This application is a 371 of PCT/EP2008/007891 filed Sep. 19, 2008. Priority is claimed on that application, and on the following application: 
     Country: Germany, Application 10 2007 045 195.6, filed Sep. 21, 2007. The entire contents of these applications are incorporated herein by reference. 
     BACKGROUND OF THE INVENTION 
     The invention concerns a method for automatically controlling a stationary gas engine, in which a speed control deviation is computed from a set speed and an actual speed, the speed control deviation is used by a speed controller to determine a set torque as a correcting variable, and the set torque is used to determine a set volume flow. In addition, the method consists in determining a mixture throttle angle for determining a mixture volume flow and for determining an actual mixture pressure in a receiver tube upstream of the intake valves of the gas engine as a function of the set volume flow. The invention further consists in determining a gas throttle angle for determining a gas volume flow as the gas fraction in a gas/air mixture, likewise as a function of the set volume flow. 
     Stationary gas engines are often used to power emergency generators or rapid-readiness units. In this connection, the gas engine is operated at a lambda value of, for example, 1.7, i.e., a lean mixture with excess air. The gas engine typically includes a gas throttle for setting the gas fraction in the gas/air mixture, a mixer for mixing the combustible gas and the air, a compressor as part of an exhaust gas turbocharger, a cooler, and a mixture throttle. The intake volume flow in the receiver tube upstream of the intake valves of the gas engine is set by the mixture throttle, and thus the mixture pressure in the receiver tube is also set. 
     EP 1 158 149 A1 describes a stationary gas engine for driving a generator. The gas engine is controlled by using a characteristic curve to compute a set lambda as a reference output from the engine output. On the basis of the set lambda, an electronic engine control unit computes a gas quantity set value, by which the gas throttle is then suitably adjusted. In a second embodiment, the set lambda value is computed from a mixture pressure control deviation. The mixture pressure control deviation is determined from the detected actual mixture pressure in the receiver tube and the set mixture pressure, which in turn is determined from the engine output by means of a characteristic curve. In a third embodiment, as a supplement to the second embodiment, the gas quantity set value is corrected to adjust the gas throttle as a function of the position of a compressor bypass valve and the speed control deviation. A common feature of all three embodiments is the adjustment of the gas throttle to a set lambda value. In practical operation, this means that when a change in the power assignment is made, first a change is made in the position of the mixture throttle as the power control element. This has the effect that the intake mixture volume flow also changes. Since the position of the gas throttle initially remains constant, there is also no change in the gas volume flow. This results in a changing actual lambda. When a mixture throttle is controlled to move, for example, in the closing direction, this causes enrichment of the mixture, which results in a change in output of the gas engine. As a response to this change in output, the set lambda value, the gas quantity set value, and the position of the gas throttle are then changed. In this type of automatic control, the response time, for example, when the load changes, is critical, since intervention in the lambda control is sluggish due to the system itself. 
     DE 103 46 983 A1 also describes a gas engine and a method for automatically controlling the fuel mixture. In this method, in a first step, an actual pressure difference of the air mass flow is determined in a venturi mixer, and, in a second step, a set pressure difference of the air mass flow is determined from the measured actual output of the gas engine. In a third step, the actual pressure difference is then brought closer to the set pressure difference by changing the amount of gas supplied by changing the position of the gas throttle. In a fourth step, the actual gas engine output that develops is detected again, and the mixture throttle is adjusted in such a way that the set/actual deviation of the pressure difference of the air mass flow in the venturi mixer is reduced. This sequential order of operations is carried out iteratively until the set/actual deviation of the pressure difference is smaller than a limit. Since a change in the position of the mixture throttle produces a change in the output of the gas engine, the position of the gas throttle must be readjusted to compensate the change in output of the gas engine. Under certain circumstances, this can cause the correcting variables to overshoot. 
     SUMMARY OF THE INVENTION 
     The objective of the invention is to design a method for automatically controlling a stationary gas engine with improved control performance. 
     This objective is achieved by a method in which a speed control deviation is computed from a set speed and an actual speed, the speed control deviation is used by a speed controller to determine a set torque as a correcting variable, and the set torque is then used to determine a set volume flow. A mixture throttle angle for determining a mixture volume flow and for determining an actual mixture pressure in a receiver tube upstream of the intake valves of the gas engine is in turn determined as a function of the set volume flow. A gas throttle angle for determining a gas volume flow as the gas fraction in a gas/air mixture is likewise determined as a function of the set volume flow. The central idea of the invention is thus the parallel control of the gas throttle and the mixture throttle as a function of the same actuating variable, in this case, the set volume flow. Advantages include not only a shortened response time but also a more precise transient oscillation with improved adjustability of the total system. In addition, due to the parallel control, lambda tracking is not necessary. All together, the invention allows uniform automatic control of the engine output. 
