Abstract:
An engine control system coordinates control of a pressure regulating mechanism associated with a turbocharger turbine and control of a variable valve actuating (VVA) mechanism for expanding the range of possible exhaust gas recirculation rates over a large portion of an engine operating map to provide EGR rates which are greater than typical present-day levels while mitigating engine pumping losses by causing the turbocharger to operate with better efficiency in some regions of the map where it otherwise would not. Turbocharger efficiency is improved by controlling the VVA mechanism to set the timing of operation of its respective cylinder valves in accordance with a predetermined correlation of operating efficiencies of a compressor to timing of operation of respective engine cylinder valves, causing the compressor to operate at points of better efficiency than it otherwise would without use of VVA.

Description:
TECHNICAL FIELD 
     This disclosure relates to an internal combustion engine which has a turbocharger (either single- or multiple-stage), external engine exhaust recirculation, and variable valve actuation. 
     BACKGROUND 
     Supercharging a diesel engine which powers a large commercial vehicle such as a truck or bus can improve engine/vehicle fuel economy and performance. A turbocharger is commonly used for supercharging such an engine. In the United States, governmental regulations also require that new vehicles comply with applicable tailpipe emission standards. Externally cooled, engine exhaust recirculation (commonly called EGR) is an effective technology for reducing oxides of nitrogen (NOx) in tailpipe emissions and may be useful in qualifying an engine design for compliance with certain tailpipe emission requirements. 
     While increasing EGR rates beyond present-day levels can further reduce NOx in tailpipe emissions, it appears that more devices would have to be added to a base diesel engine to accomplish that. The addition of such devices to a base engine may also impact other aspects of engine/vehicle operation such as engine/vehicle performance, durability, fuel economy, and/or manufacturing cost targets. 
     For example, it is known that moderate increases in EGR rates from typical present-day levels can be achieved by using one or more additional control valves, such as an intake throttle valve, for managing flow into the engine&#39;s cylinders. However, throttling the intake flow reduces engine efficiency, and the inclusion of additional components like an intake throttle valve may impair the ability to achieve EGR rate increases which are greater than moderate rates. Even if more than moderate increases in EGR rates can be achieved over a large portion of an engine operating map, significant engine pumping losses may occur in some regions of the map where the boosting system, i.e. the turbocharger, is operating with relatively poorer efficiency than in other regions. 
     EGR rate is affected by pressure in an exhaust manifold, i.e. by exhaust back-pressure. Control of exhaust back-pressure is an element of an engine control strategy because exhaust back-pressure can affect engine/vehicle performance, fuel economy, tailpipe emissions, and engine components including components in intake and exhaust systems. In a turbocharged diesel engine, a turbocharger can be used to control exhaust back-pressure. 
     Commercially available turbochargers have either single or multiple stages. Two types of turbochargers are wastegate turbochargers and variable geometry turbochargers (VGT&#39;s). In a two-stage wastegate turbocharger, a wastegate shunts a high-pressure turbine which is downstream of an exhaust manifold. The wastegate is in essence a valve that is controlled to selectively shunt engine exhaust around the high-pressure turbine. 
     When the wastegate is closed, all exhaust coming from the exhaust manifold, less any which may be recirculated as EGR in a high-pressure external EGR system, operates the high-pressure turbine. Increasingly opening the wastegate increasingly shunts engine exhaust around the high-pressure turbine. The extent to which the wastegate shunts engine exhaust affects not only exhaust back-pressure but also pressure developed in an engine intake manifold by the high-pressure turbine&#39;s operation of a high-pressure compressor in the intake system. 
     The difference between pressure in the intake manifold and pressure in the exhaust manifold affects both engine efficiency and external EGR flow. 
     Even if engine modifications like those mentioned earlier could be implemented successfully in an engine to create proper air/EGR charge over the entire engine speed-load domain for a range of EGR rates more extensive than typical present-day ranges, while at the same time meeting engine/vehicle performance and fuel economy targets for all engine operating conditions (e.g., EGR off, high altitude operation, cold start), the implementation is apt to add significant complexity, especially for a diesel engine which has a multi-stage turbocharger capable of developing high intake manifold pressure (i.e. boost). 
     Moreover, the engine modifications should not have significant adverse effects on other systems, such as exhaust after-treatment by processes such as selective catalytic reduction (SCR) and/or diesel particulate filter (DPF) which depend on proper exhaust temperature and composition. 
     SUMMARY 
     Briefly, the engine which is the subject of this disclosure comprises a turbocharger, external engine exhaust recirculation, and variable valve actuation (VVA). VVA refers to the ability to change the timing of operation of cylinder intake and/or cylinder exhaust valves. In the absence of VVA, cylinder valve timing is fixed by the shape of lobes on a camshaft which operate the cylinder valves. There are a variety of mechanisms that can be incorporated in an engine to provide VVA. 
