Abstract:
In an internal combustion engine and a method of operation the internal combustion engine which includes a high pressure and a low pressure turbocharger having exhaust gas turbines arranged in series in an engine exhaust line provided with a high pressure exhaust gas recirculation line and a low pressure exhaust gas recirculation line via which exhaust gas can be conducted to an inlet line of the engine, the arrangement is switchable depending on the engine speed between an exhaust gas recirculation by way of the high pressure line and an exhaust gas recirculation by way of both, the high pressure and the low pressure line, to achieve low emissions and low fuel consumption.

Description:
This is a Continuation in Part application of international patent application PCT/EP2010/004044 filed Jul. 3, 2010 and claiming the priority of German patent application 10 2009 036 743.8 filed Aug. 8, 2009. 
    
    
     The invention relates to a method for operating an internal combustion engine having at least two turbochargers with turbines arranged in series in the engine exhaust duct and compressors arranged in series in the engine intake duct and with exhaust gas recirculation from the engine exhaust duct to the engine intake duct and also to an internal combustion engine designed to perform the method. 
     WO 2007/107865 A2 discloses an internal combustion engine with at least one exhaust gas turbocharger which comprises on an exhaust gas side of the internal combustion engine a turbine through which an exhaust gas of the internal combustion engine can flow, wherein subject to an operating state of the internal combustion engine by means of a high pressure exhaust gas recirculation and by means of a low pressure exhaust gas recirculation exhaust gas is removed and recirculated to a suction side of the internal combustion engine. In a speed range of the internal combustion engine a switching threshold is provided, at which there is a switch from a recirculation of the exhaust gas via the low pressure exhaust gas recirculation to a recirculation of the exhaust gas via the high pressure exhaust gas recirculation and via the low pressure exhaust gas recirculation. Below a speed threshold of 2000 rpm a high pressure exhaust gas recirculation is provided, whereas, above the speed threshold, a combined operation with low pressure exhaust gas recirculation and high pressure exhaust gas recirculation is established. 
     JP 2004 150319 A discloses a high pressure and low pressure exhaust gas recirculation and a single exhaust gas turbocharger. Below a switching threshold with a low or medium load and low to average speeds a combined high pressure and low pressure exhaust gas recirculation operation is provided. Below the switching threshold at low to average speeds and a high engine load on the other hand a low pressure exhaust gas recirculation operation is provided. Above the switching threshold a high pressure exhaust gas recirculation operation is provided at high engine speeds and high engine loads. 
     Furthermore an internal combustion engine is known from DE 10 2005 046 507 A1 which discloses an internal combustion engine with two exhaust gas turbochargers arranged one behind the other, wherein a recirculation of exhaust gas of the internal combustion engine from an exhaust gas system thereof is provided from upstream of a turbine of an exhaust gas turbocharger close to the engine into an intake passage of the internal combustion engine. 
     This known internal combustion engine already has good fuel consumption and emission values but increased resources are necessary to achieve future predefined emission limits. 
     U.S. Pat. No. 7,013,879 B2 discloses an exhaust gas recirculation system, wherein a high pressure exhaust gas recirculation is provided upstream of a turbine of an exhaust gas turbocharger and a low pressure exhaust gas recirculation is provided downstream of the turbine of the exhaust gas turbocharger. 
     WO 2008/058596 A1 describes an internal combustion engine with exhaust gas recirculation, wherein likewise a high pressure exhaust gas recirculation is provided upstream of a turbine of an exhaust gas turbocharger and a low pressure exhaust gas recirculation downstream of this turbine. These known solutions have problems, however, with regard to reaching future emission limits in particular. 
     It is the principal object of the present invention to improve a method for operating an internal combustion engine of the type described above so that low emissions can be realized with low fuel consumption. 
     SUMMARY OF THE INVENTION 
     In an internal combustion engine and a method of operation the internal combustion engine which includes a high pressure and a low pressure turbocharger having exhaust gas turbines arranged in series in an engine exhaust line provided with a high pressure exhaust gas recirculation line and a low pressure exhaust gas recirculation line via which exhaust gas can be conducted to an inlet line of the engine, the arrangement is switchable depending on the engine speed between an exhaust gas recirculation by way of the high pressure line and an exhaust gas recirculation by way of both, the high pressure and the low pressure line, to achieve low emissions and low fuel consumption. 
