CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]     The present application is a continuation patent application of International Application No. PCT/SE2004/001306 filed 9 Sep. 2004 which is published in English pursuant to Article 21(2) of the Patent Cooperation Treaty and which claims priority to Swedish Application No. 0302418-9 filed 9 Sep. 2003 and Swedish Application No. 0303201-8 filed 25 Nov. 2003. Said applications are expressly incorporated herein by reference in their entireties. 
     
    
     TECHNICAL FIELD  
       [0002]     The present invention relates to a piston-type internal combustion engine having an intake line for delivering air to combustion chambers of the engine and an exhaust system for removing exhaust gases from said combustion chambers, the exhaust system comprising equipment for reducing environmentally harmful exhaust emissions from the engine, which is intended to function with variable load in order to propel a vehicle.  
       BACKGROUND OF INVENTION  
       [0003]     The statutory requirements relating to diesel engines have been tightened up and will continue to become more stringent, particularly in relation to emissions of nitrogen oxide pollutants and particulate emissions.  
         [0004]     The quantity of nitrogen oxides formed by the combustion of fuel in an engine cylinder depends on the combustion temperature. Higher temperatures lead to a greater proportion of the atmospheric nitrogen being converted into nitrogen oxides. A known engine-based method of reducing the quantity of nitrogen dioxide formed is so-called exhaust gas recirculation (EGR) and in particular cooled EGR, which makes it possible to reduce the combustion temperature. This method is normally not sufficient, however, to meet the statutory requirements when the engine is operating at high load. This method of cooled exhaust gas recirculation (EGR) places an increased load on the cooling system of the engine and the vehicle, especially at high engine loads. This constitutes a limit to the attainment of a high power output while achieving lower emissions. Another known method of reducing the quantity of nitrogen dioxide, which is based on exhaust gas after-treatment, uses a so-called NO x  trap (Lean NO x  Absorber, LNA) to store NO x  while the engine runs with excess oxygen. The NO x  trap is regenerated by allowing the engine to run with deficient oxygen; that is to say, with extra fuel admixture and/or reduced air flow, as described in U.S. Pat. No. 5,473,887, for example. The method can result in a certain increased load on the engine in the form of soot formation and contamination of the engine lubricating oil, or dilution of the lubricating oil with fuel and high exhaust gas temperatures that are harmful to the exhaust system. Furthermore, it may create certain problems for the LNA system in operating efficiently at low and partial load, since an LNA system usually functions best at exhaust gas temperatures in excess of approximately 300° C., which normally means high or medium load.  
         [0005]     Other known systems for reducing nitrogen oxides are LNC (Lean NO x  Catalyst), which continuously reduces nitrogen oxides under lean-burn conditions. Urea SCR (Selective Catalyst Reduction) is also used for NO x  reduction, see U.S. Pat. No. 5,540,047, for example.  
       SUMMARY OF THE INVENTION  
       [0006]     An object of the invention is therefore to create an internal combustion engine, which will permit a functionally improved and economic use of an exhaust after-treatment system, such as LNA, LNC, urea-SCR and soot particle filter systems.  
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0007]     The invention will be described in more detail below with reference to exemplary embodiments as shown in the drawing attached, in which  
         [0008]      FIG. 1  is a diagramatic view of an internal combustion engine configured according to a first exemplary embodiment of the invention; and  
         [0009]      FIGS. 2 and 3  are diagramatic views of a second and a third exemplary embodiment of the invention. 
     
    
     DETAILED DESCRIPTION  
       [0010]     The internal combustion engine  10  comprises (includes, but is not necessarily limited to) an engine block  11  having six piston cylinders  12  together with inlet manifold  13  and exhaust manifold  14 . Exhaust gases from the engine are fed via an exhaust line  15  to a turbine rotor  17  of a turbocharger unit  16 . The turbine shaft  18  drives the compressor wheel  19  of the turbocharger unit, which by way of an intake line  20  compresses incoming air and delivers it to the inlet manifold  13  via an air intercooler  21 . Fuel is fed to each cylinder  12  via injection devices (not shown). Although the figure illustrates a six-cylinder engine, the invention can also be used in conjunction with other cylinder configurations.  
         [0011]     Exhaust gases that have passed through the turbocharger unit  16  are led onwards by way of the exhaust line  22  to an oxidizing filter device  23  to separate particles from the exhaust gas flow. Downstream of the filter device is a three-way valve  24 , which, as appreciated by those skilled in this art, may conduct the exhaust gases through either a branch pipe  25  or via a branch pipe  26 , the two branch pipes running parallel and being reunited downstream at a point  27 . The exhaust gas flow is thereafter led onwards into the atmosphere via a so-called clean-up unit  28 , which may comprise an oxidation catalytic converter which oxidizes (burns) emission residues (HC, CO, etc). This unit may take various forms according to the demands placed on it (system design).  
