METHOD FOR AN ENGINE

Abstract

A method for an engine (10) comprising: in response to the fact that an actual soot level (306) of an exhaust particulate filter (72) is lower than a target soot level (307), and an ash level (308) at an exhaust particulate filter (72) is lower than a threshold (309) for the ash level (308), adjusting one or more of a fuel injection timing and a fuel injection pressure to increase the soot output of the engine (10) until the actual soot level (306) is at the target soot level (307), the target soot level (307) being varied based on the engine temperature (304) and the engine load.

Description

Field of InterestThe present invention relates to a method for an engine, the engine exhaust gases of which are treated with an exhaust gas particulate filter.Background / SummaryEngine combustion using gasoline fuel may produce particulate matter (PM) (such as soot and aerosols) that may be released to the atmosphere. To enable emissions requirements to be met, gasoline particulate filters (BPFs) may be included in the engine exhaust system to filter out exhaust PM before the exhaust is released to the atmosphere. For higher emission quality, a higher efficiency BPF comprising a denser filter mesh may be used, particularly in cold start conditions.To maintain the efficiency of the exhaust system particulate filter, the filter may need to be regenerated intermittently. In an example approach, shown by Neely et al. in U.S. Pat. No. 9,027,333 B2, a diesel particulate filter (DPF) is regenerated in response to a soot level exceeding the threshold. In particular, regeneration is controlled so that a low level of soot is retained on the filter to provide optimal efficiency for hydrocarbon conversion during conditions such as an upcoming cold start. In other approaches, all soot accumulated on the filter may be removed during the regeneration process.Further procedures with various possible interventions for controlling the particle formation during combustion and thus for charging the particles of the exhaust gas particle filter are known from the documents DE 10 2015 121 003 A1, DE 10 2013 210 896 A1, DE 10 2014 205 507 A1 and US 2012 / 0 053 814 A1.However, the inventors herein have recognized potential issues with such approaches. As an example, because filters tend to be least efficient when free of soot or ash, filters with higher filtering characteristics (e.g., a denser mesh rate) are typically implemented in engine systems. When the filters are clean, the pores in the substrate may be fully open, thus the particles may more easily pass through them and the likelihood of collisions and adhesion may be reduced, thereby negatively affecting the soot detection rate. However, using filters with higher filtering capacity may result in increased exhaust backpressure, which may negatively affect engine performance and increase fuel consumption. Such filters can also cause considerable additional costs. Another problem is that the optimal level of residual soot level at the filter may vary with operating conditions. For example, the residual soot level corresponding to optimal emissions control during a cold start may be higher than the residual soot level optimal for engine idle conditions. As a result, the soot level remaining on the filter after regeneration at an engine cold start during a subsequent engine idle condition may result in inefficient exhaust emissions. Still further, the amount of ash that has accumulated on the filter after a regeneration event, as well as the distribution of the ash throughout the filter, may affect the operation of the filter, as well as the resulting exhaust backpressure. For example, even though the residual soot level is lower, if a significant amount of ash is left in the filter from the previous regeneration event, the total load on the filter may be higher than the optimal soot level desired for improved emission quality.In order to alleviate the problems described, the present invention proposes a method for an engine according to claim 1 and a method for an engine according to claim 11. Preferred embodiments of the invention are the subject of the dependent claims.In one example, the issues described above may be addressed by a method comprising, responsive to the actual soot level at an exhaust particulate filter being less than a target soot level, adjusting one or more of a fuel injection timing and a fuel injection pressure to increase soot output of the engine until the actual soot level is at the target soot level, wherein the target soot level varies based on the engine temperature and the engine load. In this way, a BPF with lower filtering capability may be used to obtain lower backpressure by actively maintaining a level of residual soot on the filter.As one example, a gasoline particulate filter (BPF) with a lower filtering characteristic (such as a lower density filter mesh) may be coupled to an engine exhaust system. An optimal residual soot level (target level) to be maintained at the BPF may be determined by an engine controller based on engine operating conditions including engine temperature, engine speed, engine load, fueling schedule, etc. A soot level at the BPF may be estimated based on contributions from one or more pressure sensors coupled upstream and / or downstream of the BPF. If it is determined that the soot level at the BPF is less than the target level for the current engine operating conditions, one or more engine actuators may be adjusted to actively accumulate soot at the BPF. As one example, a beginning of fuel injection timing may be advanced and / or a fuel rail pressure may be reduced to increase soot level in exhaust flow based on the actual soot level relative to the target soot level. Also, an ash level at the BPF generated at previous regeneration events may be considered. For example, soot accumulation may be increased until a particular combined soot and ash level at the filter is at the target level. If it is determined that the current soot level at the BPF is higher than the target level for the current engine operating conditions, the BPF may be regenerated to remove the excess soot. A regeneration rate may be limited to reduce the soot level at the filter to the target level and not lower. Also, if the regeneration rate is higher than a target rate, spark timing may be retarded to increase soot production so that the soot level at the filter balances the target level at the end of regeneration and does not fall below the target level.In this way, by supporting the soot and ash level at a filter to increase the capture rate of particulate matter (PM) at an PM filter of an exhaust system, the dependence on expensive filters having a higher mesh density is reduced. By using filters with a lower mesh density, the exhaust back pressure can be reduced. As such, reducing back pressure increases engine performance and fuel efficiency. The technical effect of maintaining a residual soot level (target level) at the BPF is that the operating efficiency of the BPF can be improved. By actively adjusting the target level based on current engine operating conditions, the performance of the exhaust emission system may be optimized at all operating conditions including engine cold starts. Overall, by using a BPF with lower filtering capability and maintaining a residual soot level at the BPF, engine efficiency, emission quality, and fuel efficiency in a gasoline engine system may be improved. In addition, when a more favorable filter is used, exhaust gas soot control and exhaust gas back pressure control can be achieved.It should be understood that the foregoing summary is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not intended to identify key or key features of the claimed subject matter, the scope of which is defined uniquely in the claims that follow the detailed description. Moreover, the claimed subject matter is not limited to implementations that address the shortcomings noted above or in any part of this disclosure.Brief Description of the DrawingsFIG. 