium including a program for progressive engagement of an all-wheel drive. The computer program may include code to determine an all-wheel drive lock value based on the sensed set of vehicle parameters. The computer program may include code to determine vehicle subsystem control activation. The computer program may include code to adjust the all-wheel drive lock value based on the vehicle subsystem control activation determination. The computer program may include code to engage an all-wheel drive controller based on the adjusted all-wheel drive lock value.
The computer program may include program code to sense a set of vehicle parameters including lateral acceleration, steering wheel angle, yaw rate, and wheel speed. The computer program may include code to determine the all-wheel drive lock value based on a ratio of a sensed lateral acceleration and a surface capability value.
The computer program may include code to determine the surface capability value computed from the sensed set of vehicle parameters.
The computer program may include code to determine vehicle subsystem control activation that includes determining whether a vehicle anti-lock brake subsystem is active, and adjusting the all-wheel drive lock value to an unlocked condition when the vehicle anti-lock brake subsystem is active.
The computer program may include code to determine vehicle subsystem control activation that includes determining whether a vehicle traction control subsystem is active, and adjusting the all-wheel drive lock value based on a traction control subsystem gain value when the vehicle traction control subsystem is active. The computer program may further include code to determine the traction control subsystem gain value, which may be a function of the surface capability value.
The computer program may include code to determine vehicle subsystem control activation that comprises determining whether a vehicle stability enhancement subsystem is active, and adjusting the all-wheel drive lock value based on a stability enhancement subsystem gain value when the vehicle stability enhancement subsystem is active. The computer program may include code to determine the stability enhancement subsystem gain value, which may be a function of the surface capability value.
The computer program may include code to make no adjustment to the all-wheel drive lock value when it is determined that the anti-lock brake subsystem is not active, the traction control subsystem is not active, and the vehicle stability enhancement subsystem is not active.
Another embodiment of the current invention includes a system for progressive engagement of an all-wheel drive, including a means for sensing a set of vehicle parameters; a means for determining an all-wheel drive lock value based on the sensed set of vehicle parameters; a means for determining vehicle subsystem control activation; a means for adjusting the all-wheel drive lock value based on the vehicle subsystem control activation determination; and a means for engaging an all-wheel drive controller based on the adjusted all-wheel drive lock value.
The invention provides the foregoing and other features, and the advantages of the invention will become further apparent from the following detailed description of the presently preferred embodiments, read in conjunction with the accompanying drawings. The detailed description and drawings are merely illustrative of the invention and do not limit the scope of the invention, which is defined by the appended claims and equivalents thereof.

Method and system for progressive engagement of all-wheel drive


Concentrated and low viscosity aqueous dispersions, e.g., opaque emulsions, well suited as flocculants for water treatment, comprise the water-soluble polymerizate of at least one water-soluble hydrophilic monomer A, optionally, at least one amphiphilic comonomer B and/or, also optionally, at least one hydrophobic comonomer C, in the presence of at least one polymeric dispersant incompatible therewith, for example an imidized and salified styrene/maleic anhydride copolymer or poly(diallyldimethylammonium chloride).

1. Technical Field of the Invention
The present invention relates to water-in-water dispersions and more especially, to highly concentrated aqueous dispersions comprising water-soluble polymers and having low viscosity.
2. Description of the Prior Art
Aqueous dispersions of water-soluble polymers are used in various applications and, in particular, as flocculants for the treatment of industrial and municipal waters, the dewatering of sludges, as thickeners and agents for the treatment of soils.
It is known to this art that aqueous water-soluble polymer systems having a high solids content are in the form of a gel and exhibit very high viscosities, which renders them difficult to handle and store. Need exists in this art for such aqueous systems, but which simultaneously have a high solids content and a low viscosity. It is also necessary for such systems to be stable on storage.
U.S. Pat. No. 5,403,883 describes the polymerization, in water, of a mixture of monomers comprising one or more hydrophilic monomers or one or more hydrophobic monomers and, optionally, one or more amphiphilic monomers. The polymerization results in a water-soluble polymer having a high molecular weight. The operation is carried out in the presence of a dispersing polymer incompatible with the water-soluble polymer prepared. The dispersant has a weight-average molecular weight of less than 500,000. A typical dispersant is poly(diallyldimethylammonium chloride), hereinafter denoted as "poly DADMAC."
However, this system of aqueous dispersions exists in the form of a transparent gel having a relatively high viscosity.

Concentrated aqueous dispersions of water-soluble polymers


The invention relates to a circuit assembly for operating at least one lighting means, comprising at least one master device; at least one slave device; and a bus system having at least one bus, by means of which bus system the at least one master device and the at least one slave device are coupled; wherein the bus is designed as a two-wire cable, wherein the at least one master device has at least one feeding connection, which is coupled to the bus and is designed to place a control signal on the bus, wherein the at least one master device is coupled to a first voltage supply; wherein the at least one slave device comprises a non-feeding connection, which is coupled to the bus, wherein the slave device comprises a connection for at least one lighting means, a second voltage supply, and a read-out device for reading out the control signal on the bus, wherein the read-out device comprises a potential-isolating device and wherein the connection for the at least one lighting means and the second voltage supply are provided on the side of the read-out device isolated from the bus with regard to potential.

