Abstract:
A system and method for increasing engine heating rate to facilitate early introduction of exhaust gas recirculation after a cold start include operating an electric machine connect to the engine to apply a load to the engine. The electric machine may be operated as a generator to produce an increased engine torque demand and increase the rate of engine heating until an engine temperature and an exhaust temperature exceed corresponding thresholds. Operation of the electric machine as a generator may be stopped in response to a battery state of charge exceeding a threshold, or engine load exceeding a threshold.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims foreign priority benefits under 35 U.S.C. §119(a)-(d) to GB 1514121.1 filed Aug. 11, 2015, which is hereby incorporated by reference in its entirety. 
       TECHNICAL FIELD 
       [0002]    This disclosure relates to a method for reducing NOx emissions from an engine after a cold start. 
       BACKGROUND 
       [0003]    It is known from, for example, U.S. Pat. No. 6,829,888 that an electric machine can be used to assist with heating the exhaust gas flow from an engine to assist with catalyst light-off. 
         [0004]    The use of high pressure exhaust gas recirculation (HPEGR) or low pressure exhaust gas recirculation (LPEGR) at the earliest possible moment during a vehicle drive cycle helps to reduce NOx emissions. Early introduction of EGR after engine starting has been used to reduce NOx emissions to satisfy emission requirements. The engine operating range in terms of engine speed and engine load and the temperature range over which EGR flow is used to reduce NOx have both increased to meet more recent emission requirements. 
         [0005]    One problem associated with the early use of EGR is that it can result in combustion instability if the temperature of the gas inducted into the engine is low such as following a cold start. 
         [0006]    A second problem associated with the early use of LP EGR in the case of an engine having forced induction using a compressor is that it can result in the formation of condensation upstream of the compressor. The formation of such condensation can seriously damage the fast rotating blades of the compressor. 
       SUMMARY 
       [0007]    The inventors have recognized that a strategy similar to that used for exhaust gas heating to accelerate catalyst light-off such as proposed in U.S. Pat. No. 6,829,888 can be beneficially used to enable earlier introduction of exhaust gas recirculation. 
         [0008]    In one or more embodiments, a method of reducing engine NOx emissions after a cold start facilitates the early use of exhaust gas recirculation without causing significant combustion instability or potentially damaging condensation. 
         [0009]    According to one embodiment, a method of enabling earlier use of exhaust gas recirculation to reduce NOx emissions from an engine following a cold start of the engine includes identifying whether at least one of engine temperature and exhaust gas temperature is below a corresponding threshold for the use of exhaust gas recirculation, using an electric machine as a generator to apply a load to the engine to increase the rate at which the engine temperature and the exhaust gas temperature increase and, when the engine temperature and the exhaust gas temperature are above corresponding thresholds to permit effective use of exhaust gas recirculation, activating exhaust gas recirculation. 
         [0010]    The use of the electric machine as a generator may be terminated when the engine temperature and the exhaust gas temperature are high enough to permit effective use of exhaust gas recirculation. Alternatively, the use of the electric machine as a generator may be terminated if a state of charge (SOC) of a battery connected to the electric machine reaches a limit. 
         [0011]    The electric machine may be an integrated starter-generator drivingly connected to the engine. The engine temperature and the exhaust gas temperature may be high enough to permit effective use of exhaust gas recirculation when the use of exhaust gas recirculation will not cause combustion instability in the engine. 
         [0012]    The engine may be a forced induction engine having a compressor, the exhaust gas recirculation may be a low pressure exhaust gas recirculation that returns exhaust gas to a position upstream from the compressor and the engine temperature and the exhaust gas temperature may be high enough to permit effective use of exhaust gas recirculation if the use of low pressure exhaust gas recirculation will not cause condensation to be inducted into the compressor. 
         [0013]    In various embodiments, a motor vehicle includes an engine drivingly connected to an electric machine, an electronic controller and an exhaust gas recirculation system to recirculate exhaust gas from an exhaust side of the engine to an intake side of the engine, the exhaust gas recirculation system including an exhaust gas recirculation valve to control the flow of recirculated exhaust gas wherein the electronic controller is arranged to identify whether at least one of engine temperature and exhaust gas temperature is below a corresponding threshold for the use of exhaust gas recirculation, use the electric machine as a generator to apply a load to the engine to increase the rate at which the engine temperature and the exhaust gas temperature increase and, when the engine temperature and the exhaust gas temperature exceed corresponding thresholds to permit the effective use of exhaust gas recirculation, the electronic controller is programmed to activate exhaust gas recirculation by opening the exhaust gas recirculation valve. 