1 shows an example engine system including a gasoline particulate filter (BPF). FIG. 2 shows a flow chart illustrating a method that may be applied to maintain a desired soot and ash level at the BPF based on current engine operating conditions. FIG. 3 shows an example of adjustments to engine operating parameters to maintain the desired soot and ash level, in accordance with the present disclosure.DETAILED DESCRIPTIONThe following description relates to systems and methods for maintaining a target soot level at a gasoline particulate filter (BPF) based on current engine operating conditions to improve emissions quality. A gasoline particulate filter with lower filtering capability may be used in an engine system, as shown in FIG. 1. An engine controller may be configured to perform a control routine, such as an example routine of FIG. 2, to adjust one or more engine operating parameters to maintain a desired soot and ash level at the BPF determined by the controller. An example of such adjustments to maintain the desired soot and ash level is shown in FIG. 3.FIG. 1 illustrates an example embodiment of a combustion chamber or cylinder of the internal combustion engine 10. The engine 10 may be controlled at least in part by a control system including the controller 12 and by input from a vehicle operator 130 via an input device 132. In this example, the input device 132 includes an accelerator pedal and a pedal position sensor 134 for generating a proportional pedal position signal PP. Cylinder (i.e., combustion chamber) 14 of engine 10 may include combustion chamber walls 136 in which a piston 138 is positioned. The piston 138 may be coupled to a crankshaft 140 such that reciprocating motion of the piston is translated into rotational motion of the crankshaft. The crankshaft 140 may be coupled to at least one drive wheel of the passenger vehicle via a transmission system. Further, a cranking engine may be coupled to crankshaft 140 via a flywheel to enable a cranking operation of engine 10.The cylinder 14 may receive intake air via a series of intake air passages 142, 144, and 146. The intake air passage 146 may communicate with other cylinders of the engine 10 in addition to the cylinder 14. The intake passage 144 may include a throttle 162 with a throttle 164. In this particular example, the position of throttle 164 may be varied by controller 12 via a signal provided to an electric motor or actuator that includes throttle 162, a configuration commonly referred to as an electronic throttle controller (ETC). In this way, throttle 162 may be operated to vary the intake air provided to the combustion chamber among other cylinders of the engine. The position of throttle plate 64 may be provided to controller 12 by a throttle position signal TP. The intake air passage 142 may include the intake air temperature (IAT) sensor and the atmospheric pressure (BP) sensor. The IAT sensor estimates the intake air temperature for use during engine operation and provides a signal to the controller 12. Similarly, the BP sensor estimates the ambient pressure for engine operation and provides a signal to controller 12. Intake passage 142 may further include a mass airflow sensor and a manifold air pressure sensor 122 for providing the corresponding MAF and MAP signals to controller 12.Exhaust gas sensor 126 is shown coupled to exhaust passage 148 upstream of emission control device 70. The sensor 126 may be any sensor for providing an indication of an air-fuel ratio (AFR) of the exhaust gas, such as a linear oxygen sensor or UEGO (universal exhaust gas oxygen) sensor, a two-state oxygen sensor or EGO sensor, a HEGO (heated EGO) sensor, a NOx, HC, or CO sensor. An oxygen sensor may be used to estimate AFR for intake and exhaust gases. Based on the AFR estimate, engine operating parameters, e.g., fueling, may be regulated.Each cylinder of engine 10 may include one or more intake valves and one or more exhaust valves. For example, cylinder 14 is shown including at least one intake poppet valve 150 and at least one exhaust poppet valve 156, located in an upper portion of cylinder 14. In some embodiments, each cylinder of engine 10, including cylinder 14, may include at least two intake poppet valves and at least two exhaust poppet valves located in an upper portion of the cylinder.Intake valve 150 may be controlled by controller 12 via actuator 152. Similarly, exhaust valve 156 may be controlled by controller 12 via actuator 154. Under some conditions, the controller 12 may vary the signals provided to the actuators 152 and 154 to control the opening and closing of the corresponding intake and exhaust valves. The position of intake valve 150 and exhaust valve 156 may be determined by corresponding valve position sensors (not shown). The valve actuators may be of the electric valve actuation type or the cam actuation type, or a combination thereof. Intake and exhaust valve timing may be controlled simultaneously, or any of intake variable cam timing, exhaust variable cam timing, dual independent variable cam timing, or fixed cam timing may be used. Each cam actuation system may include one or more cams and utilize one or more of cam profile switching (CPS), variable cam timing (VCT), variable valve timing (VVT), and / or variable valve lift systems (VVL) that may be operated by controller 12 to vary valve operation. For example, cylinder 14 may alternatively include an electronic valve commanded intake valve and a cam commanded exhaust valve including CPS and / or VCT. In other embodiments, the intake and exhaust valves may be controlled by a common valve actuator or actuation system or actuator or actuation system for variable valve actuation.In some embodiments, each cylinder of engine 10 may include a spark plug 192 to initiate combustion. Ignition system 190 may provide spark to combustion chamber 14 via spark plug 192 in response to a pre-ignition signal SA from controller 12 under selected operating modes. However, in some embodiments, spark plug 192 may be omitted, such as when engine 10 may initiate combustion by auto-ignition or by injecting fuel, which may be the case with some diesel engines.In some embodiments, each cylinder of engine 10 may be configured with one or more fuel injectors to provide fuel thereto. As a non-limiting example, cylinder 14 is shown to include two injectors 166 and 170. Injector 166 is shown coupled directly to cylinder 14 for injecting fuel directly therein in proportion to the pulse width of a signal FPW- 1 received from controller 12 via an electronic driver 168. In this way, injector 166 provides what is known as direct injection of fuel into combustion chamber 14. Although FIG. 1 shows injector 166 as a side injector, it may also be disposed over the piston, such as near the position of spark plug 192. Such a position may improve mixing and combustion when the engine is operated with an alcohol-based fuel, as some alcohol-based fuels have lower volatility. Alternatively, the injector may be positioned above and