Method for reducing the heating of a particulate filter during a regeneration event

Abstract

A method for reducing the heating of a particulate filter (15) connected to receive exhaust gas from an engine (10) of a motor vehicle (5) during a regeneration event when a release of the accelerator pedal (7) is active, the method comprising checking whether a regeneration event is taking place and whether a release event of the accelerator pedal (7) is present, and, if both a regeneration event is taking place and a release event of the accelerator pedal (7) is present, the method comprising using active temperature control to reduce the temperature within the particulate filter (15) during the regeneration event by operating an electric machine (16) drivingly connected to the engine (10) in a generator mode to charge a battery (17) of the motor vehicle (5) and by supplying fuel to the engine (10) to generate torque,to compensate for the additional load applied to the engine (10) by the electric machine (16), wherein the fuel supplied to the engine (10) causes a reduction in the oxygen concentration of the exhaust gas flow to the particulate filter (15), the method further comprising using a model to predict the temperature of the particulate filter (15) during the next regeneration event, and using active temperature control during the next regeneration event if the prediction of the particulate filter temperature by the model indicates that the temperature of the particulate filter (15) will be unacceptably high during the regeneration event.

Description

[0001] This invention relates to a particulate filter adapted to receive exhaust gas from an engine of a motor vehicle, and more particularly to a method of protecting a particulate filter from overheating during a regeneration event when the engine decelerates due to release of the accelerator pedal.

[0002] It is well known to provide a particulate filter (PF) in an engine's exhaust system to filter soot from the exhaust gases flowing from the engine to the atmosphere. If the engine is a diesel engine, such a particulate filter is typically referred to as a diesel particulate filter, or 'DPF'.

[0003] A particulate filter (PF) can be damaged during what is known as a 'fuel cut-off' scenario, in which fuel to a vehicle's engine is shut off during an accelerator pedal release event to conserve fuel during vehicle deceleration. If the fuel supply to the engine is shut off when a soot combustion process, known as a 'regeneration event', is active, the percentage of oxygen in the exhaust gas flowing to the particulate filter is greatly increased. This increase in oxygen concentration often causes uncontrolled combustion to occur in the particulate filter if a large amount of soot remains in the particulate filter to be burned during the regeneration process.

[0004] When uncontrolled combustion occurs, the temperature within the particulate filter can rise to more than 1000°C, potentially causing the particulate filter to crack, melt the particulate filter substrate, or degrade the catalyst washcoat, which is present to assist in the removal of other regulated pollutants (HC, CO, or NOx). In extreme cases, this overheating condition can cause the particulate filter material to combust, potentially leading to thermal damage to the surrounding components.

[0005] A temperature likely to cause damage to the particulate filter is an unacceptably high temperature, and the particulate filter may be considered to be overheating when exposed to such a temperature.

[0006] The document DE 10 2012 200 097 A1 discloses a method for protecting a particulate filter in the exhaust system of an internal combustion engine of a motor vehicle against uncontrolled combustion of soot in the particulate filter.

[0007] It is an object of this invention to provide a method for reducing excessive heating of a particulate filter during a regeneration event when an accelerator pedal release event is active.

[0008] According to a first aspect of the invention, a method is provided for reducing the heating of a particulate filter connected to receive exhaust gas from an engine of a motor vehicle during a regeneration event when accelerator pedal release is active, the method comprising checking whether a regeneration event is taking place and whether an accelerator pedal release event is present, and, if both a regeneration event is taking place and an accelerator pedal release event is present, the method comprising using active temperature control to reduce the temperature within the particulate filter during the regeneration event by operating an electric machine drivingly connected to the engine in a generator mode to charge a battery of the motor vehicle and by supplying fuel to the engine to generate torque,to compensate for the additional load applied to the engine by the electric machine, whereby the fuel supplied to the engine causes a reduction in the oxygen concentration of the exhaust gas flow to the particulate filter.

[0009] Supplying fuel to the engine to produce torque may include increasing the amount of fuel supplied to the engine from zero to the desired amount required to produce the compensation torque.

[0010] The engine may be a direct injection diesel engine and the particulate filter may be a diesel particulate filter.

[0011] The method may further comprise using active temperature control during regeneration of the particulate filter only if a determined exhaust gas temperature at an outlet from the particulate filter or a determined particulate filter temperature indicates that the temperature of the particulate trap is unacceptably high.

[0012] The method includes using a model to predict the temperature of the particulate filter during the next regeneration event, and using active temperature control during the next regeneration event if the model's prediction of the particulate filter temperature indicates that the temperature of the particulate trap will be unacceptably high during the regeneration event.

[0013] The prediction of the particulate filter temperature during the next regeneration event can be provided by a soot combustion model.

[0014] The temperature may be unacceptably high if it exceeds a specified temperature limit.

[0015] The method may further include actively reducing the battery state of charge prior to a regeneration event if the battery state of charge is above a predetermined level and an estimate of the current soot loading of the particulate filter indicates that regeneration of the particulate filter will be required in the near future.

[0016] Regeneration of the particulate filter may be required in the near future if the estimate of the current soot loading of the particulate filter is above a first soot loading limit.