         [0012]     According to a first exemplary embodiment of the invention, the branch pipe  25  comprises a device  29  for mixing diesel fuel into the exhaust gas flow and a downstream NO x  trap in the form of an LNA reactor  30 . This comprises material which adsorbs and binds NO x  during lean-burn operation within the normal temperature range of the engine. Regeneration takes place at a higher temperature than the adsorption and when the three-way valve  24  leads the exhaust gas flow largely through the branch pipe  26  (bypass) and only a smaller, variable secondary flow through the branch pipe  25 , the device  29  delivering diesel fuel that is gasified and mixed into the exhaust gas flow, forming regeneration gas, which according to the prior art converts and releases the bound nitrogen oxide as N 2 .  
         [0013]     The engine  10  has a system for returning exhaust gases to the intake side of the engine as so-called EGR gas, via a pipeline  31 , for reducing the nitrogen oxide emission of the engine in accordance with the prior art. This line comprises a valve  32 , which serves both as shut-off valve and as regulating valve for regulating the EGR flow. There is also a cooler  33  for cooling the EGR gases. The EGR system, for example, may re-circulate flows in the order of 30-60% (the gas in the inlet housing  20  is composed of 30-60% re-circulated exhaust gases and the remaining 40-70% is fresh air). When the engine is operating at low load this is feasible without overloading the cooling system. At high engine loads, on the other hand, with an effective mean pressure on the order of pme=0-15 bar and higher, these high EGR flow rates result in increased loading of the vehicle cooling system for which it is not usually designed. The internal structure of the engine is also not designed for the high cylinder pressures that can occur with high EGR contents.  
         [0014]     When the engine is operating at low load and the composition of the gas in the inlet casing  20  is composed of 30-60% EGR, very low exhaust gas emissions, both of NO x  and soot, can be achieved, for example, through the use of so-called homogeneous charge compression ignition (HCCI) combustion. For example, NO x  levels of&lt;0.5 g/kwh and theoretically soot-free combustion can be achieved. When the engine is operating at high load, the EGR system, among other things, is restrictive and the NO x  trap is designed to provide the necessary NO x  reduction.  
         [0015]     The valves  24  and  32  are connected to an engine control unit containing control program and control data for controlling the engine with reference to input data. The engine control unit is connected, for example, to sensors which detect the engine speed and the accelerator pedal position. The engine control unit is designed to control the valve  24  so that at low load the exhaust gas flow is led through the branch pipe  26 . Within this load range the exhaust emissions lie at acceptable levels without further after-treatment. In other load ranges the exhaust gas flow is led through the branch pipe  25 , NO x  being stored in the NO x  trap with periodic regeneration according to known methods.  
         [0016]     Designing the internal combustion engine according to the invention means that the exhaust gas after-treatment system has minimal impact on engine operation. The NO x  trap can function within an advantageous temperature range (medium and full load), regeneration gas (hydrocarbons, H2 and CO) being ignited and the formation of byproducts at the same time being minimized (in NO x  conversion at lower temperatures, that is to say&lt;300° C., NH 3  and N 2 O are formed). When the engine is operating at low load, for example pme=2 bar, the exhaust gas temperature downstream of the turbocharger is in the order of 200° C. Only when the engine is operating at an effective mean pressure of approximately pme=5 bar does the exhaust gas temperature downstream of the turbocharger reach a level in the order of 300° C. Since regeneration is out of the question at low load, the fuel consumption is reduced. The NO x  trap is also subjected to less ageing and can thereby be designed with a smaller volume (less than 30 liters, for legislation according to USA EPA Heavy Duty Engine 2007 Family Emission Level (US07) approximately 20 liters for 40% NO x  conversion in combination with an engine, the displacement of which is in the order of 12 liters and with maximum power output of approximately 300-350 kW) with the ensuing reduced need for precious metals (less than 100 g/ft 3 ). Moreover, the engine does not need to be run rich for LNA regeneration, which reduces the loading that is associated with the dilution of lubricating oil by fuel, or heavy soot formation in the combustion chamber. Heavy soot formation forms sooty exhaust gases and can also lead to contamination/degradation of the lubricating oil. The fact that the NO x  trap can reduce NO x  at higher loads gives greater freedom in designing the engine cooling and supercharging system which can afford major advantages in terms of lower costs and better engine installation solutions.  
         [0017]     It is normally true to say when designing the catalytic converter capacity for an NO x  trap (LNA) that the smaller the catalytic converter capacity the greater the fuel penalty in that regeneration needs to be performed more frequently. The solution according to the present invention means that a reduction in the capacity of the NO x  trap need not be achieved at the expense of an increased fuel penalty, and that the increased fuel penalty normally associated with the effects of ageing of the NO x  trap can be minimized.  