near the intake valve to improve mixing. Fuel may be delivered to fuel injector 166 via high pressure fuel system 172 including a fuel tank, fuel pumps, a fuel rail, and driver 168. Alternatively, fuel may be supplied at a lower pressure by a single stage fuel pump, here the timing of direct fuel injection during the compression stroke may be more limited than when using a high pressure fuel system. Further, although not shown, the fuel tank may include a pressure transducer that provides a signal to the controller 12.Fuel injector 170 is shown disposed in intake passage 146, rather than in cylinder 14, in a configuration that provides so-called port injection of fuel into the intake passage downstream of cylinder 14. The injector 170 may inject fuel in proportion to the pulse width of a signal FPW- 2 received from the controller 12 via the electronic driver 171. Fuel may be provided to fuel injector 170 through fuel system 172.Fuel may be delivered to the cylinder by both injectors during a single stroke of the cylinder. For example, each injector may provide a portion of the total fuel injection that is burned in cylinder 14. Further, the distribution and / or relative amount of fuel provided by each injector may vary with operating conditions such as engine load and / or knock, as described herein below.Fuel injectors 166 and 170 may have different characteristics. This includes differences in size, for example, one injector may have a larger injection opening than the other. Other differences include, but are not limited to, different injection angles, different operating temperatures, different goals, different injection timings, different injection characteristics, different positions, etc. Moreover, different effects may be achieved depending on the distribution ratio of the injected fuel among the injectors 170 and 166.The fuel tank in the fuel system 172 may contain fuel having different properties, for example, having different fuel compositions. These differences may include various alcohol levels, various octane numbers, various vaporization heats, various fuel mixtures, and / or combinations thereof, etc.Emission control device 70 is shown disposed downstream of exhaust gas sensor 126 along exhaust passage 148. Device 70 may be a three-way catalyst (TWC), NOx trap, various other emission control devices, and combinations thereof. A gasoline particulate filter (BPF) 72 may be coupled to exhaust passage 148 downstream of emission control device 70. A first pressure sensor 76 may be coupled to the exhaust passage upstream of the BPF 72 and a second pressure sensor 78 may be coupled to the exhaust passage 148 downstream of the BPF 72. Also, a first temperature sensor 77 may be coupled to the exhaust passage upstream of the BPF 72, and a second temperature sensor 79 may be coupled to the exhaust passage 148 downstream of the BPF 72.The BPF 72 may include an outer cover enclosing therein a monolithic (honeycomb) structure. The monolithic structure may consist of individual cells with alternating orientations such that particulate matter (soot) present in the exhaust stream may be trapped in certain specific (first set) cells acting as inlet passages, while the exhaust gas may pass freely through a second set of cells acting as outlet passages. During combustion, soot may be generated in engine cylinders and the level of soot generation may increase during incomplete combustion events. The soot level at the BPF may be removed by regenerating the BPF to a desired level, and higher temperature of a circuit may be used to combust the accumulated soot level. During regeneration of the BPF while the soot burns, ash may be generated that accumulates in the BPF.Due to the dense monolithic structure of the BPF, a back pressure may be generated in the exhaust system, which may negatively affect engine performance and fuel economy. To reduce backpressure, the BPF may be configured with a lower filter parameter. For example, the BPF may have lower filtering capability, with fewer cells comprising the monolithic structure. Also, by using a BPF with lower filtering capability, component cost can be reduced. To maintain a soot detection rate at the BPF above a threshold detection rate so that emission quality may not degrade, a target soot level at the BPF may need to be maintained. The target load may be selected based on one or more engine operating conditions such as each of an engine temperature, an engine speed, an engine load, and a fueling schedule. For example, the target load may be stored in the engine controller memory in a look-up table as a function of engine load, engine speed, and engine temperature. The target load may be increased with a decrease in engine temperature, an increase in engine speed, and an increase in engine load. Also, an ash level at the BPF may be estimated in the controller based on filter regeneration parameters, and the target soot level may be further based on the estimated ash level to maintain an aggregated ash and soot level of the BPF within a threshold level. In one example, the ash level may be estimated based on each of the measured and / or estimated soot load on the BPF at the time of filter regeneration, exhaust temperature, and exhaust flow rate through the filter during filter regeneration, and a duration of filter regeneration. In this example, the determined amount of ash is an amount other than the determined amount of soot.A current soot level at the BPF may be estimated based on contributions from one or more of the pressure sensors 76, 78 and temperature sensors 77 and 79 coupled to the exhaust passage upstream and downstream of the BPF. For example, the soot load of the BPF may be determined as a function of the pressure difference and / or temperature difference by the BPF, where the estimated soot load increases as the pressure difference increases. If it is determined that the soot level at the BPF is less than the target level for the current engine operating conditions, one or more engine actuators may be adjusted to actively increase exhaust soot generation and soot accumulation at the BPF to the target level. In one example, adjusting one or more engine actuators includes adjusting one or more of a fuel injection timing and a fuel rail pressure to actively raise the actual soot level to the target soot level. As one example, a beginning of fuel injection timing may be advanced and / or a fuel rail pressure may be reduced to cause full combustion at the cylinders, thereby generating a higher soot level. A degree of advancing the beginning of the injection timing and a degree of decreasing the fuel rail pressure may be increased as the difference between the target soot level and the actual soot level increases. Also, in response to the actual soot level at the BPF being higher than the target soot level, filter regeneration may be initiated and filter regeneration may be stopped when the actual soot level is reduced to the target soot level. Also, if the regeneration rate is higher than a threshold rate, spark timing may be retarded to increase soot production so that after the regeneration process, the BPF soot level does not drop below the target level. A detailed description of a method for maintaining a desired level of soot at the BPF is discussed with reference to FIG. 2.Controller 12 is shown in FIG. 1 as a microcomputer, including microprocessor unit 106, input / output ports 108, an electronic storage medium for executable programs and calibration values, shown as read only memory 