[0017] The estimation of the current soot loading of the particulate filter can be based either on a model of soot production since the last regeneration event or on a measurement of the pressure drop across the particulate filter.

[0018] The electrical working machine can be an integrated starter generator.

[0019] According to a second aspect of the invention, a motor vehicle is provided comprising an internal combustion engine, a battery, an electric working machine drivingly connected to the engine, a storage device for electrical energy connected to the electric working machine, a particulate filter configured to receive exhaust gas from the engine, and an electronic control unit configured to control the engine and the electric working machine, wherein the electronic control unit is configured to check whether a regeneration event is taking place and whether an accelerator pedal release event is present, wherein, if both a regeneration event is taking place and an accelerator pedal release event is present, the electronic control unit is configured to use active temperature control,to control the temperature within the particulate filter during the regeneration event by operating the electric machine drivingly connected to the engine in a generator mode to charge the battery of the motor vehicle and by supplying fuel to the engine to generate torque to compensate for the additional load applied to the engine by the electric machine, wherein the increase in fuel supplied to the engine causes a reduction in the oxygen concentration of the exhaust stream to the particulate filter.

[0020] Supplying fuel to the engine to produce torque may include increasing the amount of fuel supplied to the engine from zero to a desired amount required to produce the compensating torque.

[0021] The engine may be a direct injection diesel engine and the particulate filter may be a diesel particulate filter.

[0022] The electronic control unit may be configured to use active temperature control during regeneration of the particulate filter only if a determined exhaust gas temperature from a temperature sensor located at an outlet from the particulate filter or a determined particulate filter temperature from a temperature sensor located within the particulate filter indicates that the temperature of the particulate trap is unacceptably high.

[0023] The electronic control unit includes a model to predict the temperature of the particulate filter during the next regeneration event, wherein the electronic control unit is configured to use active temperature control during the next regeneration event if the prediction of the particulate filter temperature by the model indicates that the temperature of the particulate trap will be unacceptably high during the regeneration event.

[0024] The model may be a soot combustion model used to provide a prediction of the particulate filter temperature during the next regeneration event.

[0025] The temperature of the particulate filter may be unacceptably high if it exceeds a specified temperature limit.

[0026] The electronic control unit may further be configured to actively reduce the battery state of charge prior to a regeneration event if the battery state of charge is above a predetermined level and an estimate of the current soot loading of the particulate filter indicates that regeneration of the particulate filter will be required in the near future.

[0027] Regeneration of the particulate filter may be required in the near future if the estimate of the current soot loading of the particulate filter is above a first soot loading limit.

[0028] The estimation of the current soot loading of the particulate filter can be based either on a model of soot production since the last regeneration event stored in the electronic control unit or on a measurement of the pressure drop across the particulate filter.

[0029] The pressure drop may be based on data supplied to the electronic control unit from a pressure sensor located upstream of the particulate filter and a pressure sensor located downstream of the particulate filter.

[0030] The electrical working machine can be an integrated starter generator.

[0031] The vehicle can be a mild hybrid vehicle.

[0032] The invention will now be described by way of example with reference to the accompanying drawings, in which: Fig. Figure 1 is a schematic diagram of a motor vehicle constructed according to a second aspect of the invention; Fig. Figure 2a is a first portion of a detailed flowchart of a method according to a first aspect of the invention; Fig. Figure 2b is a second section of the detailed flowchart of the method according to the first aspect of the invention; Fig. Figure 3 is a composite graph showing a relationship between temperature and time for a DPF during an accelerator pedal release event while regeneration is taking place and the relationship between oxygen concentration and time for the same event according to the prior art; Fig. Figure 4 is a composite graph showing a relationship between temperature and time for a DPF during an accelerator pedal release event while regeneration is taking place, according to this invention, and the relationship between oxygen concentration and time for the same event; and Fig. Figure 5 is a schematic graph showing the relationship between soot loading and time for a DPF during a period in which a regeneration event is taking place.

[0033] In Fig. 1 shows a mild hybrid motor vehicle 5 having four road wheels 6, a direct injection diesel engine 10 and an electronic control unit 20.

[0034] The engine 10 is designed to receive air through an intake 11 and, although not shown, the air flow to the engine 10 is in most cases compressed by a supercharger or turbocharger before flowing into the engine 10 to increase the efficiency of the engine 10.

[0035] The exhaust gas from the engine 10 flows through a first or upstream section 12 of an exhaust system to a particulate filter in the form of a diesel particulate filter (DPF) 15, wherein the exhaust gas, after passing the DPF 15, flows to the atmosphere via a second or downstream section 13 of the exhaust system.

[0036] It should be noted that other exhaust gas purification devices or a noise suppression device may be present in the exhaust gas flow path from the engine 10 to the position where it exits to the atmosphere.