         [0018]     The exhaust line  22  may be provided with a desulfurization device  34 , for example a so-called SO x  trap. This device is located between the filter device  23  and the three-way valve  24 . The desulfurization device comprises material that adsorbs and binds SO x  in lean burn operation within the normal engine temperature range. If so required, the device  34  is regenerated at increased temperature and when the two-way valve  24  leads the exhaust gas flow through the branch pipe  26 . The NO x  trap can thereby be protected from sulfur oxide contamination, so that sulfur oxide regeneration need not take place in the NO x  trap. It is known that sulfur oxide contamination and sulfur oxide regeneration are critical factors which contribute to the ageing of LNA reactors and which have a negative effect on their performance.  
         [0019]     According to a second exemplary embodiment of the invention shown in  FIG. 2  the branch pipe  25  comprises a device  29  for mixing a reducing agent, urea or ammonia into the exhaust gas flow and a downstream SCR catalytic converter  30 . Regeneration takes place continuously in that the three-way valve  24  leads the exhaust gas flow through the branch pipe  25 , the device  29  adding urea or ammonia, which react with NO x  in the SCR catalytic converter, producing N 2 .  
         [0020]     The valves  24  and  32  are connected to an engine control unit containing control program and control data for controlling the engine with reference to input data. The engine control unit is connected, for example, to sensors which detect the engine speed and the accelerator pedal position. The engine control unit is designed to control the valve  24  so that at low load the exhaust gas flow is led through the branch pipe  26 . Within this load range the exhaust emissions lie at acceptable levels without further after-treatment. In other load ranges the exhaust gas flow is led through the branch pipe  25 , the gas flow being led through the SCR catalytic converter with continuous reduction as is conventionally known.  
         [0021]     Designing the internal combustion engine according to the invention means that the exhaust gas after-treatment system has minimal effect on engine operation. The SCR catalytic converter can function within an advantageous temperature range (medium and full load). When the engine is operating a low load, for example pme=2 bar, the exhaust gas temperature downstream of the turbocharger is in the order of 200° C. Only when the engine is operating at an effective mean pressure of approximately pme=5 bar does the exhaust gas temperature downstream of the turbocharger reach a level in the order of 300° C. Since NO x  reduction is out of the question at low load, the SCR catalytic converter functions in an optimum temperature range which gives high NO x  reduction.  
         [0022]     Furthermore, at low temperature the SCR catalytic converter is prevented from storing ammonia, which otherwise might be led onwards in the exhaust system during load transients. The fact that the SCR catalytic converter can reduce NO x  at higher loads gives greater freedom in designing the engine cooling and supercharging system, which can afford major advantages in terms of lower costs and better engine installation solutions. A further advantage resides in the fact that a vehicle having this after-treatment system can be driven in accordance with the statutory requirements even if the reducing agent for the SCR catalytic converter has run out, in that the engine power output is reduced so that it is temporarily impossible to run the engine at high load.  
         [0023]     If the filter device  23  is regenerated in a way that produces exhaust gas temperatures that are harmful to the SCR catalytic converter (or LNA, or LNC) the three-way valve  24  and the branch pipe  26  conduct these exhaust gases past the NO x  reduction catalytic converter, thereby protecting this against ageing.  
         [0024]     In a third exemplary embodiment of the invention shown in  FIG. 3  the SCR system has been replaced by an LNC system. In this case the filter device  23  may be located either upstream (as shown in  FIG. 2 ) or downstream of the three-way valve  24 . By locating the LNC system upstream of the filter device  23 , the three-way valve is used to protect the NO x  after-treatment system by leading exhaust gases destined for filter regeneration past the LNC catalytic converter. In filter regeneration, temperatures in excess of 700° C. can occur which are harmful to the NO x  after-treatment system, which is located downstream of the filter device  23 . In such cases the hot exhaust gases bypass the NO x  after-treatment system via the branch pipe  26 . A desulfurization device  34  is located upstream of the valve  24 .  
         [0025]     The invention must not be regarded as being limited to the exemplary embodiments described above, a number of further variants and modifications being feasible without departing from the scope of the following claims.
A piston-type internal combustion engine ( 10) having an intake line ( 20) for delivering air to combustion chambers of the engine and an exhaust system ( 15, 22) for removing exhaust gases from said combustion chambers. The exhaust system includes equipment ( 29, 30) for reducing environmentally harmful exhaust emissions from the engine, which is intended to function with variable load in order to propel a vehicle. The exhaust system ( 15, 22) has a branch pipe ( 26) controlled by a valve ( 24) and bypassing at least one part of the equipment ( 29, 30) for reducing environmentally harmful exhaust emissions. The valve ( 24) is controlled so that it leads the exhaust gas flow through the branch pipe ( 26) over a part of the overall load range of the engine. The engine is optimized in order to give acceptable exhaust emissions over said part of the overall load range of the engine.