     The set volume flow is computed from the set torque by limiting the set torque and assigning the set volume flow to the limited set torque by an engine map as a function of the actual speed. The set torque is limited as a function of the actual speed and, in addition, as a function of a detected fault state of the system, for example, a sensor failure. A permissible mechanical maximum torque is also taken into account. The limitation of the set torque improves the operating reliability of the total system. 
     The mixture throttle angle is determined by computing a set mixture pressure from the set volume flow, determining a mixture pressure control deviation from the set mixture pressure and an actual mixture pressure in the receiver tube, and using the mixture pressure control deviation to compute, by means of a mixture pressure controller, a correcting variable for determining the mixture throttle angle. The computation of the set mixture pressure involves the use not only of the system constants but also, for example, the stroke volume, a set lambda, and a mixture temperature in the receiver tube. 
     In a V-type gas engine, the method provides that a first mixture throttle angle is computed for the A side for determining a first mixture volume flow and a first actual mixture pressure in a first receiver tube and that a second mixture throttle angle is computed for the B side for determining a second mixture volume flow and a second actual mixture pressure in a second receiver tube. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The drawings show a preferred embodiment 
         FIG. 1  is a total system diagram, 
         FIG. 2  is a functional block diagram for controlling the gas throttle and the mixture throttles, 
         FIG. 3  is a closed-loop control system for the automatic control of the mixture pressure, and 
         FIG. 4  is a program flowchart. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       FIG. 1  shows a total system diagram of a stationary gas engine  1  with a V configuration. The gas engine  1  drives a generator  5  via a shaft  2 , a coupling  3 , and a shaft  4 . The generator  5  generates electric power, which is fed into an electric network. The following mechanical components are assigned to the gas engine  1 : a gas throttle  6  for setting a supply volume flow of gas, for example, natural gas; a mixer  7  for mixing air and gas; a compressor  8  as part of an exhaust gas turbocharger; cooler  9 ; a first mixture throttle  10  on the A side of the gas engine  1 ; and a second mixture throttle  11  on the B side of the gas engine  1 . 
     The operating mode of the gas engine  1  is determined by an electronic gas engine control unit  14  (GECU). The electronic engine control unit  14  contains the usual components of a microcomputer system, for example, a microprocessor, interface adapters, buffers, and memory components (EEPROM, RAM). Operating characteristics that are relevant to the operation of the gas engine  1  are applied in the memory components in the form of engine maps/characteristic curves. The electronic engine control unit  14  uses these to compute the output variables from the input variables.  FIG. 1  shows the following input variables: a first actual mixture pressure p 1 (IST) and a mixture temperature T 1 , both of which are measured in the first receiver tube  12 , a second actual mixture pressure p 2 (IST), which is measured in the second receiver tube  13 , an actual engine speed nIST of the gas engine  1 , a set speed nSL, which is preassigned by a system controller (not shown) of the generator  5 , and an input variable IN. The other input signals, for example, the oil temperature, are combined as the input variable IN.  FIG. 1  also shows the following output variables of the electronic engine control unit  14 : the signal of a set volume flow VSL for controlling the gas throttle  6 , the signal of a first mixture throttle angle DKW 1  for controlling the first mixture throttle  10 , the signal of a second mixture throttle angle DKW 2  for controlling the second mixture throttle  11 , and a signal OUT. The signal OUT is representative of the other signals for regulating and controlling the gas engine  1 . 
     The system has the following general functionality: A gas volume flow supplied to the mixer  7  is adjusted by the position of the gas throttle  6 . The position of the first mixture throttle  10  defines a first mixture volume and thus the first actual mixture pressure p 1 (IST) in the first receiver tube  12  upstream of the intake valves of the gas engine  1 . The second mixture throttle  11  determines a second mixture volume and thus the second actual mixture pressure p 2 (IST) in the second receiver tube  13  upstream of the intake valves of the gas engine  1 . 