[0037] An electric work machine is drivingly connected to the engine 10. In the case of this example, the electric work machine is an integrated starter-generator 16, which, depending on the mode in which it operates, can be used to generate electricity or to generate torque. A battery 17, along with associated control electronics (not shown), is connected to the integrated starter-generator 16. When the integrated starter-generator 16 operates as a generator, it charges the battery 17, while when the integrated starter-generator 16 operates as a motor, the battery 17 is configured to supply electrical energy to the integrated starter-generator 16.The integrated starter generator 16 is used to start the engine 10 and, in the case of this example, may also provide a limited torque boost to the engine 10 during acceleration or cruising speed of the vehicle 5.

[0038] The electronic control unit 20, in this example, is configured to receive inputs from a number of sensors, such as, but not limited to, the following: A / an air mass flow sensor 21 used to measure the mass of air flowing into the engine 10; B / an engine speed sensor 22; C / a lambda / oxygen sensor 24 to measure the air-fuel ratio / oxygen content of the exhaust gas leaving the engine 10; D / a vehicle speed sensor 25 for measuring the speed of the vehicle 5; E / a NOx sensor 26 for measuring the NOx content in the exhaust gas from the engine 10; F / an accelerator pedal position sensor 27 designed to determine the position of an accelerator pedal 7; G / a temperature sensor 28 to measure the temperature of the exhaust gas leaving the DPF 15; H / an upstream exhaust pressure sensor 29U for determining the pressure of the exhaust gas upstream of the DPF 15; and I / a downstream exhaust pressure sensor 29D to determine the pressure of the exhaust gas downstream of the DPF 15.

[0039] Please note that in some cases, points H and I may be replaced by a single differential pressure sensor connected to two pipes. One of the pipes is connected near an inlet to the DPF 15, while the other pipe is connected near an outlet from the DPF 15.

[0040] The electronic control unit 20 is capable of controlling the operation of the engine 10 and the operating state of the integrated starter-generator 16. It is noted that the electronic control unit 20 could be formed from several separate electronic units that are electrically connected to one another, and not in the form of a single unit, as in Fig. 1 is shown.

[0041] The electronic control unit 20 is designed to prevent overheating of the DPF 15 using active temperature control during a regeneration event when an 'accelerator pedal release event' occurs.

[0042] An 'accelerator pedal release event' is an event in which a driver of the vehicle 5 has removed their foot from the accelerator pedal 7, or the accelerator pedal 7 is depressed by the driver by such a small amount that it represents a situation of zero torque demand. When such an accelerator pedal release event occurs, the fuel supply to the engine 10 is normally shut off, so that no fuel is delivered to the engine 10 and thereby increasing the fuel economy of the motor vehicle 5. Such an event is therefore often referred to as a deceleration fuel cut-off event, or DCFO. During a DCFO, no torque is produced by the engine. The engine is rotated by the inertia of the motor vehicle, which is transmitted through the driveline from the driven road wheels, which are rotating because the motor vehicle is moving.The engine therefore acts as a pump, forcing air through the connected exhaust system and consequently through the DPF. In such a situation, the engine exerts a braking force due to the torque required to rotate as a pump and the closed driveline between the engine and the driven road wheels.

[0043] The presence of an accelerator pedal release event is determined in this example by the electronic control unit 20 using the input it receives from the accelerator pedal sensor 27 associated with the accelerator pedal 7.

[0044] The electronic control unit 20 is designed to operate the engine 10 to perform regeneration of the DPF 15 when it is determined that regeneration of the DPF 15 is required.

[0045] The requirement to regenerate the DPF 15 can be determined in several ways, but is generally based on estimating whether a current soot load is above a predetermined limit.

[0046] A soot model may be used to estimate a soot load of the DPF 15 based on an estimate of the soot generated by the engine 10 since the last regeneration event, taking vehicle usage into account. The soot load estimate may then be compared to a soot load limit, and if the estimated soot load is above the soot load limit, regeneration of the DPF 15 is initiated.

[0047] Alternatively, the soot loading can be estimated by determining the pressure upstream and downstream of the DPF 15 using the two pressure sensors 29U, 29D, providing a value for the pressure drop across the DPF 15 that indicates the soot loading of the DPF. The pressure drop can then be compared to a pressure drop limit that indicates a level of soot loading at which regeneration is required. If the measured pressure drop is above the pressure drop limit, regeneration of the DPF 15 is initiated.

[0048] It should be understood that the invention is not limited to any particular method for determining when regeneration of the DPF 15 is required and that any suitable method could be used.

[0049] Regardless of the method used, when it is determined that regeneration of the DPF 15 is required, the electronic control unit 20 may either act immediately to control the temperature within the DPF 15 using active temperature control, or the electronic control unit 20 may delay initiation of active temperature control until the temperature of the DPF 15 exceeds a predetermined value.

[0050] For example, if the signal received by the electronic control unit 20 from the exhaust gas temperature sensor 28 located downstream of the DPF 15 indicates that the temperature of the exhaust gas leaving the DPF 15 is excessive. That is, if the temperature of the exhaust gas measured by the temperature sensor 28 exceeds a predetermined temperature limit (T Lim), the electronic control unit 20 acts to actively control the temperature within the DPF 15, but if the temperature of the exhaust gas leaving the DPF 15 is below this predetermined temperature limit (T Lim ), it takes no action, but instead allows the regeneration of the DPF 15 to continue without intervention. In such a case, the specified temperature limit T Lim set to a temperature above which damage is likely to occur, such as, without limitation, approximately 850°C. It is recognized that the temperature determined by the downstream temperature sensor 28 is not a measurement of the actual temperature within the DPF 15, but that the temperature within the DPF 15 can be inferred from this temperature measurement. The temperature within the DPF 15 is likely higher than this measured or modeled temperature.