Lamps or lighting means, also referred to as LE (=light engines) in the following, can include elements for automatization of lighting systems. They can for example be light sensors, which prevent turning on the LE with sufficiently large ambient brightness and automatically turn on this LE, respectively, if the ambient brightness falls below a presettable value. Furthermore, a motion sensor can also be integrated on an LE, which turns on the LE or varies the brightness thereof if a motion, for example a moving person, is detected in a target field.
In some cases of application, however, it can be reasonable to provide a relatively high number of lamps for reasons in terms of lighting, but not to provide sensors in each of these lamps. It is advantageous to install and provide sensors only in a part of the lamps, to remote control the other lamps from the lamp with sensor.
The present invention deals with the problem of describing a simple possibility, how control signals can be transmitted from a lamp with sensor, referred to as master in the following, to a lamp without sensor, referred to as slave in the following.
From the prior art, numerous bus systems or master-slave systems are known, for example under the designation I2C or DALI.
In this context, AT 11 444 U1 discloses an interface for a bus member of a lighting system, wherein the interface has a rectifier for rectifying the voltage of the bus line as well as means for potential isolation of the bus line, wherein the rectifier and the means for potential isolation are contained in an integrated interface.
GB 2 115 240 A discloses a control device for controlling the current, which is applied to a load from an alternating current supply, by phase control of an electrically triggerable switch, which is connected between load and current supply, wherein the control device includes a phase detector device for generating a succession of clock signals at respective points of time, at which the alternating current supply is at a preset point in its waveform.
Furthermore, DE 20 2005 021 023 U1 discloses a building installation system consisting of two different bus systems, which each have at least one bus apparatus, which are in communication with each other via a bus transfer module for the purpose of communication, adequate functional compliance and start-up, wherein incoming information of the first bus system is converted into transmittable commands of the second bus system and/or incoming information of the second bus system is converted into transmittable commands of the first bus system, wherein all of the bus apparatuses have such a construction that the bus apparatuses of the first bus system can only be employed as adapted bus apparatuses for use in the second bus system using an adapted physical layer.
From DE 10 2009 009 535 A1, a circuit for controlling an operating apparatus for a light application is known, including a galvanically separated transmitter, to which a control signal can be applied, and including a power part, which can be activated by the galvanically separated transmitter depending on the control signal.
However, these systems require a non-insignificant connection, installation and cost expenditure. Such a high complexity and functionality is not desired for simple applications for cost reasons.

Circuit assembly for operating lighting means via a master-slave system


An oscillator is temperature compensated by adjusting a slave oscillator with an error signal, that is produced by using the temperature of a reference oscillator to determine the frequency error of the reference oscillator, and then adjusting the frequency of the slave oscillator until its frequency coincides with an ambient reference frequency.

The invention relates to a temperature compensated ocillator.
Temperature compensation of an oscillator, such as a crystal oscillator, normally involves measuring the temperature of the crystal, determining the frequency error of the crystal at the temperature which involves complicated circuitry, and then applying a correction to a control signal which pulls the oscillator back on frequency. Many errors arise when this type of oscillator is used because crystal oscillators which can be adjusted require external components such as variable capacitors. Because of the type of crystal which is normally used in this type of circuit, there are accelerated aging rates of the crystals as well as frequency hysteresis effects which must also be compensated for.

Closed loop temperature compensated frequency reference


Some embodiments include a system and computer-implemented method for aggregating and tracking medical delivery to a patient including a non-transitory computer-readable medium in data communication with at least one processor, where the non-transitory computer-readable medium includes software instructions for a medical services tracking system and method. Upon execution of the software instructions, information from a patient database or server can be received and displayed a medical record dashboard. A user can view and edit access to the information, and a user selectable link can display medical record information. The system and method enable auto-population of medical data entry fields based at least one part on at least one claim made or billing signed off by a physician for at least one medical service or procedure previously provided to or performed on at least one patient.

Electronic medical record systems that provide computerized interfaces between medical professionals and staff and patient medical records and services or procedures have the potential to significantly improve and streamline the business of medical care. Using these systems, a medical provider can track the delivery of medical care, access a patients medical records, track billing for services provided, and follow a patient's progress. However, these systems typically include complex interfaces that require users to navigate through multiple layers, folders and/or windows to access even basic patient information.
Medical knowledge is doubling every five years, diagnostic tests and procedures are exploding, and documentation requirements for payments are increasing. Doctors are becoming burdened with documentation and administrative tasks rather than spending their time was medical providers. As a result, they have to turn their back to the patient to input their findings and have to navigate through multiple screens to do so. The potential of medical errors, over ordering or under ordering of diagnostic tests and medical, and other related mistakes occurs because information is missed or buried in the electronic interface of the EMR system. Furthermore, physicians increasingly rely on technicians, assistants and other staff to input information, and with a simple miss click of the mouse or human error, improper documentation can occur. Many current EMR systems require significant administrative overhead, and are prone to user error that can result in a discrepancy between billing, claims and payment for professional services and patient procedures. Much documentation involves non-physician input and client information that is subject to human error or carelessness.
There exists a need for a tool that allows physicians to rapidly detect potential problems, inconsistencies, medical changes, potential billing errors, review diagnostic tests and navigate through the entire patient chart history or access to outside sources, all through this one page quick review "cheat interactive notes" allowing a doctor to provide the best possible care.

Medical services tracking system and method


A circuit and method for controlling commutation of an electric motor includes first and second n-bit counters. The counters alternately serve as model reference and model following counters during alternate zero-cross intervals of a detected back-emf signal which is generated during rotation of the electric motor. The model following counter counts clock cycles occurring since a detected zero cross and continues counting until an optimal commutation switching point is detected, which is ideally located halfway through a commutation interval. A masking circuit is provided to mask spurious zero cross detections during a desired masking interval. A soft switching circuit determines commutation starting and ending points which are symmetrically located with respect to a center point of a commutation interval. Overflow protection circuits protect the counters from overflow during long commutation intervals and the like, and the internal clock is modulated to prevent overflow during certain operating conditions.