         [0014]    The electronic controller may be programmed to terminate the use of the electric machine as a generator when the engine temperature and the exhaust gas temperature exceed corresponding thresholds to permit effective use of exhaust gas recirculation. 
         [0015]    The electronic controller may be programmed to monitor the state of charge of a battery connected to the electric machine and may be further programmed to terminate use of the electric machine as a generator if a state of charge of a battery reaches a limit. 
         [0016]    The electric machine may be an integrated starter-generator. 
         [0017]    The engine temperature and the exhaust gas temperature threshold may be high enough to permit the effective use of exhaust gas recirculation when the use of exhaust gas recirculation will not cause combustion instability in the engine. 
         [0018]    The engine may be a forced induction engine having a compressor to compress the flow of air entering the engine, the exhaust gas recirculation system may be a low pressure exhaust gas recirculation system that returns exhaust gas to an intake system for the engine at a position upstream from the compressor and the thresholds for the engine temperature and the exhaust gas temperature may be high enough to permit the effective use of exhaust gas recirculation if the use of low pressure exhaust gas recirculation will not cause the induction of condensation into the compressor. 
         [0019]    The compressor may be part of a turbocharger having a turbine to drive the compressor and the exhaust gas may be extracted from an exhaust system of the engine at a position downstream from the turbine of the turbocharger. 
         [0020]    The exhaust gas system may be a high pressure exhaust gas system, the engine may have a turbocharger having a compressor driven by a turbine for increasing the flow of air through an intake system of the engine, recirculated exhaust gas may be extracted from an exhaust system of the engine at a position upstream from the turbine of the turbocharger and be returned to the intake system of the engine downstream from the compressor. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0021]      FIG. 1  is a schematic diagram of a motor vehicle according to one embodiment; and 
           [0022]      FIG. 2  is a high level flow chart illustrating operation of a system or method of enabling earlier use of exhaust gas recirculation to reduce NOx emissions from an engine following a cold start of the engine in accordance with various embodiments. 
       
    
    
     DETAILED DESCRIPTION 
       [0023]    As required, detailed embodiments are disclosed herein; however, it is to be understood that the disclosed embodiments are merely representative and may be embodied in various and alternative forms not explicitly illustrated or described. The figures are not necessarily to scale; some features may be exaggerated or minimized to show details of particular components. Therefore, specific structural and functional details disclosed herein are not to be interpreted as limiting, but merely as a representative basis for teaching one skilled in the art to variously employ the embodiments. 
         [0024]    With reference to  FIG. 1  there is shown a motor vehicle  5  having a forced induction engine in the form of a turbocharged engine  10 . The engine  10  has an intake system through which atmospheric air flows to the engine  10 . The intake system comprises a number of intake conduits, an air filter  12 , a compressor  20   c  of a turbocharger  20 , a throttle valve  13 , an intercooler  14  and an intake manifold  15 . The intake conduits are used to connect together the various components of the intake system used to flow air to the engine  10 . 
         [0025]    Air enters the intake system via the air filter  12 , is compressed by the compressor  20   c  and flows via the throttle valve  13  to the intercooler  14  and then to the intake manifold  15  of the engine  10 . Fuel is injected into the engine  10  by a number of fuel injectors (not shown) and the products of combustion in the form of exhaust gas flow via an exhaust manifold  11  to a turbine  20   t  of the turbocharger  20 . After passing through the turbine  20   t  the exhaust gas flows through one or more aftertreatment devices  16  to a tailpipe  17  and from the tailpipe  17  to atmosphere. 
         [0026]    A HPEGR circuit  30  is arranged to extract exhaust gas from a position between the exhaust manifold  11  and the turbine  20   t  and flow the extracted exhaust gas via an exhaust gas cooler  31  to a high pressure exhaust gas recirculation valve  32 . When the high pressure exhaust gas recirculation valve  32  is open, the high pressure exhaust gas can flow into the intake path upstream from the intake manifold  15  and downstream from the intercooler  14  and the compressor  20   c.    
         [0027]    A LPEGR circuit  40  is arranged to extract exhaust gas from a position between the turbine  20   t  and the aftertreatment devices  16  and flow the extracted exhaust gas via an exhaust gas cooler  41  to a low pressure exhaust gas recirculation valve  42 . When the low pressure exhaust gas recirculation valve  42  is open, the low pressure exhaust gas can flow into the intake path. 
         [0028]    It will be appreciated that if the aftertreatment devices  16  include a particulate filter then the exhaust gas for the LPEGR could be extracted downstream from the particulate filter as that would reduce the amount of soot reaching the compressor  20   c  and reduce contamination of the blades of the compressor  20   c.  Alternatively, if no aftertreatment particulate filter is present, a particulate filter could be included between the exhaust gas cooler  41  and the extraction point of the exhaust gas. 