[0051] It should be noted that instead of the downstream temperature sensor 28, a temperature sensor capable of measuring the temperature inside the DPF 15 could be used, in which case the predetermined temperature limit could be set higher than 850 °C, such as 950 °C.

[0052] It is also noted that instead of measuring the temperature of the DPF 15 or the exhaust gas flow through the DPF 15 during a regeneration event, a soot combustion model could be used to predict whether overheating of the DPF 15 is likely to occur, and if likely to occur, the electronic control unit 20 can act as soon as regeneration begins to prevent overheating of the DPF 15 during the regeneration event if an accelerator pedal release event is also present.

[0053] Assuming that the determination of the electronic control unit 20 is that the temperature within the DPF 15 is excessive, or that it is likely to be excessive, i.e. that overheating of the DPF 15 is likely, the electronic control unit 20 is designed to use active temperature control to prevent overheating of the DPF 15 during regeneration.

[0054] Active temperature control involves using the integrated starter-generator 16 to apply a load to the engine 10 by operating the integrated starter-generator 16 as a generator to charge the battery 17, and compensating for the increased load by increasing the torque output of the engine 10 by increasing the amount of fuel supplied to the engine 10. It should be noted that prior to initiating active temperature control, the engine is not producing torque if it is in a DCFO state, and therefore, upon the demand for torque by the engine 10, combustion begins by providing fuel to the engine 10.

[0055] It should be noted that the additional load applied by the integrated starter-generator 16 to the engine 10 during an accelerator pedal release event would normally cause the engine 10 to decelerate faster than it would otherwise decelerate. However, it is one of the advantages of the invention that the torque applied by the integrated starter-generator 16 to the engine 10 is offset by an increase in the torque output of the engine 10, so that the deceleration rate of the engine 10 during an accelerator pedal release event is substantially the same regardless of whether the integrated starter-generator 16 is used or not to control the overheating of the DPF 15. The driver of the vehicle 5 is therefore unaware that action is being taken to control the overheating of the DPF 15 during a regeneration event, with the deceleration of the vehicle 5 being as expected by the driver.

[0056] The effect of requesting an increase in the torque of the engine 10 during the accelerator pedal release event is that additional fuel must be supplied to the engine 10 to produce the additional torque. As before, if a DCFO is present prior to the initiation of active temperature control, then no fuel is supplied to the engine 10, and therefore no torque is produced by the engine 10. The increase in the fuel supplied to the engine 10 from zero has the effect of reducing the air / fuel ratio of the exhaust gas flowing to the DPF 15 from substantially 100% air, and thereby reducing the oxygen concentration of the exhaust gas flow to the DPF 15. It will be appreciated that the composition of the air contains approximately 21% oxygen.

[0057] The increased torque output of engine 10, in this case, is provided by increasing an engine torque setpoint within electronic control unit 20, causing the engine to run and thereby requiring a fuel supply, reducing the amount of oxygen flowing to DPF 15. Reducing the oxygen in the exhaust gas entering DPF 15 slows the soot combustion rate within DPF 15, thereby reducing the temperature of DPF 15.

[0058] The electronic control unit 20 is designed to increase the torque output of the engine 10 to match the load applied by the integrated starter-generator 16, that is, the torque increase requested by the electronic control unit 20 from the engine 10 is substantially equal to the torque applied to the engine 10 by the integrated starter-generator.

[0059] In the case of one example, the concentration of oxygen in the exhaust stream to the DPF 15 was reduced from 21% in a case where a conventional fuel shutoff was used during an accelerator pedal release event to 5% oxygen concentration when the integrated starter-generator 16 was used to load the engine 10 and fuel was supplied to the engine 10 to counteract the load applied by the integrated starter-generator 16.

[0060] It should be noted that the use of the battery 17 as a load for the integrated starter-generator 16 instead of any other type of electrical load has the advantage that the energy stored in the battery 17 during use of the integrated starter-generator 16 for the purpose of active temperature control can be returned from the battery 17 to the vehicle 5 at a later time.

[0061] In order to provide sufficient charge capacity or 'headroom' to accommodate the electrical charge generated by the integrated starter-generator 16 while it is being used to load the engine 10, the electronic control unit 20 is further advantageously operable to take measures to reduce the state of charge (SOC) of the battery 17 prior to regenerating the DPF 15, which is known as 'active SOC reduction' if the SOC is above a predetermined SOC limit.

[0062] The SOC of battery 17 can be actively reduced by: A / using the integrated starter-generator 16 more frequently than a motor to provide a torque boost to the engine 10 and thereby increase the torque output of the engine 10 during an acceleration event; or B / using the integrated starter-generator 16 as a motor to enable the torque output of the engine 10 to be reduced while driving at cruising speed of the vehicle and thereby reducing the fuel consumption of the engine 10; or C / inhibiting charging of the battery 17 to cause the battery 17 to discharge over time until the SOC 17 of the battery falls below the predetermined SOC limit.