     A switching threshold is provided in a speed range of the internal combustion engine, at which there is a switch from a recirculation of the exhaust gas via the low pressure exhaust gas recirculation to a recirculation of the exhaust gas via the high pressure exhaust gas recirculation and via the low pressure exhaust gas recirculation. 
     The switching threshold is provided in a speed range of 1000≦rpm≦2000 of the internal combustion engine, at which there is a switch from a recirculation of the exhaust gas via the low pressure exhaust gas recirculation to a recirculation of the exhaust gas via the high pressure exhaust gas recirculation and via the low pressure exhaust gas recirculation. In a speed range of the internal combustion engine below the switching threshold the exhaust gas is recirculated exclusively via the low pressure exhaust gas recirculation, and in a speed range of the internal combustion engine above the switching threshold the exhaust gas is recirculated in a combined manner via the high pressure exhaust gas recirculation and via the low pressure exhaust gas recirculation. In an upper speed range and/or load range of the internal combustion engine the exhaust gas recirculated exclusively via the high pressure exhaust gas recirculation. 
     This switching threshold lies in a speed range of greater than, or equal to 1000 revolutions per minute to less than or equal to 2000 revolutions per minute, in particular, a speed range of greater than or equal to 1200 revolutions per minute to less than or equal to 1600 revolutions per minute, and especially at substantially 1250 revolutions per minute. With this exhaust gas recirculation from the exhaust gas side of the internal combustion engine to the intake side via two exhaust gas recirculation paths it is possible to reach current emission limits but in particular also future emission limits provided by legislation. In particular compliance with EU6 emission limits without active nitrogen oxide exhaust gas aftertreatment (NO x  exhaust gas aftertreatment) is thereby possible without a disadvantageous fuel consumption increase of the internal combustion engine. 
     The invention additionally relates to an internal combustion engine with at least one exhaust gas turbocharger which comprises on an exhaust gas side of the internal combustion engine a turbine through which an exhaust gas of the internal combustion engine can flow, wherein a high pressure exhaust gas recirculation and a low pressure exhaust gas recirculation are provided, by means of which exhaust gas can be recirculated from the exhaust gas side to a suction side of the internal combustion engine. A control device is assigned to the internal combustion engine, by means of which there is a switch in a speed range of the internal combustion engine from a recirculation of the exhaust gas by means of the low pressure exhaust gas recirculation to a recirculation of the exhaust gas via the high pressure exhaust gas recirculation and additionally via the low pressure exhaust gas recirculation. It is thereby provided according to the invention that the at least one exhaust gas turbocharger is a high pressure exhaust gas turbocharger, and that a low pressure exhaust gas turbocharger is connected in series downstream thereof, both respectively comprising at an exhaust gas side of the internal combustion engine a turbine through which an exhaust gas of the internal combustion engine can flow, wherein downstream of at least one of the two turbines of the two exhaust gas turbochargers the low pressure exhaust gas recirculation is arranged, and wherein the high pressure exhaust gas recirculation is arranged between the internal combustion engine and the turbine of the high pressure exhaust gas turbocharger, wherein the low pressure exhaust gas recirculation is arranged downstream of the turbine of the low pressure exhaust gas turbocharger. 
     By means of the control device, in a speed range 1000≦rpm≦2000 of the internal combustion engine there is of the internal combustion engine there is a switch from a recirculation of the exhaust gas via the low pressure exhaust gas recirculation to a recirculation of the exhaust gas via the high pressure exhaust gas recirculation and also via the low pressure exhaust gas recirculation, wherein, below the speed range the exhaust gas is recirculated exclusively via the low pressure exhaust gas recirculation. 
     The control device may be for example a separate control unit. It is also possible for the control device to be integrated into an existing motor control unit which is provided on an engine and thus included in the internal combustion engine. 