     The engine which is the subject of this disclosure has a control system which coordinates control of a pressure regulating mechanism associated with a turbocharger turbine and control of the VVA mechanism for expanding the range of possible EGR rates over a large portion of an engine operating map to provide EGR rates which are greater than typical present-day levels when requested by an engine operating strategy, while mitigating engine pumping losses by causing the turbocharger to operate with better efficiency in some regions of the map where it otherwise would not. 
     Turbocharger efficiency is improved by controlling a VVA mechanism to set the timing of operation of its respective cylinder valves in accordance with a predetermined correlation of operating efficiencies of a compressor stage to timing of operation of its respective cylinder valves, causing the compressor stage to operate at points of better efficiency than it otherwise would without use of VVA. 
     Engine efficiency losses are mitigated when exhaust back-pressure is to be changed from an existing back-pressure to a different one. Mitigation is achieved by using VVA alone without adjusting the current setting of a turbocharger wastegate or adjusting current position of turbocharger vanes. The wastegate setting is changed or vanes adjusted, only when VVA by itself is unable to change existing exhaust back-pressure to a different one. 
     This strategy of coordinating use of VVA and control of turbocharger wastegate or vanes can accomplish engine volumetric efficiency manipulation appropriate to changes in engine operation that mitigate efficiency losses while providing proper control over EGR rate, boost pressure, and intake airflow rate and also obeying all relevant engine design constraints including peak cylinder pressure, compressor outlet temperature, exhaust manifold gas temperature, and turbocharger speed. 
     The strategy can be used during different engine operating conditions such as EGR off, high altitude operation, cold start, engine braking, and after-treatment device regeneration. 
     For an engine which can have a range of EGR rates extending from no EGR to a rate which is significantly greater than typical present-day EGR rates, a turbocharger sized for the greater EGR rates can be sized relatively smaller. The smaller size can be helpful by improving “spool-up” capability for example, but at relatively lower EGR rates, the smaller sized device may be subject to certain unfavorable operating conditions. For example, temperature and speed may exceed specified limits for the particular turbocharger selected. To avoid exceeding those limits, EGR can be increased and/or VVA can be used. Using VVA in preference to increasing EGR rate reduces cooling demand, and that is helpful when the engine is operating at higher altitudes, for example those at which tailpipe emissions may not be regulated, because it can avoid having to reduce engine power. Use of VVA is also helpful for handling certain transients in engine operation, and in minimizing or even eliminating use of wastegate or vane-adjustment. 
     When no EGR is being used, the entire exhaust flow is directed toward the turbine, and that can elevate exhaust back-pressure sufficiently to call for use of the wastegate or vane adjustment. However in a two-stage turbocharger, using the wastegate or vanes forces more of the flow coming from the exhaust manifold around a high-pressure turbine to a low-pressure turbine, increasing the latter turbine&#39;s speed. VVA can be used to increase boost, which reduces the pressure differential across the engine and can mitigate temperature/speed increases. 
     One general aspect of the claimed subject matter relates to the method defined by independent claim  1 . 
     Another general aspect of the claimed subject matter relates to the method defined by independent claim  4 . 
     The foregoing summary is accompanied by further detail of the disclosure presented in the Detailed Description below with reference to the following drawings that are part of the disclosure. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a general schematic diagram of an engine which is the subject of this disclosure. 
         FIG. 2  shows an example of the effect of VVA on cylinder intake valve closing. 
         FIG. 3  is a compressor operating efficiency diagram for different engine operating conditions at fixed base timing of opening and closing of cylinder intake valves. 
         FIG. 4  is a compressor operating efficiency diagram showing more efficient operating points for the engine operating conditions shown in  FIG. 3  as a result of using VVA to set cylinder intake valve closing timing in accordance with a predetermined correlation of timing of cylinder intake valve closing to compressor operating efficiency. 
         FIGS. 5, 6, and 7  are diagrams relating VVA timing to EGR %, fresh air flow, and engine heat rejection respectively. 
     
    
    
     DETAILED DESCRIPTION 
     In  FIG. 1  an internal combustion engine  10  comprises structure which forms engine cylinders  12  within which fuel combusts with air to operate the engine. Engine  10  further comprises an intake system  14  serving engine cylinders  12  through an intake manifold  16 , and cylinder intake valves  18  controlling admission of a fluid mixture which has an air component and an engine exhaust component from intake manifold  16  into engine cylinders  12 . 