[0063] To prevent the battery 17 from discharging when no regeneration event is imminent, the electronic control unit 20 is operable to monitor the soot loading of the DPF 15 and to permit active discharging of the battery 17 only when the soot loading in the DPF 15 approaches a level where regeneration is required. That is, the electronic control unit 20 is operable to permit active SOC reduction of the battery 17 only when a first predetermined soot loading limit is exceeded.

[0064] The first predefined soot loading limit is slightly lower than a second higher soot loading limit, which is used as a trigger for regeneration.

[0065] In summary, if an accelerator pedal release event occurs during a DPF regeneration event, DPF overheating is therefore prevented or mitigated using active temperature control, in which a load is applied to the engine by the electric machine and the applied load is counteracted by increasing the engine torque output, which requires additional fuel to be supplied to the engine.

[0066] In the Fig. 2a and Fig. 2b specifically shows a method 100 for protecting a particulate filter from overheating during a regeneration event when an engine decelerates due to accelerator pedal release.

[0067] When accelerator pedal release is active, a driver of the motor vehicle 5 has removed his foot from the accelerator pedal 7 or a driver of the motor vehicle 5 depresses the accelerator pedal 7 by such a small amount that the effective torque request from the accelerator pedal 7 to the engine 10 is zero.

[0068] The method begins at box 105 with the engine 10 running normally. That is, an electronic control unit, such as the electronic control unit 20, acts on the engine 10 to produce torque in response to a driver torque request via input from the accelerator pedal 7, or in response to a torque request from a cruise control control unit.

[0069] From box 105, the method proceeds to box 110, where it is checked whether a soot load (SL) of a particulate filter, such as the diesel particulate filter (DPF) 15, is above a first predetermined limit.

[0070] With reference to Fig. Figure 5 shows a schematic diagram illustrating the filling and regeneration of a particulate filter, such as the DPF 15. It should be noted that in practice, the DPF 15 is not filled in a linear manner, and that regeneration does not result in a linear removal of soot from the DPF 15. The amount of soot in the DPF 15, referred to as the 'soot loading', is indicated by the line SL. It can be seen that the soot loading SL increases until a regeneration event occurs at time t2, whereby most of the soot is removed from the DPF 15.

[0071] In the case of this example, two predefined limits or trigger levels (SL Lim1 and SL Lim2 ) is present. The first specified soot limit SL Lim1 is set to a soot loading of approximately 75% of the full soot loading (SL = 100%).

[0072] This first specified soot limit SL Lim1is used to provide an indication that regeneration of the DPF 15 is likely to be required in the near future, being exceeded at a time t1 in the example shown.

[0073] The second specified soot limit SL Lim2 is set to a soot load of approximately 85% of the full soot load. This second specified soot limit SL Lim2 is used to provide an indication that regeneration of the DPF 15 is required and is sometimes referred to as a 'regeneration trigger limit'. Provided that the conditions for regeneration are present, the second predefined soot limit SL Lim2 as a trigger to start the regeneration of the DPF 15 when the soot load exceeds this limit, which in the case of this example occurs at time t2.

[0074] It is noted that the values ​​of 75% and 85% are exemplary and that the invention is not limited to the use of such limits.

[0075] If, again referring to Fig. 2a, the estimated soot level in the DPF 15 below the first specified soot limit SL Lim1 the method returns from box 110 to box 105 because regeneration of the DPF 15 is not currently required or is not expected to be required in the near future.

[0076] However, if the test in box 110 shows that the estimated soot level in the DPF 15 is above the first specified soot limit SL Lim1 the method proceeds from box 110 to box 120 because it is likely that regeneration of DPF 15 will be required in the near future.

[0077] In box 120, it is checked whether the state of charge (SOC) of the battery 17 is above a predetermined SOC limit SOC Lim which is set to provide a predetermined amount of headroom for charging the battery 17. It should be noted that if the battery 17 is fully charged or nearly fully charged, it cannot be used as an effective electrical load for the electric machine 16, so it is therefore desirable to reduce the SOC of the battery 17 if regeneration of the DPF 15 is likely to be required in the near future. However, it should also be noted that maintaining a high charge level in the battery 17 is generally beneficial, so it is only desirable to discharge the battery 17 if the DPF 15 is likely to require regeneration in the near future.

[0078] If the SOC of the battery 17 is above the SOC limit SOC Lim then the procedure proceeds to box 125, otherwise it proceeds to box 130.

[0079] That is, if SOC > SOC Lim , go to 125, otherwise go to 130.

[0080] In box 125, the SOC of the battery 17 is actively reduced in an economical and efficient manner. The SOC of the battery 17 can be reduced by using the integrated starter-generator 16 as a motor to provide supplemental torque and thereby either increase the total torque output available for accelerating the vehicle 5, or to allow the torque output of the engine 10 to be reduced while driving at cruising speed to reduce the fuel consumption of the engine 10, or by inhibiting charging of the battery 17 to thereby allow it to be slowly discharged until the SOC falls below the predetermined SOC limit SOC Lim falls, can be actively reduced.