         [0029]    An electric machine is drivingly connected to the engine  10  and in the case of this example is in the form of an integrated starter-generator  18 . The integrated starter-generator  18  is, in the case of this example, belt driven from a crankshaft of the engine  10  but it will be appreciated that it could alternatively be chain driven or gear driven. 
         [0030]    The integrated starter-generator  18  can be used to generate electricity or generate torque depending upon the mode in which it is operating. A battery  19  is connected to the integrated starter-generator  18  along with associated control electronics formed as part of a central electronic controller  50 . When the integrated starter-generator  18  is operating as a generator it charges the battery  19  and, when the integrated starter-generator  18  is operating as a motor, the battery  19  provides electrical energy to the integrated starter-generator  18 . The electronic controller  50  monitors the state of charge (SOC) of the battery  19  and controls the integrated starter-generator  18  to ensure that the state of charge of the battery remains within safe upper and lower limits. 
         [0031]    The electronic controller  50  is arranged or programmed to control operation of the integrated starter-generator  18 , the operating state of the high pressure exhaust gas recirculation valve  32 , the operating state of the low pressure exhaust gas recirculation valve  42  and the rotary position of the throttle valve  13 . It will be appreciated that the high and low pressure exhaust gas recirculation valves  32  and  42  will have at least fully open and fully closed operating states and in most cases partially open/closed operating states. The electronic controller  50  is also used to control normal operation of the engine. 
         [0032]    The electronic controller  50  receives inputs from a number of sensors such as, for example, a mass air flow sensor (not shown), an engine speed sensor (not shown), an accelerator pedal position sensor (not shown), a Lambda sensor (not shown), three exhaust gas temperature sensors  6   a,    6   b,    6   c,  an intake temperature sensor  7  and an engine cylinder head temperature sensor  8 . The connections between the electronic controller  50  and the various sensors and components it controls are not shown in  FIG. 1  to simplify and improve understanding of the figure. 
         [0033]    It will be appreciated that the electronic controller  50  may comprise several interconnected electronic controllers and need not be a single unit as shown in  FIG. 1 . Control logic, functions, algorithms, or methods performed by controller  50  may be represented by a flow chart such as illustrated in  FIG. 2 . This flowchart provides a representative control strategy, algorithm, and/or logic that may be implemented using one or more processing strategies such as event-driven, interrupt-driven, multi-tasking, multi-threading, and the like. As such, various steps or functions illustrated may be performed in the sequence illustrated, in parallel, or in some cases omitted. Although not always explicitly illustrated, one of ordinary skill in the art will recognize that one or more of the illustrated steps or functions may be repeatedly performed depending upon the particular processing strategy being used. Similarly, the order of processing is not necessarily required to achieve the features and advantages described herein, but is provided for ease of illustration and description. The control logic may be implemented primarily in software executed by a microprocessor-based vehicle, engine, and/or powertrain controller, such as controller  50 . Of course, the control logic may be implemented in software, hardware, or a combination of software and hardware in one or more controllers depending upon the particular application. When implemented in software, the control logic may be provided in one or more non-transitory computer-readable storage devices or media having stored data representing code or instructions executed by a computer to control the vehicle or its subsystems. The computer-readable storage devices or media may include one or more of a number of known physical devices which utilize electric, magnetic, and/or optical storage to keep executable instructions and associated calibration information, operating variables, and the like. 
         [0034]    In one embodiment, the electronic controller  50  is programmed with instructions to implement one or more algorithms to operate as described below. 
         [0035]    When the temperature of the engine  10  is sensed to be below a normal operating temperature following an engine start, the vehicle is controlled to provide heating of the engine to reduce the time delay between engine start and the time that EGR can be used to help reduce NOx emissions. When deciding whether exhaust gas recirculation can be used effectively, controller  50  determines when combustion is likely to be unstable and whether the recirculated exhaust gas will be hot enough to help with combustion stability. 
         [0036]    Combustion will be unstable if in-cylinder gas temperatures are low which will be the case if the temperature of the engine is low. The engine temperature can be deduced by using the output from the cylinder head temperature sensor  8  (or an engine coolant sensor) which provides an indication whether the cylinder walls are cold and by using the intake temperature sensor  7  which provides an indication of whether the temperature of the inducted gas is low. A prediction can be made as to whether unstable combustion would be likely from the use of EGR based on these measurements. 