110 in this particular example, random access memory 112, keep alive memory 114, and a data bus. Controller 12 may receive various signals from sensors coupled to engine 10, in addition to those signals previously discussed, including measurement of BPF soot level from exhaust system pressure and temperature sensors 76, 77, 78, and 79, inducted mass air flow (MAF) from mass air flow sensor 122, engine coolant temperature (ECT) from temperature sensor 116 coupled to cooling sleeve 118, a profile ignition pickup signal (PIP) from Hall effect sensor 120 (or other type) coupled to crankshaft 140, a throttle position (TP) from a throttle position sensor, and an intake manifold absolute pressure signal (MAP) from sensor 124. Engine speed signal RPM may be generated by controller 12 from signal PIP. The manifold pressure signal (MAP) from the manifold pressure sensor may be used to provide an indication of vacuum or pressure in the intake manifold.The controller 12 receives signals from the various sensors of FIG. 1 and employs the various actuators of FIG. 1 to adjust engine operation based on the received signals and instructions stored on a memory of the controller 12. In one example, controller 12 may estimate a soot level at the BPF based on signals received from one or more exhaust passage pressure and temperature sensors 76, 77, 78, and 79, and based on a difference between the estimated BPF soot level and a target soot level, the controller may send a signal to one or more engine actuators to maintain the BPF soot level at the target level. As one example, if the estimated BPF soot level is less than the target level, the controller 12 may send a pulse width signal to an electronic driver coupled to the fuel injector to prefer the beginning of the injection timing. If the estimated BPF soot level is higher than the target level, the controller 12 may send a signal to an electrical switch of a circuit coupled to the BPF to close the circuit and initiate a BPF regeneration process to lower the BPF soot level to the target level. Also, during the regeneration process, the controller 12 may send a signal to the spark plug 192 to retard spark timing.In this way, the system of FIG. 1 provides for an engine system, comprising: an engine including a cylinder, an exhaust passage receiving combustion gases from the cylinder, a gasoline particulate filter (BPF) having a filter mesh density less than threshold coupled to the exhaust passage, one or more pressure sensors coupled to the BPF, one or more temperature sensors coupled to the BPF, a fuel delivery system including a fuel rail, a fuel tank, a fuel pump, and a fuel injector for delivering fuel to the engine cylinder, and a controller with computer readable instructions stored on non-transitory memory to: infer an actual soot load of the BPF based on contributions from the one or more pressure sensors and temperature sensors, and, when the actual soot load falls below the target load, increase soot output by the engine by advancing a time of fuel injector actuation or reducing a pressure of the fuel rail until the actual soot load is at the target load.FIG. 2 illustrates an example method 200 that may be implemented to maintain a desired soot and ash level at a gasoline particulate filter (BPF) (such as BPF 72 in FIG. 1 ) based on engine operating conditions. Instructions for carrying out method 200 and the remaining methods included herein may be executed by a controller based on instructions stored on a memory of the controller and in conjunction with signals received from sensors of the engine system, such as the sensors described above with reference to FIG. 1. The controller may employ engine actuators of the engine system to adjust engine operation according to the methods described below.At 202, the routine includes estimating and / or measuring current engine operating conditions. Conditions being judged may include, for example, driver torque demand, engine temperature, engine load, engine speed, throttle position, exhaust pressure, exhaust air to fuel ratio, etc.Based on the current engine operating conditions, an optimal soot level at the BPF may be selected and maintained to provide improved BPF performance and emissions quality. By maintaining a target soot level at the BPF, the rate of detection of the soot particles generated by the engine at the BPF may be increased. Specifically, at 204, a target soot level desired at the BPF may be determined based on the current engine operating conditions and the fueling schedule. In one example, the routine determines the target BPF soot level based on one or more of engine temperature, engine load, and engine speed. The controller may determine the target BPF soot level by a determination that directly accounts for an estimated engine temperature, engine load, engine speed, and a current fueling schedule. Engine temperature, engine load, and engine speed may be measured directly via engine sensors such as an exhaust temperature sensor, Hall effect sensor, etc., or determined based on engine operating conditions. The controller may alternatively determine the target BPF soot level based on a calculation using a look-up table, where the contribution is one or more of engine temperature, engine load, and engine speed, and the power is the corresponding target BPF soot level. In one example, the target soot level is set to a first target soot level during engine start conditions when the engine temperature is below a threshold temperature and the engine speed is below a threshold speed, the target soot level is set to a second target soot level during engine idle conditions when the engine temperature is above the threshold temperature and the engine speed is below the threshold speed, and the target soot level is set to a third target soot level when the engine temperature is above the threshold temperature and the engine speed is above the threshold speed. The first, second, and third target soot levels may be different from each other. In one example, the first target soot level may be less than the second target soot level and the second target soot level may be less than the third target soot level. In another example, the second target soot level may be less than the first target soot level and higher than the third soot level. Also, variables including BPF mid-bed temperature (measured and / or modeled), air change temperature, fuel injection schedule (e.g., use of split injection, presence of one or more of intake and compression direct injections) may affect the target soot level. Based on each of the noted variables, there may be a unique soot and / or ash target that would result in an optimal combination of engine soot output and particulate filter efficiency. As one example, a first target soot level may be based on a fuel injection schedule during engine start conditions, a second target soot level may be based on the fuel injection schedule during engine idle conditions, and a third target soot level may be based on the fuel injection schedule when the engine temperature is above the threshold temperature and the engine speed is above the threshold speed, the fuel injection schedule including one or more of a split ratio of port injected fuel relative to direct injected fuel, a split ratio of compression stroke direct injected fuel relative to intake stroke direct injected fuel, and a number of compression stroke direct injections per fueling event.At 206, a current soot level at the BPF may be estimated based on one or more exhaust