[0081] If the SOC of the battery 17 is below the SOC limit SOC Lim, then the method continues from box 120 to box 130, where it is checked whether the estimated soot level within the DPF 15 is above the second predetermined soot limit SL Lim2 is located.

[0082] If the soot level in the DPF 15 is below the second specified soot limit SL Lim2 , the test has failed and there is no current need to regenerate the DPF 15, so the method then returns to box 105. In such a case, the method then cycles through steps 105 to 130 until the test in box 130 is finally passed.

[0083] If the test according to Box 130 is passed, indicating that the current estimated soot level in the DPF 15 is above the second specified soot limit SL Lim2 regeneration of the DPF 15 is required and the method proceeds to box 140 where regeneration of the DPF 15 begins.

[0084] It should be noted that in practice, the start of regeneration may be delayed until the required operating conditions for regeneration have been achieved. Techniques for regenerating particulate filters are well known in the art, see, for example, EP1744042; GB2496876 and GB2506660.

[0085] From box 140, the method proceeds to box 150, where it is checked whether an accelerator pedal release event is present. This can be checked by the electronic control unit 20, for example, using the output from the accelerator pedal position sensor 27 to measure whether the accelerator pedal 7 is currently in a rest position or depressed by the driver of the motor vehicle 5 to request torque from the engine 10.

[0086] It should be noted that the operations in boxes 140 and 150 could be reversed, so that pedal release is checked before regeneration begins, or could be performed simultaneously.

[0087] If no accelerator pedal release event is in progress, then the method proceeds from box 150 to box 170 where regeneration of the DPF 15 continues without temperature control intervention, then proceeds from box 170 to box 175 to check if regeneration is complete.

[0088] If regeneration is complete, the method returns from box 175 to box 105 with the DPF 15 regenerated, otherwise it returns to box 150 and cycles through boxes 150, 170 and 175 until either an accelerator pedal release event occurs or the DPF 15 is regenerated.

[0089] If, returning to box 150, it is determined in box 150 that an accelerator pedal release event is present, the method proceeds from box 150 to box 160.

[0090] In box 160, a check is made to determine whether the particulate filter in the form of the DPF 15 is overheating. As previously described, this may be achieved using the temperature sensor 28 to measure the temperature of the exhaust gas leaving the DPF 15, or a temperature sensor (not shown) located within the DPF 15 could be used. However, as an alternative to this approach, the temperature within the DPF 15 could be modeled, for example, using a model of the soot combustion process to estimate the temperature within the DPF 15.

[0091] The use of such a soot combustion model has the advantage that there is no delay between the time at which the temperature in the DPF 15 is predicted to be excessive and the start of temperature control by the electronic control unit 20, whereas there is a small delay when the increase is detected by the downstream temperature sensor 28 because the temperature of the exhaust gas must increase before its increase can be detected, and therefore the system then acts retroactively.

[0092] Furthermore, if a soot combustion model such as the one disclosed in US patent application 2012 / 0031080 is used, the increase in temperature can be predicted so that the system can act proactively, resulting in the necessary steps to control the temperature being taken earlier.

[0093] For example, if in an accelerator pedal release situation the prediction indicates that the temperature within the DPF 15 is likely to be unacceptably high during regeneration, i.e., above a predetermined limit set based on a need to prevent damage occurring to the DPF 15, then the DPF 15 is likely to overheat and active temperature control may be initiated once regeneration begins.

[0094] Regardless of the method used to determine whether overheating is likely to occur or is occurring, the estimated or determined temperature is usually compared to a predetermined temperature limit, such as 850 °C.

[0095] If the result of the test in box 160 is that the DPF 15 is not currently overheating or is predicted not to overheat, ie, the measured or predicted temperature is below 850°C, the method proceeds to box 170, whereupon it continues as previously described unless a vehicle key-off event occurs, whereupon it ends.

[0096] However, if it is determined in box 160 that the result is that the DPF 15 is overheating, or, if a soot combustion model is used, that the overheating of the DPF 15 is likely, then the method goes from box 160 via boxes 180 and 200 to the one in Fig. Continue to box 210 shown in Figure 2b.

[0097] Box 210 represents the active temperature control steps taken to prevent the DPF 15 from overheating during a regeneration event during an accelerator pedal release event.

[0098] The first step, indicated in box 212, is to switch the electric machine, which in this case is the starter-generator 16 driven by the engine 10, into a battery charging mode. This process causes an increase in load in the form of torque applied to the engine 10.