(1) Field of the Invention
The present invention relates to a model reference following commutation circuit and an adjusting method thereof, and in particular, to a model reference following commutation circuit for driving a brushless direct current motor and an adjusting method thereof, which optimally controls a commutation delay time and a soft switching operation to mask spurious zero-cross detections and to optimize motor performance according to individual applications.
(2) Description of Related Art
In conventional circuits for driving brushless and sensorless direct current (DC) motors, it is generally desired that an actual back-emf zero-cross point be located out of phase from an commutation switching point. For a typical three-phase DC motor, the commutation switching point is ideally located 30 eletrical degrees from the zero-cross point. Advanced methods are needed to find the optimal switching point in order to drive a motor effectively.
In addition, multi-phase DC motors have induced load characteristics which are represented by a motor electrical constant, L/R. in general, because the time constant of a motor is longer than switching time of an electric device used to control commutation in the motor, such as a field effect transistor (FET), this time difference may cause switching noise, called spikes, and may even cause current re-circulation back to a power line. Moreover, spikes may adversely cause a comparator in a circuit to detect a false zero-cross of the back-emf. Therefore, masking is required to get rid of this disturbance.
Furthermore, soft switching is needed to adjusting a turn on/off time of a transistor controlling commutation and to eliminate the disturbance caused by spikes. As shown in FIGS. 1A and 1B, one can adjust the turn on/off time so that commutation starts from a certain point before the center point and continues to a certain point after the center point, and through this commutation a soft switching can be performed. In other words, the center point is ideally halfway through a commutation interval and aligned with a center reference axial, so that it is located symmetrically with the starting point and extended point at the left and right sides of the center reference axial.
Related art exists for finding an optimal switching point for commutation and for masking fake zero-cross points incorrectly detected due to noise caused by phase commutation. Techniques for detecting an optimal switching point and masking false detections using three counters are disclosed in U.S. Pat. No. 5,221,881, issued Jun. 22, 1993, and entitled "Method and Apparatus for Operating Polyphase DC Motors" and U.S. Pat. No. 5,317,243, issued May 31, 1994, and entitled "Method and Apparatus for Detecting Velocity Profiles of a Spinning Motor of a Polyphase DC Motor."
Among the three counters used in the related art, one is an up-counter that counts the zero-cross period. The other two are down-counters of which the first down-counter counts down to zero to generate a commutation delay signal; the second down-counter starts counting down after the first down-counter completed its counting to generate a masking time. When a zero-cross is detected, the counting result of the up-counter is loaded into both of the first and the second down-counters, then the up-counter is rested until a new commutation period.
Although the digital counters used in the above techniques are an advanced approach, there are four problems worth noting. The first problem is that the up-counter must load the counting results to the down-counter, and loading errors may occur.
The second problem is that the up-counter requires operations such as stopping, loading, resetting, and restarting of the counting, all of which are time-consuming. Therefore, these operations are performed in an off-line state and not in a real time signal process to prevent losing the track of the disk and to provide accurate control.
The third problem is that four different signals are needed to consecutively execute the procedures. However, these cannot be performed at the same time. If the pulse width of a control signal is 0.01 .mu.s, from stop counting to restart counting, approximately 0.04 .mu.s is required to issue the signals. This 0.04 .mu.s is wasted, and when performed off-line, may result in lost tracking. Furthermore, miscommunication for 0.04 .mu.s is caused while the period counter counts zero-crossings, which affects the rotation speed.
And the fourth problem is that it is difficult to find the rate of acceleration for the motor. For this purpose, SGS-Thomson uses an additional N+1 bits counter to control the clock frequency for the mask and delay counters based on the detected rate of acceleration. However, this is almost useless in high-density disk drive (HDD) applications, because in steady states, the motor speed is locked up to be quite stable, and the deviation of rotor velocity is typically less than 0.05%, which is practically negligible and difficult to measure.
In order to solve these problems, a conventional circuit, using a delay including at least one resistor-capacitor, is disclosed in U.S. Pat. No. 5,233,275, issued Aug. 3, 1993, and entitled "A Simplified Sensorless DC Motor Commutation Control Circuit Using Analog Timing Techniques."
However, as is known, once a resistor-capacitor RC constant time is set, there is no way that one can change this time constant during motor operations. For both low and high frequency motor rotation speeds, this fixed time constant may be neither sufficient nor appropriate for the dynamic motor commutation. In other words, a user must always take the trouble to find an optimal value for their type of motors. That includes the difficulty in deciding upon a set-of values for the resistor and capacitor according to the un-unified motor parameters.
Accordingly, to solve the above-mentioned problems, a circuit for adjusting the commutation delay time comprising a feedback circuit between the motor and the driver, is introduced in U.S. Pat. No. 5,285,135, issued Feb. 8, 1994, and entitled "Automatic Adjustment of Commutation Delay for Brushless DC Motor for Improved Efficiency." This circuit adjusts the commutation delay by performing a transfer function to modify a signal for adjusting the delay time. However, detecting the center position using that circuit without a position sensor is not an easy task. And, if a position sensor or other hardware is used, it could make the drive circuit more complicated in terms of the electronic components of the drive IC either internally or externally.

Model reference following commutation circuit and adjusting method thereof


A removable wrist strap having a compartment containing a potable liquid held to the user's wrist by a VELCRO fastener. The wrist strap includes a tube or spout for the user to access the liquid during physical activity.