passage temperature and pressure sensors (such as sensors 76, 77, 78, and 79 in FIG. 1 ) coupled upstream and / or downstream of the BPF. For example, the pressure difference and / or temperature difference may be estimated by the BPF (based on contributions of the pressure and temperature sensors) and the soot load of the BPF may be determined as a function of the pressure difference and / or temperature difference. As the soot load on the BPF increases, there may be a corresponding increase in pressure and temperature difference by the BPF.At 208, the current (estimated) BPF soot level may be compared to the target soot level, and a difference between the current BPF soot level and the target soot level may be determined. At 210, the routine includes determining if the current BPF soot level is equal to the target soot level. Alternatively, it may be determined whether the difference between the current BPF soot level and the target soot level is less than a threshold.If it is determined that the current BPF soot level is equal to the target soot level or that the difference is less than the threshold, it may be inferred that no further change in the BPF soot level is required to improve emission quality. Thus, at 212, current engine operation may continue with existing engine operating parameters. This includes continuing to accumulate exhaust soot at the BPF as engine operation changes, and initiating filter regeneration when the soot level at the BPF is higher than a regeneration threshold level. During this time, exhaust soot is generated during engine operation, but exhaust soot generation is not actively increased.If it is determined that the BPF soot level is not equal to the target soot level or that the difference (between the current BPF soot level and the target soot level) is higher than the threshold, it may be inferred that the BPF operation may be affected. At 214, the routine may include determining if the current BPF soot level is higher than the target soot level. If it is determined that the current BPF soot level is higher than the target soot level, one or more motor actuators, such as an electrical switch on a circuit coupled to the BPF, may be actuated to reduce the current BPF soot load. Also, a plurality of engine operating parameters may be adjusted to increase the exhaust temperature, which may be used as a heat source to start the regeneration process.To reduce the BPF soot level to the target level, at 216, the controller may initiate the BPF regeneration. To start BPF regeneration, the controller may send a signal to close the switch on a circuit coupled to the BPF. By closing the switch, electrical current can flow through the circuit and through the BPF. The electric current may increase the temperature at the BPF, which may facilitate burning of the soot deposited at the BPF. As regeneration progresses, the soot level at the BPF may steadily reduce. The duration of the regeneration, the temperature (directly proportional to the electric current flowing through the circuit) of the BPF, and the oxygen supply to the BPF may regulate the regeneration rate of the BPF (degree of decrease in soot level). In one example, the regeneration rate (rate of decrease of soot level) may increase with one or more of an increase in duration of regeneration, an increase in BPF temperature (magnitude of electric current flowing through the circuit and / or degree of change in engine operating parameters), and an increase in oxygen supply to the BPF. Therefore, one or more of the regeneration duration, the BPF temperature, and the BPF oxygen supply may be increased accordingly to increase the regeneration rate of the BPF. As the soot is burned at the BPF, ash may be generated, which may deposit in the BPF.Once BPF regeneration is initiated by actuation of the switch, the temperature of the BPF may increase to a higher than a desired level, causing an increased rate of soot regeneration. If the regeneration rate increases beyond a desired level, a greater than expected soot load may be burned and the BPF soot level may thus fall below the target level. Therefore, during BPF regeneration at 217, a rate of soot generation (at the engine) may be opportunistically increased by retarding spark timing to maintain (and no longer reduce) the BPF soot level at the target level. The controller may send a signal to an actuator coupled to the spark plug to retard spark timing. In one example, a rate of BPF regeneration may be inferred based on the BPF temperature and / or regeneration duration, and in response to the rate of filter regeneration being higher than a threshold rate (with an increased risk of the BPF soot level falling below the target level), spark timing may be retarded to increase soot generation at the engine while continuing to regenerate the filter. The amount of spark retard applied may be increased when the rate of filter regeneration exceeds the threshold rate.At 218, the routine includes determining if the current BPF soot level has reached the target soot level due to the regeneration process. If it is determined that the BPF soot level has not reached the target level and is still higher than the target level, then at 220, BPF regeneration may continue. If it is determined that the current BPF soot level has reached the target level, the regeneration process may be ended at 222. To end the regeneration, the controller may send a signal to actuate the switch to an open position to suspend flow of the electric current through the BPF.If it is determined at 214 that the BPF soot level is not greater than or equal to the target soot level, it may be inferred at 224 that the current BPF soot level is less than the target soot level. For optimal performance of the BPF, the soot level at the BPF may be actively increased to the target level. Since BPF operation may also depend on the BPF ash level, at 226, an ash level at the BPF may be estimated based on the durations and regeneration temperatures used for previous (one or more) regeneration events at the BPF during which ash may have been produced by burning the soot. The duration and temperature of a regeneration event may be based on a desired regeneration rate such that an amount of soot may be burned that allows the BPF soot level to be reduced to the target level. As the duration of each regeneration event increases and as the exhaust temperature increases at the time of regeneration, the amount of soot burned increases and also the amount of ash that accumulates on the BPF correspondingly increases. Also, BPF levels may be accounted for prior to initiation of the regeneration event to determine the amount of soot burned and the amount of ash produced during each regeneration event. For example, soot load decreases and ash load on the BPF increases as the amount of soot burned during regeneration increases. Additionally or alternatively, a combination of ash and soot load on the BPF may be estimated based on contributions from one or more pressure and temperature sensors coupled to the exhaust passage upstream and / or downstream of the BPF. A one-time ash model may be used to estimate the ash load on the BPF. After multiple regenerations over an extended period of time, a significant amount of ash may be deposited on the BPF, which may have a significant impact on BPF operation. Also, due to natural oil consumption during various engine operating conditions, a significant amount of ash may be deposited on the BPF. Therefore, by considering the ash load, the exhaust back