       FIG. 2  shows a functional block diagram for controlling the two mixture throttles  10  and  11  and the gas throttle  6 . The system controller of the generator is identified by reference number  15 . Reference number  14  identifies the electronic engine control unit in the form of a reduced block diagram, in which the depicted elements represent the program steps of an executable program. The input variables of the electronic engine control unit  14  in this representation are the set speed nSL and optionally an actual torque MIST, which are supplied by the system controller  15 , the actual speed nIST, and an additional variable E. The additional variable E combines the following: a set lambda, a stroke volume of the cylinders of the gas engine, the volumetric efficiency in terms of a cylinder cutoff, and the fuel quality. The output variables are the first mixture throttle angle DKW 1  for controlling the first mixture throttle  10 , the second mixture throttle angle DKW 2  for controlling the second mixture throttle  11 , and the set volume flow VSL for controlling the gas throttle  6 . 
     The set speed nSL, for example, 1,500 rpm, which corresponds to a frequency of 50 Hz, is preset by the system controller  15  as the desired output. At a point A, a speed control deviation dn is computed from the set speed nSL and the actual speed nIST. A speed controller  16  uses the speed control deviation do to compute a set torque MSL as a correcting variable. In practice, the speed controller  16  is realized as a PIDT 1  controller. A torque limiter  17  limits the set torque MSL to a minimum and maximum value. The output value represents a limited set torque MSLB. The parameters for the limits of the torque limiter  17  are the actual speed nIST and a fault signal FM, which is set when an error in the total system is detected, for example, if a defective pressure sensor is detected. A permissible mechanical maximum torque can also be provided as an additional parameter. If the value of the set torque MSL is in the permitted range, then the value of the limited set torque MSLB is the same as the value of the set torque MSL. A set volume flow VSL is assigned by an efficiency unit  18  to the limited set torque MSLB as a function of the actual speed nIST. For this purpose, a suitable engine map is stored in the efficiency unit  18 . The set volume flow VSL is the input variable of the mixture quantity unit  19  and at the same time is the input variable of the gas throttle  6 . The mixture quantity unit  19  computes the first mixture throttle angle DKW 1  and the second mixture throttle angle DKW 2  from the set volume flow VSL as a function of the actual speed nIST and the input variable E. The mixture quantity unit  19  will be explained in greater detail in connection with  FIG. 3 . The first mixture throttle  10  is controlled with the first mixture throttle angle DKW 1 . The first mixture throttle  10  sets a first mixture volume flow V 1  and the first actual mixture pressure p 1 (IST). The second mixture throttle  11  is controlled with the second mixture throttle angle DKW 2  and sets a second mixture volume flow V 2  and the second actual mixture pressure p 2 (IST). The gas throttle  6  is also controlled with the set volume flow VSL. The gas throttle  6  has an integrated electronic processing unit  20 , by which the value of the set volume flow VSL is assigned a corresponding cross-sectional area and a corresponding angle. The gas throttle  6  sets a gas volume flow VG as the gas fraction of the gas/air mixture. 
     As shown in  FIG. 2 , the two mixture throttles  10  and  11  and the gas throttle  6  are controlled parallel to each other as a function of the same setpoint value, in this case, the set volume flow VSL. Compared to the prior art with sequential control and lambda tracking, the method of the invention offers the advantages of a shortened response time and a more precise transient oscillation with improved adjustability of the total system. In addition, due to the parallel control, lambda tracking is not necessary. All together, the invention allows uniform automatic control of the engine output. 
       FIG. 3  shows a first closed-loop control system  21  for automatically controlling the first actual mixture pressure p 1 (IST) in the first receiver tube and a second closed-loop control system  22  for automatically controlling the second actual mixture pressure p 2 (IST) in the second receiver tube. Reference number  23  identifies a computing unit for computing the set mixture pressure pSL. The input variable of the first closed-loop control system  21  is the set mixture pressure pSL. The output variable of the first closed-loop control system  21  is the first actual mixture pressure p 1 (IST). The first closed-loop control system  21  comprises a comparison point A, a first mixture pressure controller  24 , a first characteristic curve  25 , and, as the controlled system, the first mixture throttle  10  for determining the supplied mixture volume flow and the first actual mixture pressure p 1 (IST). The input variable of the second closed-loop control system  22  is also the set mixture pressure pSL. The output variable of the second closed-loop control system  22  is the second actual mixture pressure p 2 (IST). The second closed-loop control system  22  comprises a comparison point B, a second mixture pressure controller  26 , a second characteristic curve  27 , and, as the controlled system, the second mixture throttle  11  for determining the supplied mixture volume flow and the second actual mixture pressure p 2 (IST). The computing unit  23 , the two comparison points (A, B), the two mixture pressure controllers ( 24 ,  26 ), and the two characteristic curves ( 25 ,  27 ) are integrated in the mixture quantity unit  19 , as is indicated by a dot-dash line. 
     The computing unit  23  computes the set mixture pressure pSL from the preset volume flow VSL by the following formula:
 