Athletes and other individuals who compete or who are required to complete rigorous physical activities within a short specified time frame such as for military training are often required to complete exercises requiring running or walking. Sources of water or other potable liquids are often not available and the user may not want to lose precious time by leaving the local or stopping for a drink. Conventional storage containers are too bulky to wear or carry. Portable storage containers known in the art utilize a water reservoir and method of holding to the user such as an elastic strip of VELCRO attached headband, wristband or the like. However, none of the known containers utilize a container having a replaceable and disposable reservoir of liquid therein attachable to the arm of the user.


A plug for use in a wellbore, said plug (201; 301) having a body with an upstream end and a downstream end, characterized in that a major portion of the length of said body is hollow and open to the upstream end of said plug. A plug set for use in a wellbore, said plug set comprising a top plug and a bottom plug, characterized in that a major portion of the volume of said bottom plug is hollow and a major portion of the length of said top plug can be accommodated therein. A plug for use in a wellbore, said plug comprising a body and at least a first fin which is made of resilient material and which projects radially outwardly from said body to engage, in use, the inner wall of a tubular, characterized in that said first fin is segmented and is provided with means to inhibit liquid passing through the gaps between said segment. The body and fin(s) of the plug can be made as a one piece injection moulding. Alternatively, the plug may be of modular construction, each module comprising a body part and a fin and each module being separately injection moulded.