     With the internal combustion engine and the method according to the invention compliance with said emission limits is achieved with only low expenditures, so that the costs for the internal combustion engine and for the method and thus for a motor vehicle are relatively low. 
     The internal combustion engine according to the invention is in particular a diesel engine, wherein avoidance of nitrogen oxide generation is very important. Nonetheless the internal combustion engine can also be another internal combustion engine, for example a gasoline engine. 
     With the inventive embodiment of the internal combustion engine and the method disclosed, an increase in the charging level is achieved by increasing an exhaust gas mass flow through the turbine of the corresponding exhaust gas turbocharger with a simultaneous increase in an exhaust gas recirculation. For the distribution of a mass flow of the exhaust gas recirculation the best possible compromise is to be sought between the fuel consumption and nitrogen oxide emissions of the internal combustion engine. 
     If at least one exhaust gas turbocharger of the internal combustion engine is a high pressure exhaust gas turbocharger, and a low pressure exhaust gas turbocharger is connected in series downstream of the high pressure exhaust gas turbocharger, wherein the exhaust gas turbochargers respectively comprise on an exhaust gas side of the internal combustion engine a turbine through which an exhaust gas of the internal combustion engine can flow, it is possible for the first exhaust gas recirculation to be arranged between the internal combustion engine and the turbine of the high pressure exhaust gas turbocharger. The at least one further exhaust gas recirculation device can then be arranged downstream of the turbine of the low pressure exhaust gas turbocharger and downstream of an exhaust gas aftertreatment system, in particular downstream of a particle filter, which in turn is arranged downstream of the turbine of the low pressure exhaust gas turbocharger. With this low pressure exhaust gas recirculation, higher charging pressures for the internal combustion engine are obtained due to a higher turbine power output because of higher mass flows of the exhaust gas. Due to these higher charging pressures in turn a higher air-fuel ratio can be achieved with the same oxygen mass concentration or, respectively, exhaust gas recirculation rate. This results in a better high pressure combustion process of the internal combustion engine. 
     Any disadvantages during a charge cycle of the internal combustion engine as a result of a higher exhaust gas counter pressure are more than compensated for by this better high pressure combustion process. Within the scope of the internal combustion engine according to the invention it is thus possible to achieve an improved fuel consumption of the internal combustion engine with constant oxygen mass concentration or a lower oxygen mass concentration with a constant air—fuel ratio and thus lower nitrogen oxide emissions with substantially constant fuel consumption. 
     In addition, a best possible compromise is achieved between soot and nitrogen oxide emissions with an expansion of the exhaust gas of the internal combustion engine ahead of the turbine of the high pressure exhaust gas turbocharger as a result of lower mass flows, whereby also a reduction in the charging cycle work of the internal combustion engine is obtained. As already indicated, by using a combined exhaust gas recirculation operation from the two exhaust gas recirculation devices a best possible compromise between emissions and fuel consumption is possible. 
     The invention will become more readily apparent from the following description of a preferred embodiment thereof with reference to the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  shows a schematic representation of an internal combustion engine with a high pressure exhaust gas turbocharger and a low pressure exhaust gas turbocharger connected in series therewith, wherein on an exhaust gas side of the internal combustion engine upstream of a turbine of the high pressure exhaust gas turbocharger a first exhaust gas recirculation device is provided and downstream of a turbine of the low pressure exhaust gas turbocharger a second exhaust gas recirculation device is provided, 
         FIG. 2  is a representation of an operating strategy for the internal combustion engine according to  FIG. 1 , wherein a mean effective pressure of the internal combustion engine is shown over a speed thereof, 
         FIG. 3  shows an operating strategy for a low pressure exhaust gas recirculation of the internal combustion engine according to  FIG. 1 , wherein the mean effective pressure of the internal combustion engine is shown over the engine speed, 
         FIG. 4  shows an operating strategy of a low pressure exhaust gas recirculation build-up valve of the internal combustion engine according to  FIG. 1 , wherein the mean effective pressure of the internal combustion engine is shown over the engine speed and 
         FIG. 5  shows an operating strategy for a high pressure exhaust gas recirculation valve of the internal combustion engine according to  FIG. 1 , wherein the mean effective pressure of the internal combustion engine is shown over the engine speed. 