[0099] This increased load would normally cause the engine speed to decrease more rapidly because, in a conventional accelerator pedal release event by the driver, no torque is requested and the engine 10 decelerates. However, in the case of this invention, the electronic control unit 20 is configured to balance the load applied to the engine 10 by the integrated starter-generator 16 with an increase in the engine output torque request. The result of applying the torque applied by the integrated starter-generator 16 in box 212 is therefore counteracted by the electronic control unit 20 increasing the engine torque command, as indicated in box 214.The effect of increasing the engine torque setpoint is to generate a torque output from the engine 10 by injecting fuel into the engine 10, thereby reducing the amount of oxygen in the exhaust stream to the DPF 15, as indicated in box 220. Additionally, by matching the increase in torque output from the engine 10 to the load applied by the integrated starter-generator 16, the deceleration rate of the engine 10 is the same as would be the case with a natural deceleration due to a conventional accelerator pedal release event. This has the advantage that a driver of the motor vehicle 5 receives the same feedback from the motor vehicle 5, whether the accelerator pedal release is a conventional one or one where the temperature of the DPF 15 is managed through the use of the integrated starter-generator 16 to prevent the DPF 15 from overheating.

[0100] From box 220, the method proceeds to box 230 to check whether DPF regeneration is complete. If DPF regeneration is complete, i.e., if it is estimated that the majority of soot has been removed from the DPF 15, then the method proceeds to box 240, where the integrated starter-generator 16 is returned to normal operation and the engine torque setpoint for normal operation is restored. The method then returns to box 105 via box 245 and repeats all subsequent steps until a key-off event occurs, at which point it ends.

[0101] However, if it is checked in box 230 that DPF regeneration is not complete, the method proceeds to box 250 where it is checked whether the DPF 15 is overheating and whether an accelerator pedal release event is still present, and if these two conditions are met, the method returns to box 210 and the steps referred to previously are repeated, continuing to reduce the oxygen supply to the engine 10.

[0102] However, if it is verified at box 250 that either the DPF 15 is not overheating or there is no longer an accelerator pedal release event, the method proceeds to box 260, where the integrated starter-generator 16 is returned to normal operation and the engine torque setpoint for normal operation is restored. The method then returns from box 260 via box 270 to box 170, repeating all subsequent steps unless a key-off event occurs, at which point it ends.

[0103] It should be understood that the invention is not limited to the precise steps described or the precise order in which these steps are performed.

[0104] The effect of carrying out a method according to this invention is by comparing the Fig. 3 shown prior art situation with the situation when the method 100 is used as in Fig. 4 is shown.

[0105] In the Fig. In the prior art case shown in Figure 3, an accelerator pedal release event during a DPF regeneration event results in an oxygen concentration in the exhaust gas of approximately 21%, indicated by the line (O2), which leads to a rapid increase in temperature (T) within the DPF due to the availability of oxygen for fuel combustion of the soot. Note that the atmospheric air has an oxygen concentration of approximately 21%. The temperature continues to rise in this case because no temperature control is active, eventually leading to overheating of the DPF during the regeneration event, with a peak temperature of approximately 1000°C.

[0106] In the case of this invention, an accelerator pedal release event during a DPF regeneration event initially generates an oxygen concentration (O2) of about 21%, resulting in a sudden increase in DPF temperature until the conditions for active temperature control are met at time 't', as shown in Fig. 4. This means that regeneration is taking place, an accelerator pedal release event is present and the temperature of the DPF 15 has reached the specified temperature limit T lim reached or it is predicted that the temperature of the DPF 15 will exceed the specified temperature limit T lim which in this case is set at 850 °C.

[0107] In the case of this example, at time 't' the specified temperature limit T limis reached and active torque control is activated, applying torque to the engine 10 by the integrated starter-generator 16 and increasing the engine torque setpoint. After implementing these changes, the oxygen concentration drops to approximately 5%, resulting in a reduction in the increase in temperature (T) within the DPF 15 due to the limited availability of oxygen for fuel combustion of the soot in the DPF 15.

[0108] Although the invention has been described with respect to a mild hybrid vehicle, it is understood that it could be usefully applied to other vehicles having an electric machine capable of applying a sufficiently large load to the engine to require an increase in engine output torque of sufficient magnitude to produce the desired reduction in the oxygen concentration of the exhaust gas flowing to the particulate filter.

[0109] Although the invention has been described with respect to a diesel engine having a diesel particulate filter for removing particulate matter from an exhaust gas stream, it is understood that it could be applied to other types of engines having a particulate filter for reducing their particulate emissions.

[0110] It will be appreciated by those skilled in the art that, although the invention has been described by way of example with respect to one or more embodiments, it is not limited to the disclosed embodiments and that alternative embodiments could be constructed without departing from the scope of the invention as defined by the appended claims.