This invention relates to a plug and a plug set for use in a wellbore.
During the construction of oil and gas wells a borehole is drilled in the ground. A casing string is then lowered down the borehole and the annular space between the casing and the borehole filled with cement.
In order to facilitate cementation a float shoe and/or a float collar is inserted in or adjacent the bottom of the casing.
At the commencement of the cementation process fluid is pumped down the casing and allowed to flow through the annular space between the casing and the borehole. The fluid is introduced to remove any debris from the annular space and clean the outside of the casing. This process is often referred to as xe2x80x9ccirculationxe2x80x9d.
After circulation is complete, which typically takes several hours, a bottom plug is inserted into the casing and pumped down the casing by cement. After sufficient cement has been introduced into the casing a top plug is inserted into the casing and the column of cement, bounded by the top plug and the bottom plug, is pumped down the casing by drilling mud until the bottom plug lands on the float shoe. When the bottom plug lands on the float shoe the pressure on the top plug is increased until a diaphragm in the bottom plug ruptures thereby allowing the cement to pass through the float shoe and/or float collar and flow around the bottom of the casing and upwardly through the annular space between the casing and the wellbore.
When the top plug lands on the bottom plug the casing is immediately pressure tested by increasing the pressure of the drilling mud to the test pressure and, after stopping the pumps, checking to see whether any drop in pressure in a given time exceeds a design limit.
Once the test is completed the pressure inside the casing is relaxed and the cement (which is still wet) is allowed to set. The float shoe and/or float collar are essentially check vales which allow the flow of cement from the inside of the casing to the annular space between the casing and the borehole whilst inhibiting return flow therethrough.
After the cement has set the top plug, bottom plug and any cement set in the casing are drilled out before extending the borehole.
Conventionally, top plugs and bottom plugs have been extremely sturdy and massive construction.
One well accepted plug is described and shown in applicants"" PCT Patent Publication No. WO 94/15062. As can be seen from this publication the plug comprises a central core of hard, drillable polyurethane surrounded by a relatively flexible outer sheath having a plurality of fins extending therefrom. The top fin is extremely sturdy and is designed to withstand a pressure of typically 11,400 psi (780 bar).
The central core of the bottom plug is designed to withstand the pressure applied to the top plug.
Whilst these plugs have performed very acceptably they are not without their problems. In particular, they are relatively heavy and expensive. Furthermore, there is a tendency for the plugs to wobble as they travel downhole.
In the case of the bottom plug this can result in the bottom plug not landing properly on the float valve or collar. When the diaphragm bursts the overlap between the opening in the bottom plug and the opening in the float collar can reduct flow through the float valve thereby prolonging the cementing operation.
In the case of the top plug, if this does not land properly on the bottom plug and ends up skewed in the casing the upstream fin will not seal. This usually prevents the well being pressure tested since any attempt to pressurise the drilling mud above the top plug results in the drilling mud leaking, past the top plug, passing through the hole in the bottom plug and the float valve and entering fissures in the borehole or extruding the annulus of cement upwardly, which is highly undesirable.
Dealing firstly with the alignment problem, the tops of prior art plugs are generally substantially planar with the consequence that they are generally pushed down a wellbore from behind. The applicants believe that this is largely responsible for the wobble. A possible analogy is pushing a bicycle by holding the saddle. It will be appreciated that this is quite difficult as the front wheel tends to move away from the intended direction of travel, particularly if it encounters a stone or similar obstruction on the ground (corresponding to mud cake on the wall of casing). If the bicycle is steered from the front handlebars then the saddle follows.
By analogy, the Applicants submit that if a top plug is pushed from a position at or adjacent the downstream end thereof the upstream portion will follow and the chances of the plug being misaligned when it lands are reduced.
According to a first aspect of the present invention there is provided a plug for use in a wellbore, said plug comprising a body and at least a first fin which is made of a resilient material and which projects radially outwardly from said body to engage in use, the inner wall of a tubular, characterised in that said first fin is segmented and is provided with means to inhibit liquid passing through the gap between said segments.
In its simplest form said means may be provided by an extension piece integral with or attached to each segment. However, said means preferably comprises a second fin which is mounted downstream of said first fin and is segmented in such a way that, in use, said segments in said second fin engage said segments in said first fin and overlap the gaps therebetween.
If desired further segmented fins may be provided. Preferably, the segments of the first fin are mounted to pivot about a line which is disposed radially outwardly of a line about which the segments of said second fin pivots. This helps ensure that the segments of the second fin effectively engage the segments of the first fin.
Advantageously, the radial extremities of the segments of the second fin are provided with means which, in use, facilitate the removal of mud cake from the inside of casing. Such means may comprise, for example, ribs or protrusions on the radial extremity of at least some of the segments or even forming an angled surface on the radial extremity of the segments which acts like a chisel as the plug passes through a tubular.
By suitably dimensioning the segments it is conceivable that a very strong fin could be formed. However, in the context of the preferred embodiment it is anticipated that the fins will have a thickness of less than 3 mm.
Plugs in accordance with the present invention lend themselves to production in one piece particularly by injection moulding.
A suitable moulding material would be a polyamide, for example a polyamide currently sold under the trademark xe2x80x9c!!PETER!!xe2x80x9d by BASF of Germany.
It will be appreciated that a one piece injection moulding is far less expensive than the two stage moulding previously used in the production of conventional plugs.
The preferred embodiment satisfies certain needs which are now emerging. In particular, because of the greater depths of modern wells and the greater radial reach of many wells, it is becoming increasingly difficult to ensure that the cement is in a usable state by the time it reaches the bottom of the casing (or liner). In particular, it can take two hours for the bottom plug to travel from the surface to the float collar. If the bottom plug does not align properly with the float collar then the flow rate of the cement through the float collar can be restricted to such an extent that the cement passes through its xe2x80x9cfirst setxe2x80x9d before reaching the annulus.
High capacity float valves are being developed to reduce this problem. However, another solution is to use special chemical mixtures which precede and follow the slug of cement. These chemicals are strong solvents and tend to dissolve rubber which is most commonly used in the construction of plugs. Furthermore, such chemicals are particularly effective solvents at high temperatures.
Whilst the preferred materials of the present invention are, in their own right, resistant to chemical attack at high temperatures we have found that a plug having a body and fins formed in one piece by injection moulding has outstanding characteristics.
According to a second aspect of the present invention there is provided a plug for use in a wellbore, said plug having a body with an upstream end and a downstream end, characterised in that a major portion of the length of said body is hollow and open to the upstream end of said plug.
The plug may be a top plug, a bottom plug or an intermediate plug. In the case of the bottom plug or the intermediate plug a bursting disk or similar rupturable membrane will be provided at or adjacent the downstream end of the plug.
Advantageously, the major portion of the volume of said body is hollow and open to the upstream end of said plug.
It will be noted that by increasing the volume of the hollow portion of the plug the total amount of material required to manufacture the plug is reduced and the cost of manufacturing and transporting the plug reduced accordingly. However, using conventional concepts it is still important that the bottom plug should be able to support the top plug, the bodies of which are of equal diameter.
According to a third aspect of the present invention there is provided a plug set for use in a wellbore said plug set comprising a top plug and a bottom plug, characterised in that a major portion of the volume of said bottom plug is hollow and a major portion of the length of said top plug can be accommodated therein.
Advantageously, a major portion of the length of said top plug is hollow and open to the upstream end of said top plug.
Preferably, the downstream end of the bottom plug is provided with a nose which is preferably tapered and which can be received in a correspondingly tapered entrance to a float shoe or float collar. The nose is preferably provided with a seal to sealingly engage in the tapered entrance to the float valve.
In a similar manner the top plug is preferably provided with a tapered nose which can enter the tapered nose of the bottom plug and seat thereon. The tapered nose is preferably provided with a seal to facilitate the making of a seal between the tapered nose on the top plug and the tapered nose on the bottom plug.
By adopting this nesting construction the need for a strong core disappears. Typically, after a prior art cementing operation is complete and the cement set the top plug and the bottom plug are drilled out. This typically takes 15 minutes which is extremely expensive when it is recalled that a large offshore platform can cost over $1,000,000 a day to operate. In contrast, the nested top plug and bottom plug described above offer negligible resistance to the immense weight and power of a large drill which will simply pass through the nested top plug and bottom plug before drilling out the float collar and/or float shoe and any cement therebelow before drilling onwards.
The next area of investigation is the fin design. As indicated above the upstream fin on conventional plugs is usually massive. This is because the top of the top plug forms the platform on which the casing is pressure tested. The top plug itself must be supported by a substantial bottom plug which rests on the float collar or shoe.
If the upstream fin is not required to perform such an onerous duty then the design of the fin can be changed.
According to a fourth aspect of the present invention there is provided a plug for use in a wellbore, said plug comprising a body and fins, characterised in that said body and fins are formed in one piece by injection moulding.


A recycled exhaust gas cooling and condensate regulation system for a natural gas fired internal combustion engine driven co-generation plant, which allows efficient exhaust recycled gas combustion while maintaining lower head temperatures to reduce thermal NOx emissions and delivering increased process/utility heat to a proximate co-generation client, is provided. The recycled exhaust gas cooling and condensate regulation system has primary and secondary, air-to-gas, exchanger units, coupled in series, such that the first unit, designed to take exhaust gas exiting the engine, cools the gas to a first exhaust gas temperature effectively super saturating the water vapor in the cooled exhaust gas; and, the second unit, designed to handle lower inlet temperatures, condenses the water vapor and super cools the exhaust gas to <130 F. In one embodiment, the engine intake mixture including the exhaust gas is supercharged. In another embodiment, a separate intercooler circuit is used to cool the compressed intake charge containing the recycled gas prior to entry into the intake engine manifold to further reduce head temperatures and control thermal NOx emissions.