     Intake system  14  further comprises an air inlet  20  through which the air component of the mixture enters intake system  14 , a compressor  22 C in downstream flow relation to air inlet  20  and a compressor  24 C in downstream flow relation to compressor  22 C. When operating, compressors  22 C,  24 C cooperate to elevate the pressure of the mixture in intake manifold  16  to superatmospheric pressure. Some heat of compression of air that has been compressed by compressor  22 C is removed by an inter-stage cooler  23  between the two compressors, and some heat of compression of air that has been compressed by compressor  24  is removed by a charge air cooler  25 . 
     Engine  10  further comprises an exhaust system  26  for conveying exhaust created by combustion of fuel in engine cylinders  12  away from engine cylinders  12 . Exhaust system  26  comprises an exhaust manifold  28  serving engine cylinders  12 . Engine  10  comprises cylinder exhaust valves  30  controlling admission of exhaust from engine cylinders  12  into exhaust manifold  28  for further conveyance through exhaust system  26 . 
     Exhaust system  26  comprises a turbine  24 T in downstream flow relationship to exhaust manifold  28  and a turbine  22 T in downstream flow relationship to turbine  24 T. Turbine  24 T is coupled by a shaft to operate compressor  24 C so that the two collectively form a high-pressure turbocharger stage. Turbine  22 T is coupled by a shaft to operate compressor  22 C so that the two collectively form a low-pressure turbocharger stage. An after-treatment system, not shown in  FIG. 1 , is typically present downstream of turbine  22 T for treating exhaust before it passes through a tailpipe to the surrounding atmosphere. 
     The two turbine-compressor stages form a multi-stage turbocharger  32 , which may be either a wastegate type turbocharger or a two-stage variable geometry type turbocharger (VGT).  FIG. 1  illustrates a wastegate type turbocharger having a wastegate  34  shunting turbine  24 T. If the turbocharger were a single-stage type, turbine  22 T, compressor  22 C, and inter-stage cooler  23  would not be present. Some two-stage turbochargers other than the one shown in  FIG. 1  might include a second wastegate shunting turbine  22 T. 
     Engine  10  further comprises an exhaust gas recirculation (EGR) system  36  which serves to provide the exhaust component of the mixture by conveying a portion of exhaust from exhaust system  26  to intake system  14 .  FIG. 1  shows EGR system  36  to be a high-pressure type EGR system because the point of EGR diversion from exhaust system  26  is upstream of turbine  24 T and the point of introduction into intake system  14  is downstream of compressor  24 C. EGR system  36  comprises an EGR valve  36 V for selectively restricting exhaust flow from exhaust system  26  to intake system  14 , and a heat exchanger (sometimes called an EGR cooler)  36 C through which some heat can be rejected from recirculated exhaust to circulating coolant and finally rejected to outside air at a radiator. 
     Downstream of the point at which recirculated exhaust is introduced into intake system  14  is an intake throttle  37  that can be operated to throttle intake flow into intake manifold  16 . 
     Engine  10  comprises respective mechanisms  38 ,  40  sometimes referred to as variable valve actuation (VVA) mechanisms, for controlling the timing of opening and/or closing of cylinder intake valves  16  and cylinder exhaust valves  30  respectively during engine cycles. An example of a VVA mechanism is contained in U.S. application Ser. No. 12/540,828, filed 13 Aug. 2009 and incorporated herein by reference. 
     In the absence of VVA, exhaust back-pressure would be controlled by wastegate  34 . Increasingly opening the wastegate increasingly relieves exhaust back-pressure by increasing the quantity of exhaust shunted around turbine  24 T, thereby reducing exhaust back-pressure. Although it does pass through turbine  22 T, the shunted exhaust does not operate turbine  24 T, and so a significant portion of the heat energy is not recovered by either turbine, thereby decreasing engine efficiency. 
     In order to mitigate the decrease in engine efficiency when exhaust back-pressure is to be changed, the engine uses a strategy involving one or both VVA mechanism  38 ,  40 . 
     One aspect of the strategy comprises changing the timing of operation of cylinder intake valves  18  and/or cylinder exhaust valves  30  by the respective VVA mechanisms without changing the existing setting of wastegate  34 . When the respective VVA mechanism by itself is unable to change the timing of operation of cylinder intake valves  18  and/or cylinder exhaust valves  30  enough within allowable timing limits to change exhaust back-pressure to a requested back-pressure, then the existing setting of wastegate  34  is changed to a different setting. In other words, preference is given to exclusive use of VVA to accomplish the change to the requested exhaust back-pressure, but when VVA alone is unable to satisfy the request, then wastegate  34  is used. 
     An example of using VVA to reduce exhaust back-pressure is to advance the closing time of cylinder intake valves  18  as portrayed by  FIG. 2 . Baseline intake valve opening is shown by the valve lift trace T 1  with closing occurring at some number of degrees before top dead center (TDC). Traces T 2 , T 3 , and T 4  show several earlier valve closing times. A VVA mechanism which can provide such a range of cylinder valve closings is a hydraulically controlled mechanism which interacts with the cylinder valve as the valve is being operated by a camshaft lobe. 