pressure in the engine can be more controlled.At 228, the routine includes determining if the ash level at the BPF is higher than a threshold ash level. If it is determined that the ash level at the BPF is higher than the threshold level, it may be inferred that even if the BPF soot level is lower than the target level, the ash level higher than the threshold may be sufficient to provide the desired BPF functionality. In the presence of an ash level at the BPF that is higher than the threshold, active production of soot may not be desired, as this may produce an increase in undesired exhaust backpressure. Thus, responsive to detecting an ash level at the BPF that is higher than the threshold, current engine operation may continue at 230 without making changes to engine operating parameters.If it is determined that the current ash level is less than the threshold level, at 232, the BPF soot level may be actively increased by adjusting one or more engine operating parameters to increase exhaust soot production. In one example, at 234, the start time of fuel injection may be advanced, which may result in incomplete combustion of gasoline, resulting in higher soot production. The controller may send a pulse width signal to an electronic driver coupled to the fuel injector to prefer the beginning of the injection timing. Advancing the fuel injection timing may include advancing one or more of a beginning of the injection timing, an end of the injection timing, and an average injection time. Also, at 236, the fuel rail pressure may be reduced to change the fueling schedule, which may result in incomplete combustion and increased soot production. In one example, the controller may send a signal to the fuel pump to vary pump power to reduce fuel rail pressure. The degree of advancing the beginning of the injection timing and / or the amount of fuel rail pressure reduction may be based on the difference between the current BPF soot level and the target soot level, and may be continuously changed as the difference changes. In one example, the amount of advancement of the start of injection timing and / or the amount of reduction in fuel rail pressure may be increased with an increase in the difference between the actual BPF soot level and the target soot level. Similarly, the degree of advancing the beginning of injection timing and / or the amount of fuel rail pressure reduction may be reduced with a reduction in the difference between the actual BPF soot level and the target soot level. In this way, one or more engine operating parameters may be adjusted until the actual BPF level reaches the target soot level.Once the actual BPF soot level reaches the target soot level, at 238, for optimal performance of the BPF, the combined BPF soot level and the ash level may be maintained at the target soot level. The target level may change based on engine operating conditions and soot generation and / or BPF regeneration may be actively adjusted to maintain the current soot level at the corresponding target level.In alternative examples, instead of individually evaluating the soot load and the ash load, an aggregated soot and ash load may be observed at the BPF and maintained at a threshold level for optimal operation of the BPF. The ash load on the BPF may be estimated based on factors such as durations and regeneration temperatures used for previous BPF regeneration events and BPF soot levels prior to initiation of the regeneration event. The amount of soot burned and the amount of ash produced can be inferred from the above factors. The soot load may be inferred based on a pressure differential through the particulate filter. In response to an aggregated ash and soot load of an exhaust particulate filter being less than a threshold load, engine soot output may be increased until the aggregated load is at the threshold load, and in response to the aggregated load being higher than the threshold load, the filter may be regenerated while spark timing is retarded until the aggregated load is at the threshold load. The spark timing may be retarded during the regeneration process to increase soot production at the engine such that a higher than desired soot level is not burned and a higher level of ash is not produced during an increased regeneration rate. Thus, regenerating the filter while retarding spark timing includes retarding spark timing by an amount proportional to a regeneration rate of the filter while continuing to regenerate the filter. A rate of soot generation (by retarding spark timing) and BPF regeneration (causing a decrease in soot level and an increase in ash level) may be simultaneously adjusted to maintain a combined soot and ash target level at the BPF. For a given engine operating condition, the target level for the combined soot and ash level may be different from the target soot level.FIG. 3 shows an example operating sequence 300 illustrating adjusting engine operating conditions for maintaining a target soot level at a gasoline particulate filter (BPF). The horizontal (x-axis) denotes time and the vertical markings t1-t9 denote important points in time during the operation of the BPF.The first plot, line 302, shows variation over time in engine speed. Dashed line 303 shows a threshold engine speed above which the target soot level at the BPF may change. In one example, dashed line 303 corresponds to an engine speed at idle. The second plot, line 304, shows the change in engine temperature over time, and dashed line 305 indicates a threshold temperature below which the engine may be considered cold, such as during cold start conditions. Once the engine temperature increases above threshold 305, the engine may be considered warm enough for exhaust catalyst activation (light off). The third plot, line 306, shows a current soot level at the BPF estimated based on one or more pressure and temperature sensors coupled to the exhaust passage upstream and / or downstream of the BPF. Dashed line 307 shows a target soot level at the BPF as determined based on current engine operating conditions, including engine temperature and engine speed. The fourth plot, line 308, shows an ash level at the BPF. Ash is produced at the BPF during BPF regeneration processes when soot is burned at higher temperature. Dashed line 309 shows a threshold ash level above which the ash load may affect the target soot level at the BPF. The fifth plot, line 310, shows BPF regeneration to reduce the current BPF soot level to the target soot level. The sixth plot, line 312, shows advancing the injection timing to actively increase soot production to increase the current soot level to the target soot level.Prior to time t 1, the engine starts from a rest state after a period of inactivity in which the vehicle was not driven. Due to a lower engine temperature (below threshold temperature 305) at the time of engine start, the engine may undergo a cold start. During engine cold start, fuel delivery is resumed and engine speed gradually increases. The engine is operated under lower load conditions. Due to cold start conditions, the exhaust catalyst may not have reached its light-off temperature, and during this time, emission quality may be improved by maintaining the soot level at the GPF at a target level. The target level of soot at the BPF may be determined based on conditions such as engine temperature, engine speed, and engine load. During cold start conditions, the exhaust soot level may be higher and the current BPF soot level may be observed to be substantially higher than