         [0037]    A further factor to be considered is whether the temperature of the exhaust gas is high enough to increase the inducted gas temperature if EGR is used. The temperature of the exhaust gas can be deduced from the outputs from the exhaust gas sensors  6   a,    6   b  and  6   c  which provide an estimate of the effective exhaust gas temperature. Although three sensors are used in the case of this example it will be appreciated that only a single exhaust gas temperature sensor can be used. 
         [0038]    It will be appreciated that there is a trade-off between combustion stability and mixture dilution that may be taken into account by controller  50  when deciding whether to use EGR, and the amount of EGR to use. That is to say, adding hot recirculated exhaust gas will increase the mobility of the molecules of fuel and air and therefore helps improve combustion stability but the addition of inert gas to the intake fuel/air mixture will dilute it and so the benefit of adding the hot recirculated exhaust gas has to be weighed against the diluting effect of the inert gas. 
         [0039]    However, in all cases it is advantageous to speed up the process of heating of the engine  10  and the electronic controller  50  is programmed to operate the integrated starter-generator  18  as a generator to do this. 
         [0040]    The use of the integrated starter-generator  18  as a generator will increase the load upon the engine  10  and this will result in an increased torque demand for the engine  10  which is met by supplying more fuel to the engine  10 . The extra fuel increases the temperature of the engine and the temperature of the exhaust gas more quickly than it would if the air/fuel ratio were to be optimized for good fuel economy as is normally the case. 
         [0041]    As referred to above, the electronic controller  50  is arranged to use the temperature sensors  6   a,    6   b  and  6   c  to sense the temperature of the exhaust gas and, when the sensed exhaust gas temperature is above a temperature limit (T HP ) below which combustion instability will likely be caused if HPEGR is used, the electronic controller  50  is programmed to open the high pressure exhaust gas recirculation valve  32 . 
         [0042]    The electronic controller  50  in the case of this example is further programmed to, when the sensed exhaust gas temperature is above a temperature limit (T LP ) below which condensation is likely to be caused if LPEGR is used, open the low pressure exhaust gas recirculation valve  42 . 
         [0043]    It will be appreciated that the temperature of the LPEGR gas will be lower than the temperature of the HPEGR gas for the same exhaust gas temperature due to the difference between the HPEGR gas flow path from the exhaust side of the engine  10  to the intake side of the engine  10  and the LPEGR gas flow path from the exhaust side of the engine  10  to the intake side of the engine  10 . 
         [0044]    Electronic controller may be programmed to use LPEGR rather than HPEGR whenever possible because LPEGR provides a potential fuel economy benefit and is better mixed with atmospheric air. As such, HPEGR is sometimes terminated as soon as LPEGR can be utilized. The electronic controller  50  is further programmed to stop the use of the integrated starter-generator  18  as a generator when at least one of several conditions is present as described immediately below. 
         [0045]    The use of the electric machine to increase engine load and an engine torque request may be stopped in response to the temperature of the exhaust gas exiting the engine  10  being above the low pressure and high pressure temperature limits (T LP ) and (T HP ), respectively, and the temperature of the engine  10 , as sensed by the cylinder head sensor  8  and the intake temperature sensor  7 , exceed predefined temperature limits. In addition, use of the electric machine to increase engine loading may be stopped in response to the load applied to the engine  10  increasing to a level where the engine  10  is warming up sufficiently rapidly that additional heating is not required. Furthermore, use of the electric machine to increase engine loading may be stopped if the state of charge (SOC) of the battery  19  reaches an upper charge limit indicating that there is substantially no capacity left in the battery  19  to store further electrical energy. 
         [0046]    It will be appreciated that the systems and methods of various embodiments may include an engine having only low pressure EGR or only high pressure EGR. Similarly, various embodiment may include an engine with forced induction or without forced induction. 
         [0047]    With reference to  FIG. 2 , a flowchart illustrates operation of a system or method for reducing NOx emissions from an engine such as the engine  10 . The method starts in box  110  with the engine  10  not running that is to say, the engine  10  is ‘Off’. The method advances to box  115  to check whether a key-on event has occurred. A ‘key-on’ event is an event that will change the state of the engine  10  from ‘Off’ to ‘On’. That is to say, the engine  10  following a ‘key-on’ event will be running. Although in some cases a ‘key-on’ event is the result of a driver of a vehicle, such as the vehicle  5 , activating an ignition switch, it will be appreciated that the specific mechanism used to start the engine  10  is not important to the operation of the system or method. 
         [0048]    If box  115  determines there has been no ‘key-on’ event it will return to box  110  and the engine  10  remains in the ‘Off’ state. If box  115  determines there has been a ‘key-on’ event then the engine  10  will be started and the method advances to box  120  where the engine  10  is now running. 