     
    
    
     DESCRIPTION OF EXEMPLARY EMBODIMENTS 
     While  FIG. 1  shows schematically an internal combustion engine with a two way exhaust gas recirculation, by which compliance with EU6 emission limits is easily possible,  FIGS. 2 to 5  show operating strategies of the internal combustion engine or components of the internal combustion engine and the two way exhaust gas recirculation in order to achieve low emissions and also low fuel consumption. 
       FIG. 1  shows an internal combustion engine  10  which is in the form of a diesel engine and comprises four cylinders  12 ,  14 ,  16  and  18 , into which fuel is injected by a high pressure injection system  20 . The internal combustion engine  10  comprises a high pressure exhaust gas turbocharger  22  and a low pressure exhaust gas turbocharger  24  arranged downstream thereof in series. Air drawn in according to direction arrows  26  by the internal combustion engine on an intake side  34  thereof and filtered by an air filter  39  is pre-compressed by a compressor  36  of the low pressure exhaust gas turbocharger  24 . Subject to a position of a circulation valve  38  of a bypass line  40  the pre-compressed air is further compressed by a compressor  42  of the high pressure exhaust gas turbocharger  22 . Direction arrows  28 ,  29 ,  30 ,  32  and  37  indicate the intake air flow of the intake side  34  of the system. 
     Downstream of the compressor  42  of the high pressure exhaust gas turbocharger  22  the compressed and thus heated air flows through a charging air cooler  44 , wherein the air is cooled. 
     On an exhaust gas side  50  of the internal combustion engine  10  an exhaust gas flows following a combustion of a fuel—air mixture according to direction arrows  52 ,  54 ,  56 ,  58  and  60  from the internal combustion engine  10  through the exhaust gas side  50 . The exhaust gas thereby drives a turbine  62  of the high pressure exhaust gas turbocharger, wherein the turbine  62  is connected via a shaft to the compressor  42 , and wherein the compressor  42  is driven by the exhaust gas. A charging pressure which is provided by the high pressure exhaust gas turbocharger  22  can be adjusted by a bypass line  64  which comprises a control valve  66 , by means of which an exhaust gas mass flow which flows around the turbine  62  through the circulation device  64  can be adjusted. 
     In order to drive the compressor  36  of the low pressure exhaust gas turbocharger  24  a turbine  68  of the low pressure exhaust gas turbocharger  24  is arranged downstream of the turbine  62  on the exhaust gas side  50  which turbine is connected via a shaft to the compressor  36 . 
     In order to bypass the turbine  68 , a bypass line  70  is likewise provided which comprises a control valve  72  which is also described as a waste gate and can adjust—in a similar way to the control valve  66 —an exhaust gas mass flow which turbine is to bypass the turbine  68 . 
     According to the direction arrow  54  the exhaust gas flows further through an exhaust gas aftertreatment system in the form of an oxidation catalyst which purifies the exhaust gas of hydrocarbons (HC) and CO x  emissions and also in the form of a particle filter  73 , through which the exhaust gas is purified in particular of particles before it leaves—according to a direction arrow  56 —the exhaust gas side  50  of the internal combustion engine  10  and is released to the environment. 
     In order to reduce emissions, in particular nitrogen oxide emissions (NO x  emissions) the internal combustion engine  10  comprises a first exhaust gas recirculation device in the form of a high pressure exhaust gas recirculation arrangement  74  which removes the exhaust gas—according to a direction arrow  58 —of the internal combustion engine directly downstream of the internal combustion engine and recirculates it—according to a direction arrow  60 —to the intake side  34  directly upstream of the internal combustion engine  10 . In order to adjust an exhaust gas mass flow of the recirculated exhaust gas a high pressure exhaust gas recirculation valve  76  is provided. Furthermore the high pressure exhaust gas recirculation arrangement  74  comprises an exhaust gas recirculation cooler  78  which reduces the temperature of the recirculated exhaust gas. The recirculated exhaust gas acts upon combustion of the internal combustion engine  10  as an inert gas and reduces the formation of nitrogen oxides. 