the target soot level.Therefore, at time t 1, to reduce the current soot level to the target soot level for improved emissions quality, BPF regeneration may be initiated by adjusting a plurality of engine operating parameters to increase exhaust temperature and / or by actuating a switch on a circuit coupled to the BPF. In one example, adjusting the engine operating parameters includes operating the engine richer than stoichiometric over a duration to increase exhaust temperature. As another example, the engine may be operated with a retarded spark timing for a duration. By closing the switch, electrical current may flow through the circuit and through the BPF, which may increase the temperature at the BPF and facilitate burning of the soot deposited at the BPF. As regeneration progresses, the soot level at the BPF may steadily reduce. The duration of the regeneration, the temperature (directly proportional to the electric current flowing through the circuit) of the BPF, and the air supply to the BPF may be based on the difference between the actual BPF soot level and the target soot level, such that an optimal amount of soot may be burned during the regeneration process. As the soot is burned at the BPF, ash may be generated, which may result in an increase in the ash level at the BPF. An actual ash level of the BPF may be inferred based at least on a duration of the operation of the switch. Between time t 1 and t 2, the regeneration process may continue and thus a steady decrease in BPF level may be observed.At time t 2, it may be observed that the engine temperature has increased above the threshold temperature and it may be determined that the exhaust catalyst is fully operational. Also, at this time, the engine speed may be lower than the idle speed. In response to the increase in engine temperature (at a lower than idle engine speed), the target BPF level may be reduced to a level suitable for optimal performance of the emission control system. Also, at this time, it may be observed that the current BPF soot level is equal to the target BPF soot level. Therefore, no further BPF regeneration may be desired. The regeneration process may be suspended by re-adjusting engine operating parameters (e.g., to continue stoichiometric engine combustion or to continue spark timing around or around MBT) and / or by opening the switch on the circuit coupled to the BPF to suspend the flow of electricity through the BFP.Between time t 2 and t 3, it may be observed that the current BPF soot level is equal to the target BPF level, and therefore no further change in the current BPF soot level may be desired. At time t 3, the engine speed may increase above the threshold speed and accordingly the target BPF level may also be increased. However, between time t 3 and t 4, the current BPF level may be observed to be substantially lower than the target level, which may negatively impact BPF operation.Therefore, at time t 4, active generation of soot may be initiated to increase the current BPF soot level to the target level. To increase the soot level at the BPF, soot production at engine exhaust may be increased by advancing a beginning of fuel injection timing, which may result in multiple incomplete combustion events. The degree of advancement of the injection timing may be based on the difference between the current BPF soot level and the target soot level, wherein the degree of advancement increases as the difference increases.Between time t 4 and t 5, the current BPF soot level may steadily increase due to advancing fuel injection timing and the resulting increase in soot production. At time t 5, it may be inferred that the current soot level is at the target BPF level corresponding to the current engine operating conditions. Therefore, the start of the injection timing at this time no longer needs to be advanced, and a normal fuel supply schedule can be continued.Also, at time t 5, engine speed may reduce (e.g., below an idle speed) and the target BPF level may be correspondingly reduced. Between time t 5 and t 6, the current BPF level may be higher than the target level, which may result in reduced performance of the exhaust system. Therefore, at time t 6, BPF regeneration may be initiated by closing the switch to flow electric current through the circuit and through the BPF, which may increase BPF temperature. Between time t 6 and t 7, the regeneration process may continue and thus a steady decrease in BPF level may be observed.At time t 6, the difference between the target soot level and the current BPF soot level may be less than the difference between the target soot level and the current BPF soot level upon onset of the previous regeneration event (at time t 1); therefore, it may be desired to combust a lesser amount of soot at the BPF to reach the target level compared to the amount of soot burned during the previous regeneration event. Once BPF regeneration is initiated by actuation of the switch, the temperature of the BPF may increase to a higher than a desired level, thereby causing an increased regeneration rate, which may result in removal of a greater than expected soot load. To maintain the soot level at the target level (and not reduce to below the target level), a rate of soot generation (at the engine) may be increased by retarding spark timing. In other words, a regeneration rate may be limited by retarding spark timing to increase engine soot output until the actual soot level is at the target level. As such, soot generation during the previous BPF regeneration event (between time t 1 and t 2) may not have been performed by retarding spark because a higher amount of soot (greater difference between the target soot level and the current BPF soot level) should be removed from the BPF. Also, during this time, as the soot is burned on the BPF, ash may be generated, which may result in an increase in the ash level on the BPF.At time t 7, it may be observed that the current soot level at the BPF has reduced to the target soot level and thus the regeneration process may be ended. Also, at this time, it may be observed that the ash level at the BPF has increased to above the threshold level. A higher than threshold BPF ash level may significantly impact BPF operation in trapping exhaust soot.Between time t 7 and t 8, the engine may operate at a higher than the threshold engine speed and the target soot level at the BPF may also increase. During this time, there may be a difference between the current BPF soot level and the target soot level. However, due to the ash level of the BPF being higher than the threshold, an accelerated increase in soot level may not be desired by active adjustments to one or more engine operating parameters. The combined ash level and soot level may be sufficient for optimal operation of the emission control system.At time t 8, the engine may be turned off and the vehicle may not be operated. The vehicle and engine may continue to be inactive for a substantial period of time, between time t 8 and t 9. At time t 9, the engine may start from rest under cold start conditions with less than the threshold engine temperature. Also, engine speed may be below the threshold speed. Based on engine operating conditions, the target soot level may be determined and it may be observed that due to a previous controlled regeneration, the current soot level at the BPF is equal to the target soot level. Therefore, no further adjustments to the BPF soot and ash levels (regeneration or active soot generation) may be desired at this