1. Field of the Invention
The present invention relates generally to recycled exhaust gas cooling systems for natural gas fueled internal combustion engines; and, more particularly, to systems for controlling temperature and condensate in cooled exhaust gas for recycle to natural gas fueled internal combustion engines to effect engine operating characteristics and emission control.
2. Description of Related Art
Electric energy generation in this country has lagged behind demand. There are a number of reasons for this, but chief among them is failure of traditional energy producers to replace spent units and capitalize new plants. This has been, in part, due to increased air quality regulations. In addition, new challenges face electric generation-security. Events of Sep. 11, 2001 showed this nation its vulnerability to terrorist attack. Vital operations, such as police, medical and civil defense that relied upon the electric power "grid" for service, realized that their needs were susceptible to disruption and viewed stand-alone unit alternatives as well as micro grids as a possible solution. These alternatives, however, have their own problems.
One reason for the growth in demand is the increased use of computers and other technology for industrial and business purposes, as well as personal use. As computer usage continues to grow, the use of power-consuming peripheral technologies, such as printers, cameras, copiers, photo processors, servers, and the like, keep pace and even expand. As business use of computer based equipment continues to rise, as do the number of in-house data servers, outsourced data storage facilities, financial systems, and Internet-related companies requiring constant electrical uptime and somewhat reducing traditional peak demand times, requirement for reliable, cheap, environmentally compliant electrical power continues to grow.
Other technological advances have also increased electrical energy demand. Increased use of power consuming devices in every aspect of life from medical to industrial manufacturing robots, as well as innovations in almost every research and industrial field are supported by increasingly complex technology, which requires more electrical power to function. CAT scans, NMRs, side looking X-rays, MRIs and the like all take electrical power.
As a result, the Federal Government deregulated power generation, and a number of states have begun to establish competitive retail energy markets. Unfortunately, the deregulation process has not provided adequate incentives for industry entities to construct generating facilities, upgrade the transmission grid, or provide consumers with price signals to enable intelligent demand-side management of energy consumption. With the deregulation in the utility market, energy (kWh) has become a commodity item that can be bought or sold. However, swings in supply and demand leave end users open to fluctuations in the cost of electricity.
According to the ETA, to meet projected increases in demand over the next 20 years, at least 393 GW of additional generating capacity must be added. In some areas, the growth in demand is much higher than the projected two percent average (e.g., California's peak electricity demand grew by 18 percent between 1993 and 1999, while generating capacity increased by only 0.3 percent.) Despite California's highly publicized energy situation, a similar problem exists for other states as well; the New York Independent System Operator recently stated that 8600 MW of additional generating capacity (a 25 percent increase) must be added by 2005 to avoid widespread shortages that may lead to blackouts.
In addition to the mismatch between demand and generating capacity, the physical transmission infrastructure necessary to deliver power from geographically remote generating facilities to the consumer's location is unable to support the increased load. Even under today's operating conditions, the transmission grid is subject to stress and occasional failure.
Additionally, security and reliability of source has become of increasing concern. Vulnerability of grid systems and blackouts has become more commonplace. Strategic industries are looking to cut energy costs, increase reliability, and assure security. This has lead to an interest in distributed market technologies. The potential market for distributed generation has become vast without adequate means for fulfilling this need. Again, inefficiency, reliability, and environmental concerns are major barriers. The compelling economics are made on engine efficiency without the financial benefit of waste heat usage, yet with all of the same customer reluctance to accept hassles. Industry estimates indicate that the existing market for distributed generation is $300 billion in the United States and $800 billion worldwide.
The need to leverage existing technology while transitioning to alternative energy sources is an important driver for meeting this challenge. Although most existing distributed generation sites use small gas turbine or reciprocating engines for generation, there are many alternatives that are being considered over the longer term. Technologies, such as micro turbines, are currently available, but only used at a relatively small number of sites. These newer generators offer some inherent advantages, including built-in communications capabilities. It is anticipated that fuel cells will be available in the next five years, which will provide some highly appealing, environmentally friendly options.
As it stands today however, small gas turbine and reciprocating engines comprise a substantial proportion of existing generator technology in the market and will for some time to come for a number of reasons. Engines provide the best conversion efficiency (40%), and they can operate using non-pressurized gas. Micro turbines, on the other hand, require compressed gas and conversion efficiency is lower (approximately 30%). These latter generators tend to be used in wastewater, landfill, and other specialty sites, where a conventional prime mover is unable to stand up to poor fuel quality. Therefore, for utilities to truly benefit from a distributed generation scheme over the short term, they must look to the existing generator technology to provide a sustainable and affordable solution.
Waste heat utilization or co-generation is one way to meet this challenge. In the case of power generation, the waste heat is not used, and the economics are based largely on the cost of the electricity produced (i.e. heat rate is paramount), with little consideration for improved reliability or independence from the electric grid. The anticipated fluctuation in energy costs, reduced reliability, and increasing demand has led end users to consider maximizing efficiency through use of heat from generation of on-site generating-heat capture systems, i.e. co-generation, or "Combined Heating and Power" (CHP).
Co-generation of electricity and client process/utility service heat to provide space heating and/or hot water from the same unit is one solution. Co-generation provides both electricity and usable process or utility heat from the formerly wasted energy inherent in the electricity generating process. With co-generation, two problems are solved for the price of one. In either case, the electricity generation must meet stringent local air quality standards, which are typically much tougher than EPA (nation wide) standards.