     Another aspect of the strategy involving the use of one or both VVA mechanisms  38 ,  40  is to set the timing of operation of the respective cylinder valves in accordance with a predetermined correlation of operating efficiency of compressors  22 C,  24 C to the timing of operation of the respective cylinder valves. The correlation is determined by plotting compressor operating efficiency points on a compressor operating efficiency diagram such as the ones shown in  FIGS. 3 and 4  for a variety of different operating conditions as a function of intake valve timing and/or exhaust valve timing. Intake valve timing and/or exhaust valve timing is/are varied over timing ranges that are defined by allowable timing limits for cylinder valve opening and closing. As timing is varied for each engine operating condition, compressor operating efficiency is measured. At some timings, compressor operating efficiency is higher and at other timings, compressor operating efficiency is lower. 
       FIG. 3  shows various data points which are representative of a turbocharger&#39;s operation without use of VVA. In the case of a two-stage turbocharger,  FIG. 3  could represent either stage or the combined stages. The line marked  50  represents compressor operating efficiency when engine  10  is operating at a first engine speed. The line marked  52  represents compressor operating efficiency when engine  10  is operating at a second engine speed. The line marked  54  represents compressor operating efficiency when engine  10  is operating at a third engine speed. The data points marked on each line, such as  50 A,  50 B,  50 C,  50 D;  52 A,  52 B,  52 C,  52 D; and  54 A,  54 B,  54 C,  54 D, represent different engine loads at the respective speed. 
     A compressor operating efficiency diagram such as  FIG. 3  is characterized by zones of different efficiencies, commonly called efficiency islands, which lie between the compressor surge line SL and the compressor choke line CL. One such efficiency island EI is marked in  FIG. 3 . Also marked in  FIG. 3  are various turbocharger speed lines such as S 1 , S 2 , S 3 , S 4 , S 5 , and S 6 . 
     The zone inside the boundary of efficiency island EI can be considered a relatively greater efficiency zone in comparison to zones lying outside. Line  50  lies substantially on the crest of a ridge running generally centrally within efficiency island EI as shown. Lines  52  and  54  do not. The crest of the ridge lies along a line of greatest compressor operating efficiency. The crest is sometimes referred to the spine of a compressor efficiency map. For greatest compressor operating efficiency at all engine speeds all operating points should lie of the spine. 
     By appropriate use of VVA, lines  52  and  54  can be brought substantially onto the spine, as shown in  FIG. 4 . 
     In order to develop a correlation of VVA to compressor operating efficiency for relocating lines  52  and  54  from their relatively lower efficiency locations shown in  FIG. 3  to the spine, VVA is varied at different combinations of engine speed and engine load during engine development to find a value for VVA timing, such as timing of cylinder intake valve closing, which for each speed/load combination, places compressor operation at least within the efficiency island EI, and ideally substantially on the spine. The correlation is used to create a map of VVA timing which is a function of engine speed and engine load. The map can then be programmed into an engine control system as a look-up table which is used by the engine control strategy to enable the compressor to operate at relatively greater efficiency over a larger portion of its efficiency diagram that it otherwise would in the absence of VVA. 
     When engine  10  is operating at higher altitudes above which current tailpipe emission regulations may not apply, engine cooling is hampered by the reduced density of ambient air which passes through heat exchangers in comparison to density at sea level. 
     When cooled EGR is being used at higher altitudes, the cooling load can be reduced by using less EGR. However, for a turbocharger which is sized for EGR rates significantly greater than typical present-day EGR rates, reducing EGR too much can overwork/overspeed/overheat the turbocharger. 
     An appropriate EGR rate may mitigate overworking/overspeeding/overheating the turbocharger, but that increases the cooling load on the engine. If the cooling system cannot handle the cooling load, engine power may have to be reduced. 
     Model simulation shows that VVA can be used to control EGR rate at different engine loads L 1 , L 2 , L 3 , L 4  to sufficiently mitigate overworking/overspeeding/overheating the turbocharger.  FIG. 5  shows that EGR rate is reduced when intake valve closing is either advanced or retarded a suitable number of degrees from a baseline timing. L 1  represents the largest of the four loads, with L 2 , L 3 , and L 4  being progressively smaller. 
       FIG. 6  shows that VVA, when used to reduce EGR rate, doesn&#39;t significantly affect fresh air flow rate within a range of VVA timings on either side of baseline valve timing at engine loads L 1 , L 2 , L 3 , L 4 . This shows that sufficient combustion air will enter the engine cylinders. 
       FIG. 7  shows how use of VVA influences engine cooling load. Over portions of the range of VVA timing at the different loads, cooling load is not seriously affected, making it unnecessary to limit engine power.