 pSL={VSL− 2[1 +L MIN−LAM( SL )]· T 1− p NORM}/[ nIST·VH·LG·T NORM]
 
where:
 
pSL set mixture pressure
 
VSL set volume flow
 
LMIN fuel quality
 
LAM(SL) set lambda
 
T 1  temperature in the first receiver tube
 
pNORM standard air pressure at mean sea level (1,013 mbars)
 
nIST present actual speed
 
VH stroke volume of the engine
 
LG volumetric efficiency (cylinder cutoff)
 
TNORM standard temperature 273.15 K
 
     The set mixture pressure pSL is the reference input for the two closed-loop control systems  21  and  22 . The set mixture pressure pSL is compared with the first actual mixture pressure p 1 (IST) at comparison point A. The result corresponds to the first mixture pressure control deviation dp 1 . The first mixture pressure controller  24 , which is typically a PIDT 1  controller, uses this control deviation dpi to compute a first cross-sectional area QF 1  as a correcting variable. The first mixture throttle angle DKW 1  is assigned to the first cross-sectional area QF 1  by the first characteristic curve  25 . The first mixture throttle  10 , which is the controlled system, is then controlled with the first mixture throttle angle DKW 1 . The output variable of the first mixture throttle  10  is the first actual mixture pressure p 1 (IST), which is the controlled variable. The first actual mixture pressure p 1 (IST) is returned to the comparison point A through an optional filter (not shown). The first closed-loop control system  21  is thus closed. 
     The set mixture pressure pSL is compared with the second actual mixture pressure p 2 (IST) at comparison point B. The result corresponds to the second mixture pressure control deviation dp 2 . The second mixture pressure controller  26  uses this control deviation dp 2  to compute a second cross-sectional area QF 2  as a correcting variable, to which is assigned the second mixture throttle angle DKW 2  by the second characteristic curve  27 . The second mixture throttle  11 , which is the controlled system, is then controlled with the second mixture throttle angle DKW 2 . The output variable of the second mixture throttle  11  is the second set actual mixture pressure p 2 (IST), which is the controlled variable. The second set actual mixture pressure p 2 (IST) is returned to the comparison point B through an optional filter (not shown). The second closed-loop control system  22  is thus closed. 
       FIG. 4  is a program flowchart, which is part of the executable program implemented in the electronic gas engine control unit  14 . At S 1  the set speed nSL and the actual speed nIST are read in, and at S 2  the speed control deviation dn is computed. At S 3  the speed controller uses the speed control deviation dn to determine the set torque MSL as the correcting variable. The set torque MSL is then limited to an upper and a lower limit. The output value corresponds to the limited set torque MSLB. If the value of the set torque MSL lies within the permitted range, then the value of the limited set torque MSLB is the same as the value of the set torque MSL. At S 5  the efficiency unit ( FIG. 2 , reference number  18 ) uses an engine map to assign a set volume flow VSL to the limited set torque MSLB as a function of the actual speed nIST. Then at S 6  the value of the set volume flow VSL, the actual speed nIST, the temperature T 1  in the first receiver tube, and the system constants are read in. At S 7  the set mixture pressure pSL is computed by the computing unit ( FIG. 3 , reference number  23 ) with the formula described above. At S 8  the first mixture pressure control deviation dp 1  and the second mixture pressure control deviation dp 2  are determined. Then at S 9 A the first mixture throttle angle DKW 1  and the second mixture throttle angle DKW 2  are computed as a function of the first mixture pressure control deviation dp 1  and the second mixture pressure control deviation dp 2  and then output. At S 9 B the value of the set volume flow VSL is simultaneously supplied to the gas throttle. At S 10  a check is made to determine whether the engine has stopped. If this is not the case, i.e., interrogation result S 10 : no, then program control is returned to point A, and the program continues at S 1 . If an engine shutdown is detected at S 10 , interrogation result S 10 : yes, then the program is terminated. 
     REFERENCE NUMBERS 
     
         
           1  gas engine 
           2  shaft 
           3  coupling 
           4  shaft 
           5  generator 
           6  gas throttle 
           7  mixer 
           8  compressor 
           9  cooler 
           10  first mixture throttle 
           11  second mixer throttle 
           12  first receiver tube 
           13  second receiver tube 
           14  electronic gas engine control unit (GECU) 
           15  system controller 
           16  speed controller 
           17  torque limiter 
           18  efficiency unit 
           19  mixture quantity unit 
           20  electronic processing unit 
           21  first closed-loop control system 
           22  second closed-loop control system 
           23  computing unit 
           24  first mixture pressure controller 
           25  first characteristic curve 
           26  second mixture pressure controller 
           27  second characteristic curve
The invention proposes a method for controlling a stationary gas motor ( 1), wherein a rotational speed control deviation is calculated from a target rotational speed (nSL) and a current rotational speed (nIST), a target torque is determined from the rotational speed control deviation as the controlled variable, wherein a mixture throttle angle (DKW 1, DKW 2) is determined for the determination of a mixture volume flow and of a current mixture pressure (p 1 (IST), p 2 (IST)) in a receiver pipe ( 12, 13) upstream of the intake valves of the gas motor ( 1) as a function of the target volume flow, and wherein a gas throttle angle is determined for determining a gas volume flow as the gas content in a gas/air mixture, also as a function of the target volume flow.