Claims

[1] A method for reducing the heating of a particulate filter (15) connected to receive exhaust gas from an engine (10) of a motor vehicle (5) during a regeneration event when a release of the accelerator pedal (7) is active, the method comprising checking whether a regeneration event is taking place and whether a release event of the accelerator pedal (7) is present, and, if both a regeneration event is taking place and a release event of the accelerator pedal (7) is present, the method comprising using active temperature control to reduce the temperature within the particulate filter (15) during the regeneration event by operating an electric machine (16) drivingly connected to the engine (10) in a generator mode to charge a battery (17) of the motor vehicle (5) and by supplying fuel to the engine (10) to generate torque,to compensate for the additional load applied to the engine (10) by the electric machine (16), wherein the fuel supplied to the engine (10) causes a reduction in the oxygen concentration of the exhaust gas flow to the particulate filter (15), the method further comprising using a model to predict the temperature of the particulate filter (15) during the next regeneration event, and using active temperature control during the next regeneration event if the prediction of the particulate filter temperature by the model indicates that the temperature of the particulate filter (15) will be unacceptably high during the regeneration event. [2] The method of claim 1, wherein the engine (10) is a direct injection diesel engine and the particulate filter (15) is a diesel particulate filter. [3] A method according to any one of the preceding claims, wherein the method further comprises using an active temperature control during regeneration of the particulate filter (15) only if a determined exhaust gas temperature at an outlet from the particulate filter (15) or a determined particulate filter temperature indicates that the temperature of the particulate trap is unacceptably high. [4] The method of claim 1, wherein the prediction of the particulate filter temperature during the next regeneration event is made using a soot combustion model. [5] A method according to claim 3 or claim 1, wherein the temperature is unacceptably high if it is above a predetermined temperature limit. [6] The method of any one of claims 1 to 5, further comprising actively reducing the state of charge of the battery (17) prior to a regeneration event if the state of charge of the battery (17) is above a predetermined level and an estimate of the current soot loading of the particulate filter (15) indicates that regeneration of the particulate filter (15) will be required in the near future. [7] The method of claim 6, wherein regeneration of the particulate filter (15) is required in the near future if the estimate of the current soot loading of the particulate filter (15) is above a first soot loading limit. [8] A method according to claim 6 or claim 7, wherein the estimation of the current soot loading of the particulate filter (15) is based either on a model of soot production since the last regeneration event or on a measurement of the pressure drop across the particulate filter (15). [9] Method according to one of claims 1 to 8, wherein the electrical working machine (16) is an integrated starter-generator. [10] A motor vehicle (5) comprising an internal combustion engine (10), a battery (17), an electric working machine (16) drivingly connected to the engine (10), a storage device for electrical energy connected to the electric working machine (16), a particulate filter (15) designed to receive exhaust gas from the engine (10), and an electronic control unit (20) designed to control the engine (10) and the electric working machine (16), wherein the electronic control unit (20) is designed to check whether a regeneration event is taking place and whether a release event of the accelerator pedal (7) is present, wherein, if both a regeneration event is taking place and a release event of the accelerator pedal (7) is present, the electronic control unit (20) is designed to use active temperature control,to control the temperature within the particulate filter (15) during the regeneration event by operating the electric machine (16) drivingly connected to the engine (10) in a generator mode to charge the battery (17) of the motor vehicle (5) and by supplying fuel to the engine (10) to generate torque to compensate for the additional load applied to the engine (10) by the electric machine (16), wherein the increase in fuel supplied to the engine (10) causes a reduction in the oxygen concentration of the exhaust gas flow to the particulate filter (15), wherein the electronic control unit (20) includes a model to predict the temperature of the particulate filter (15) during the next regeneration event, wherein the electronic control unit (20) is configured to use the active temperature control during the next regeneration event if the prediction of the particulate filter temperature by the model indicates that the temperature of the particulate filter (15) will be unacceptably high during the regeneration event. [11] Vehicle (5) according to claim 10, wherein the engine (10) is a direct injection diesel engine and the particulate filter (15) is a diesel particulate filter. [12] A vehicle (5) according to claim 10 or claim 11, wherein the electronic control unit (20) is adapted to use the active temperature control during regeneration of the particulate filter (15) only if a determined exhaust gas temperature from a temperature sensor (28) located at an outlet from the particulate filter (15) or a determined particulate filter temperature from a temperature sensor (28) located within the particulate filter (15) indicates that the temperature of the particulate trap is unacceptably high. [13] A vehicle (5) according to claim 10, wherein the model is a soot combustion model used to provide a prediction of the temperature of the particulate filter (15) during the next regeneration event. [14] A vehicle (5) according to claim 12 or claim 10, wherein the temperature of the particulate filter (15) is unacceptably high if it is above a predetermined temperature limit. [15] Vehicle (5) according to one of claims 10 to 14, wherein the electronic control unit (20) is further configured to actively reduce the state of charge of the battery (17) prior to a regeneration event if the state of charge of the battery (17) is above a predetermined level and an estimate of the current soot loading of the particulate filter (15) indicates that regeneration of the particulate filter (15) will be required in the near future. [16] Vehicle (5) according to claim 15, wherein the regeneration of the particulate filter (15) is required in the near future if the estimate of the current soot loading of the particulate filter (15) is above a first soot loading limit. [17] A vehicle (5) according to claim 15 or claim 16, wherein the estimate of the current soot loading of the particulate filter (15) is based either on a model of soot production since the last regeneration event stored in the electronic control unit (20) or a measurement of the pressure drop across the particulate filter (15). [18] Vehicle (5) according to one of claims 10 to 17, wherein the electric working machine (16) is an integrated starter generator. [19] Vehicle (5) according to one of claims 10 to 18, wherein the vehicle (5) is a mild hybrid vehicle.

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