     The high pressure exhaust gas recirculation  74  comprises a circulation device  46  which is assigned to the exhaust gas recirculation cooler  78 . Circulation is possible around the exhaust gas recirculation cooler  78  by way of the bypass line  46 , whereby this circulation or mass flow of the circulating exhaust gas is controlled by a bypass valve  48  of the bypass line  46 . 
     In order to comply with stricter emission limits, in particular nitrogen oxide limits, such as the EU6 standard, the internal combustion engine  10  additionally comprises a second exhaust gas recirculation device in the form of a low pressure exhaust gas recirculation line  80 , by means of which exhaust gas of the internal combustion engine  10  can be removed directly after the particle filter  73  on the exhaust gas side  50  and recirculated to the intake side  34  directly before the compressor  36  of the low pressure exhaust gas turbocharger  24 . In order to adjust an exhaust gas mass flow of this recirculated exhaust gas a further exhaust gas recirculation valve  82  is provided in the form of a low pressure exhaust gas recirculation valve  82 . The low pressure exhaust gas recirculation line  80  also comprises an exhaust gas recirculation cooler  84  for cooling the recirculated exhaust gas. 
     The low pressure exhaust gas recirculation line  80  is further controllable by exhaust gas back-up valve  86 , by means of which the exhaust gas of the internal combustion engine can be restricted directly downstream of a removal point of the low pressure exhaust gas recirculation line  80  and thus creates a further control possibility for the exhaust gas mass flow. 
     By an operating point-dependent mixing of the exhaust gas mass flow of the exhaust gas recirculated through the low pressure exhaust gas recirculation line  80  an exhaust gas turbocharger charging can be significantly increased in comparison with a high pressure exhaust gas recirculation. An exhaust gas recirculation through the low pressure exhaust gas recirculation line  80  is thus a charging means. 
       FIG. 2  shows an operating strategy of the internal combustion engine  10  according to  FIG. 1  in a diagram  88 , wherein a mean effective pressure of the internal combustion engine  10  is indicated on the Y axis  90  and a speed of the internal combustion engine  10  on the X axis  92  according to  FIG. 2 . The mean effective pressure is thereby given in bar as a measurement unit, the speed in 1 per minute as a measurement unit. The values indicated in the diagram  88  are thereby to be understood merely by way of example. A region  96  characterizes an operating region of the internal combustion engine  10 , in which an exhaust gas recirculation is carried out through the low pressure exhaust gas recirculation line  80 . In regions  98  and  100  a combined exhaust gas recirculation is carried out, whereby this constitutes a combination of exhaust gas recirculation through the low pressure exhaust gas recirculation line  80  and the high pressure exhaust gas recirculation arrangement  74 . A cooling of the exhaust gas recirculated through the low pressure exhaust gas recirculation line  80  is thereby necessary in region  100 . 
     In a further region  102 , thus in an upper load range of the internal combustion engine  10 , an exhaust gas recirculation is carried out through the high pressure exhaust gas recirculation arrangement  74 . 
     With the operating strategy shown in  FIG. 2  compliance with EU6 emission limits is possible. 
       FIG. 3  shows an operating strategy of the flow through the low pressure exhaust gas recirculation line  80  according to  FIG. 1  in a diagram  88 ′, wherein the mean effective pressure of the internal combustion engine  10  is indicated on the Y axis  90 ′ and the speed on the X axis  92 ′ in the respective measurement units mentioned in connection with  FIG. 2 . The isolines shown in the diagram  88 ′ and provided with numbers describe a low pressure exhaust gas recirculation rate, whereby in a speed range below a switching threshold  104  an exhaust gas recirculation is carried out through the low pressure exhaust gas recirculation line  80  and above the switching threshold  104  an exhaust gas recirculation is carried out through a combined exhaust gas recirculation operation, as already explained in connection with diagram  88  of  FIG. 2 . 