         [0049]    From box  120  the method advances to box  130  to check whether the current engine temperature as sensed by the intake temperature sensor  7  and the cylinder head sensor  8  and the current exhaust gas temperature as measured by one or more exhaust gas sensors  6   a,    6   b,    6   c  are below minimum effective EGR usage temperatures. 
         [0050]    It will be appreciated that, in the case of the exhaust gas temperature, there could be a single temperature limit or multiple temperature limits. For example, if the engine  10  has, as shown in  FIG. 1 , both low pressure and high pressure EGR circuits, then the exhaust gas temperature required to prevent unstable combustion could be different to that required to prevent condensation upstream from a compressor, such as the compressor  20   c.    
         [0051]    Box  130  therefore represents tests to determine whether the current measured temperatures are above or below predefined limits and based upon the result of these tests the method either advances to box  140  or  160 . 
         [0052]    If one or more of the current measured temperatures is below the predefined limit for that parameter, the method advances to box  140  where EGR flow is prevented by, for example, closing or keeping closed EGR flow control valves such as the HPEGR and LPEGR valves  32  and  42  shown on  FIG. 1 . Opening the HPEGR and LPEGR valves  32  and  42  at this time would likely result in combustion instability and/or condensation being inducted into the compressor  20   c.    
         [0053]    The method then advances from box  140  to box  150  where an electric machine, such as the integrated starter-generator  18 , is operated as a generator to load the engine  10 . The additional load applied by the integrated starter-generator  18  will cause the engine temperature and the exhaust gas temperature to increase more rapidly than would normally be the case. 
         [0054]    From box  150  the method returns to box  130  to recheck the measured engine and exhaust gas temperatures and will cycle around the boxes  130 ,  140  and  150  until the measured engine and exhaust gas temperatures exceed the predefined temperature limit for that respective parameter. 
         [0055]    If in box  130  all of the measured engine and exhaust gas temperatures exceed their respective temperature limit, the method will advance to box  160 . 
         [0056]    In box  160  when exhaust gas heating by the use of the integrated starter-generator  18  is already active then it is deactivated and the integrated starter-generator  18  is then controlled normally to meet the demands of the vehicle  5 . 
         [0057]    However, if no exhaust gas heating is active when box  160  is entered, because the route to box  160  is from box  120  to box  130  and then to box  160 , the integrated starter-generator  18  is controlled normally to meet the demands of the vehicle  5 . 
         [0058]    Irrespective of the route followed to reach box  160 , from box  160  the method advances to box  170  where EGR is activated. That is to say, exhaust gas recirculation flow is permitted. It will be appreciated that if the engine  10  is fitted with low and high pressure EGR then in box  170  it could be LPEGR that is enabled, HPEGR that is enabled, or both, depending upon the temperature limit that has been reached. 
         [0059]    From box  170  the method advances to box  180  where it is checked whether a ‘key-off’ event has occurred and if it has the method returns to box  110  with the engine  10  ‘Off’ and if it has not, the method returns to box  120  with the engine  10  running. 
         [0060]    It will be appreciated that as soon as the engine  10  has warmed up sufficiently, the tests in box  130  will always result in the method advancing to box  160  and during normal hot running of the engine the method will therefore cycle continuously through boxes  120 ,  130 ,  160 ,  170  and  180 . 
         [0061]    It will be appreciated that in addition to the tests described in box  130  there could be additional tests used to determine whether the use of the integrated starter-generator  18  should be terminated such as, for example, whether the state of charge of the battery  19  has reached an upper limit or whether the torque demand upon the engine  10  is sufficiently high that additional heating is no longer required. 
         [0062]    While representative embodiments are described above, it is not intended that these embodiments describe all possible forms of the claimed subject matter. Rather, the words used in the specification are words of description rather than limitation, and it is understood that various changes may be made without departing from the spirit and scope of the disclosure. Additionally, the features of various implementing embodiments may be combined to form further embodiments that are not explicitly described or illustrated. While various embodiments may have been described as providing advantages or being preferred over other embodiments or prior art implementations with respect to one or more desired characteristics, as one of ordinary skill in the art is aware, one or more features or characteristics may be compromised to achieve desired overall system attributes, which depend on the specific application and implementation. These attributes include, but are not limited to: cost, strength, durability, life cycle cost, marketability, appearance, packaging, size, serviceability, weight, manufacturability, ease of assembly, etc. Embodiments described as less desirable than other embodiments or prior art implementations with respect to one or more characteristics are not necessarily outside the scope of the disclosure and may be desirable for particular applications.