On-site co-generation represents a potentially valuable resource for utilities by way of distributed generation. A utility can increase capacity by turning to a "host" site (e.g. industrial user) with an existing generator, and allow them to parallel with the grid and use their generator capacity to handle peak volumes. From the utility's point of view, the key advantages to a distributed generation solution are twofold: improved system reliability and quality; and the ability to defer capital costs for a new transformer station.
For customers who can use the process/utility waste heat, the economics of co-generation are compelling. The impediment to widespread use is reliability, convenience, and trouble-free operation. Co-generation products empower industrial and commercial entities to provide their own energy supply, thus meeting their demand requirements without relying on an increasingly inadequate public supply and infrastructure. Unfortunately, to date, the most widespread and cost-effective technologies for producing distributed generation and heat require burning hydrocarbon-based fuel. Other generating technologies are in use, including nuclear and hydroelectric energy, as well as alternative technologies such as solar, wind, and geothermal energy. However, burning hydrocarbon-based fuel remains the primary method of producing electricity. Unfortunately, the emissions associated with burning hydrocarbon fuels are generally considered damaging to the environment and the Environmental Protection Agency has consistently tightened emissions standards for new power plants. Green house gases, as well as entrained and other combustion product pollutants, are environmental challenges faced by hydrocarbon-based units.
Of the fossil fuels, natural gas is the least environmentally harmful. Most natural gas is primarily composed of methane and combinations of Carbon Dioxide, Nitrogen, Ethane, Propane, Iso-Butane, N-Butane, Iso-Pentane, N-Pentane, and Hexanes Plus. Natural gas has an extremely high octane number, approximately 130, thus allowing higher compression ratios and broad flammability limits. Natural gas is the most popular fuel choice for engine cogeneration because it is relatively clean, already widely distributed, safe, and it provides favorable engine power and durability. However, many of the markets that would be best served by the economics of engine-based co-generation have such poor air quality that strict exhaust emission limits have been instituted by air quality regulating agencies. The exhaust emissions limits on oxides of nitrogen, carbon monoxide, and non-methane hydrocarbons are so restrictive that no technology exists to allow raw exhaust emissions from any engine operating on any hydrocarbon fuel to enter the atmosphere without exhaust aftertreatment and a variety of other strategies. problem area is the emissions produced by these natural gas engines. Never-the-less, natural gas fueled engines provide a valuable power source for distributed generation.
Internal combustion engines utilized for combined heat and power are designed so that heat generated during combustion can be recovered from the engine coolant and exhaust and then transferred to a cogeneration client. Prior art co-generation systems have had to comply with strict emissions limits by either altering the air/fuel ratio from an excess-air strategy to a stoichiometric strategy to facilitate the successful operation of non-selective three-way catalysts or by applying selective catalytic reduction (SCR) exhaust aftertreatment technologies to the exhausts of excess-air fueled engines. Each approach has undesirable consequences compared to the original excess-air, or lean-burn, operation. The stoichiometric air/fuel ratio with three-way catalyst strategy increases combustion temperatures to such an extent that the engine must be derated to control detonation and mitigate accelerated wear. This scenario also results in reduced fuel efficiency compared to a lean-burn engine. The SCR emissions compliance approach allows a lean-burn engine to operate at full load with excellent fuel efficiency, but at the expense of having to store chemicals on site and then inject them in a very controlled fashion into the lean-burn exhaust stream. After injection the exhaust becomes compatible with catalytic emissions reduction techniques.
Thus, applying SCR technology is expensive, complicated, and generally not considered an option for engines producing less than one megawatt of electric power. Prior art co-generation systems employing internal combustion engines, and specifically, natural gas fueled engines, have suffered from the myriad of problems including elevated head temperatures and inability to deliver large quantities of process and/or utility heat not true to the co-generation client. Excessive head temperatures lead to inefficient operation and unacceptable environmental conditions, which include excessive use of fuel as well as significant thermal NOx production.
It is well known that emission reduction for natural gas engines can be accomplished by recycling of exhaust gases to make the engines "run lean." This is the same term used to describe the diluted air/fuel condition that exists when excess-air is introduced into combustion. This method of combustion combines the positive cool combustions and high fuel efficiency attributes of operating with excess air with the compatibility between simple non-selective three-way catalysts and exhaust that stoichiometric engines benefit from without incurring the need to derate the output of the engine. This method of emission control has decided advantages over exhaust gas after treatment such as a catalyst. When catalysts are still required they are minimized.
For this reason, numerous systems have been devised to recycle exhaust gas into the fuel-air induction system of an internal combustion engine for the purposes of pre-heating the air-fuel mixture to facilitate its complete combustion in the combustion zone, for re-using the unignited or partially burned portions of the fuel which would otherwise pass to exhaust and into the atmosphere, and for reducing the oxides of nitrogen emitted from the exhaust system into the atmosphere. It has been found that approximately 15% to 20% exhaust gas recycling is required at moderate engine loads to substantially reduce the nitrogen oxide content of the exhaust gases discharged in the atmosphere, that is, to below about 1,000 parts per million.
Although the prior art systems have had the desired effect of reducing nitrogen oxides in the exhaust by reducing the maximum combustion temperature as a consequence of diluting the fuel-air mixture with recycled exhaust gases during certain operating conditions of the engine, these systems have not been experienced broad market application in the field of cogeneration where continuously high engine output is the normal mode of operation and the level technological implementation on engine controls often lacks the automotive market.