       FIG. 4  also shows in a diagram  88 ′″ an operating strategy and indeed the operating strategy of the exhaust gas build-up valve  86  in the low pressure exhaust gas recirculation line  80 , whereby the isolines shown in the diagram  88 ′″ indicate a duty cycle of the exhaust gas build-up valve  86  in a percentage as a measurement unit. A duty cycle of 0% thereby corresponds to an open position of the exhaust gas build-up valve  86  while a duty cycle of 100% corresponds to a closed position of the exhaust gas build-up valve  86 . An A Y axis  90 ′″ of the diagram  88 ′″ indicates in turn values for the mean effective pressure of the internal combustion engine  10  in bar, while the speed of the internal combustion engine  10 ′ is indicated on the X axis  92 ′″ in the measurement unit of 1 per minute. With regard to the switching threshold  104  indicated in the diagram  88 ′″ the indications given in the context of the previous diagrams  88 ′ and  88 ″ apply accordingly. 
       FIG. 5  shows in a diagram  88 ″″ an operating strategy of the high pressure exhaust gas recirculation valve  76  of the high pressure exhaust gas recirculation line  74 , whereby the mean effective pressure of the internal combustion engine  10  in bar as the measurement unit, is indicated on the Y axis  90 ″″ and the speed of the internal combustion engine in rpm as the measurement unit, is indicated on the X axis  92 ″″. The isolines shown in the diagram  88 ″″ describe a duty cycle of the high pressure exhaust gas recirculation valve  76  as a percentage. In this case a duty cycle of 0% corresponds to a closed position of the high pressure exhaust gas recirculation valve  76 , while a duty cycle of 100% corresponds to an open position thereof. As can be deduced from the diagram  88 ″″ and can be recognized in comparison with diagram  88 ′″, the high pressure exhaust gas recirculation valve has intermediate positions between a completely closed and a completely open position. In contrast, the low pressure exhaust gas recirculation valve  82  is either completely closed or completely open. It is, however, also possible to provide intermediate positions of the low pressure exhaust gas recirculation valve  82 . 
     It is further clear from  FIG. 5  that the high pressure exhaust gas recirculation valve  76  is activated merely in the speed range above the switching threshold  104 , as it is in this range—as already mentioned—that a combination of an exhaust gas recirculation by means of the high pressure exhaust gas recirculation arrangement  74  and by means of the low pressure exhaust gas recirculation line  80  is carried out, while below the switching threshold  104  merely an exhaust gas recirculation through the low pressure exhaust gas circulation line  80  is carried out. In this lower speed range an activation of the high pressure exhaust gas recirculation valve  76  is not therefore necessary and is not provided. 
     It should be noted at this point that in particular the values of the isolines of the diagrams  88 ,  88 ′,  88 ′″,  88 ″″ are to be regarded as an example and can by all means fluctuate within certain boundaries. The same applies to the mean effective pressure and the speed of the internal combustion engine  10 . 
     LIST OF REFERENCE NUMERALS 
     
         
           10  Internal combustion engine 
           12  Cylinder 
           14  Cylinder 
           16  Cylinder 
           18  Cylinder 
           20  High pressure injection system 
           22  High pressure exhaust gas turbocharger 
           24  Low pressure exhaust gas turbocharger 
           26  Direction arrow 
           28  Direction arrow 
           29  Direction arrow 
           30  Direction arrow 
           32  Direction arrow 
           34  Intake side 
           36  Compressor 
           37  Direction arrow 
           38  Circulation valve 
           39  Air filter 
           40  Bypass line 
           42  Compressor 
           44  Charging air cooler 
           46  Bypass line 
           48  Bypass valve 
           50  Exhaust gas side 
           52  Direction arrow 
           54  Direction arrow 
           56  Direction arrow 
           58  Direction arrow 
           60  Direction arrow 
           62  Turbine 
           64  Circulation device 
           66  Control valve 
           68  Turbine 
           70  Bypass line 
           72  Control valve 
           73  Exhaust gas purification device 
           74  Recirculation arrangement 
           76  Recirculation valve 
           78  Recirculation cooler 
           80  Recirculation line 
           82  Recirculation valve 
           84  Recirculation cooler 
           86  Exhaust gas backup valve 
           88  Diagram 
           100  Region in diagram 
           104  Threshold line