The formation rate of nitrogen oxide emission is a direct function of combustion temperature. Any incremental increase in rate of cooled EGR applied during combustion at any load results in lower peak combustion temperatures and hence lower untreated NOx emissions. The propensity for detonation, another temperature dependent phenomenon, is also reduced for each incremental increase in cooled EGR. EGR rates from 20–25% are generally required to achieve similar detonation control characteristics and raw engine-out NOx formation rates as compared to high excess-air strategies.
Thus, natural gas engine survivability with regards to detonation at high load is largely dependent on the success of appropriately metering and cooling the recirculated exhaust gas. While there are numerous products available commercially for the accurate control and metering of the amount of excess-air used in traditional lean-burn (lambda>1) engines, the same is not true for recirculated exhaust gas metering and cooling in non-automotive applications. The industrial nature of the cogeneration market does not benefit from the research and development budgets of the mass produced automotive products and thus the application of advanced emissions control techniques such as cooled EGR has thus far been limited. One challenges for applying EGR highly loaded natural gas engines includes is to provide sufficient cooling of the recirculated exhaust gas such that impacts on volumetric efficiency of air induction are minimized. The higher the temperature of the recirculated exhaust gas as it enters the air/fuel stream, the more difficult it becomes to induce adequate air flow to support full load combustion. Furthermore, the higher the EGR temperature, the higher the compressed intake charges temperature from the turbocharger both before and after the charge-air intercooler. The higher the EGR temperature induced into the air stream, the more this higher temperature offsets the benefits of EGR with regards to detonation mitigation.
Natural gas fired, internal combustion driven, co-generation systems have previously suffered from one or more disadvantages. Specifically, the EGR system did not recycle exhaust gas to the intake engine manifold at sufficiently low temperature to foster low cylinder head temperatures. Simultaneously, turbo charged fuel systems, because of the compression, increase intake fuel manifold temperatures causing the same effect. This combination of disadvantages made natural gas fueled, internal combustion driven co-generation systems a marginal candidate for client based distributed generation complexes.
Additionally, since combustion of natural gas (methane) produces large amounts of water, this water is entrained as vapor in the exhaust gas. Although reducing the temperature of the exhaust gas allows more volume of gas to be introduced into the intake gas in the manifold, it can also present a water vapor saturated exhaust gas mixture, which precipitates at the carburetion site. Thus, cooling of the gas in a non-regulated manner can present condensation problems in the intake venturi. If the gas is cooled in the EGR cooler, so that the vapor pressure in the exhaust gas approaches saturation, further cooling by introduction of air and fuel causes an immediate condensation within the intake manifold, which can lead to erratic engine performance, corrosion, and the like.
In addition, the presence of large amounts of water vapor in the recycled exhaust gas displaces other combustion gases in the intake manifold making the engine unstable. Problematically, introducing the recycle gas at a temperature above the dew point (to prevent induction condensation) leads to detonation and NOx production causing the engine to burn out of NOx emission ranges as well as shorting engine life. Contrawise, some water vapor in the recycled exhaust gas is beneficial to the engine performance.
The previous solution was to increase the capacity of the primary cooler. However, in addition to being hard to "tune", large primary coolers are expensive because they must handle very high temperatures and corrosive media. Because the volume of very hot gas varies as the engine accelerates or decelerates, the water vapor in the exhaust gas exiting such primary coolers, is difficult to regulate. When the cooler is a single unit, it is difficult to incrementally vary the composition of the output gas.
Typical prior art exchangers are gas-to-liquid, not gas-to-air. These heat exchangers are found commercially on heavy-duty truck and bus engines for the purpose of cooling re-circulated exhaust gas for diesel emissions compliance. However, the dynamics of these systems are different. For all the reasons mentioned above, these engines have different operating characteristics than natural gas fueled engines. First, the engine's coolant is typically the cooling medium in order to achieve exhaust gas cooling in a minimum amount of space. This inherently limits the minimum EGR temperature that can be achieved to well over 200° F. even with the best of liquid-to-gas heat exchangers. For diesel engines, this does not pose a serious threat to engine life because detonation is not a concern with compression ignition engines. Second, gas and diesel engine, liquid-to-gas, EGR cooling heat exchangers are typically of stainless steel construction in flat-plate and tube-in-shell configurations. This design is very expensive, but necessary to handle the exhaust gas heat.
In addition, certain spark ignited engines, such as gasoline and natural gas fueled engines, have different EGR constraints than compression ignition engines, such as diesel. In automotive gasoline engines, EGR is a prevalent emissions reduction technique. However, gasoline engine using EGR emission control do not require dedicated EGR cooling because: 1) the EGR rate is usually limited to 10–12% minimizing the impact on charge-air temperatures; and; 2) the EGR is not employed at high load where temperature-influenced ignition is of most concern.
It would, therefore, be advantageous to have an air-to-gas cooled system wherein recycled exhaust gas temperature and vapor pressure could be regulated to maintain engine performance and emission compliance through out the load range while preventing inadvertent condensation within the intake manifold which causes sporadic engine performance and reduces engine life. It would be further advantageous to have an air-to-gas cooled system which allows the natural gas fired internal combustion engine using EGR to operate at higher shaft power output without inducing detonation; allows the engine to operate with better fuel efficiency due to increased detonation tolerance when advancing ignition timing; and, allows the engine to operate at the same maximum continuous power output. In addition, it would be advantageous to have an EGR cooling system which could be "tuned" to facilitate using recycled exhaust gas for a lean burning natural gas fired engine which results in not only a lean burn but also reduced head temperatures leading to reduces thermal emissions and greater efficiency without unwanted detonations and/or water condensation within the intake manifold.

EGR cooling and condensate regulation system for natural gas fired co-generation unit