time. In this way, by maintaining a target soot level at the BPF even during cold start conditions when the catalyst may not be fully functional, the rate of detection of soot at the BPF may be maintained at an optimal level.In this way, by maintaining a target soot level at a gasoline particulate filter (GPF) by actively adjusting soot production and BPF regeneration, the emission control system may operate with a higher accuracy even under conditions such as cold start. By determining the target soot level based on current engine operating conditions, BPF performance may be maintained at different engine operating conditions. The technical effect of using a lower filtering capacity with a lower density filter mesh is that backpressure in the exhaust system may be reduced and, consequently, engine performance and fuel efficiency may be increased. Also, by using a filter grating with lower filter density, component cost for the BPF can be reduced.An example method for an engine includes, responsive to the actual soot level at an exhaust particulate filter being less than a target soot level, adjusting one or more of a fuel injection timing and a fuel injection pressure to increase the soot output of the engine until the actual soot level is at the target soot level, the target soot level varying based on the engine temperature and the engine load. In the preceding example, additionally or optionally, the target soot level is set to a first target soot level during engine start conditions when the engine temperature is below a threshold temperature and the engine speed is below a threshold speed, the target soot level is set to a second target soot level during engine idle conditions when the engine temperature is above the threshold temperature and the engine speed is below the threshold speed, and the target soot level is set to a third target soot level when the engine temperature is above the threshold temperature and the engine speed is above the threshold speed. In any or all of the preceding examples, additionally or optionally, the first target soot level is based on a fuel injection schedule during engine start conditions, the second target soot level is based on the fuel injection schedule during engine idle conditions, and the third target soot level is based on the fuel injection schedule when the engine temperature is above the threshold temperature and the engine speed is above the threshold speed, the fuel injection schedule including one or more of a split ratio of port injected fuel relative to direct injected fuel, a split ratio of compression stroke direct injected fuel relative to intake stroke direct injected fuel, and a number of compression stroke direct injections per fueling event. In any or all of the preceding examples, additionally or optionally, the target soot level is varied to maintain a detection rate of soot at the filter above a threshold detection rate. Any or all of the preceding examples, further comprising, additionally or optionally, estimating an ash level of the filter based on filter regeneration parameters, and further comprising varying the target soot level based on the estimated ash level to maintain an aggregated ash and soot load of the filter within a threshold load. In any or all of the preceding examples, additionally or optionally, adjusting one or more of the fuel injection timing and the fuel injection pressure includes advancing the fuel injection timing and reducing the fuel rail pressure to actively increase the soot output of the engine. In any or all of the preceding examples, additionally or optionally, advancing the fuel injection timing includes advancing one or more of a beginning of the injection timing, an end of the injection timing, and an average injection time. In any or all of the preceding examples, additionally or optionally, a degree of advancing the beginning of the injection timing and a degree of decreasing fuel rail pressure are increased as a difference between the target soot level and the actual soot level increases. Any or all of the preceding examples, further comprising, additionally or optionally, responsive to the actual soot level at the exhaust particulate filter being higher than the target soot level, initiating filter regeneration, and responsive to a rate of filter regeneration being higher than a threshold rate, retarding spark timing to generate soot at the engine while continuing to regenerate the filter. In any or all of the preceding examples, additionally or optionally, an applied amount of spark retard is increased when the rate of filter regeneration exceeds the threshold rate. In any or all of the preceding examples, additionally or optionally, the engine is fueled and wherein the filter is a gasoline particulate filter.Another example method for an engine includes, responsive to an aggregated ash and soot load of an exhaust particulate filter being less than a threshold load, increasing engine soot output until the aggregated load is at the threshold load, and responsive to the aggregated load being greater than the threshold load, regenerating the filter while retarding spark timing until the aggregated load is at the threshold load. In the preceding example, additionally or optionally, increasing engine soot output includes one or more of advancing a start of injection timing of a cylinder fuel injection and reducing a fuel rail pressure to actively generate soot on the engine and increase soot load on the exhaust particulate filter. In any or all of the preceding examples, additionally or optionally, an amount of advancing the injection timing and an amount of reducing the fuel rail pressure are increased when the aggregated load falls below the threshold load. In any or all of the preceding examples, additionally or optionally, regenerating the filter while retarding spark timing includes retarding spark timing by an amount proportional to a regeneration rate of the filter while continuing to regenerate the filter. In any or all of the preceding examples, additionally or optionally, the ash load is estimated based on a duration of filter regeneration, and wherein the soot load is estimated based on a pressure differential through the particulate filter.In yet another example, an engine system includes an engine including a cylinder, an exhaust passage receiving combustion gases from the cylinder, a gasoline particulate filter (BPF) having a filter mesh density less than threshold coupled to the exhaust passage, one or more pressure sensors coupled to the BPF, one or more temperature sensors coupled to the BPF, a fuel delivery system including a fuel rail, a fuel tank, a fuel pump, and a fuel injector for delivering fuel to the engine cylinders, and a controller with computer readable instructions stored on non-transitory memory to: infer an actual soot load of the BPF based on contributions from the one or more pressure sensors and temperature sensors, and, when the actual soot load falls below the target load, increase soot output by the engine by advancing a time of fuel injector actuation or reducing a pressure of the fuel rail until the actual soot load is at the target load. In the preceding example, additionally or optionally, the controller comprises instructions to: select the target load based on each of an engine temperature, an engine speed, and an engine load, wherein the target load is increased upon a reduction in engine temperature, an increase in engine speed, and an increase in engine load. In any or all of the preceding examples, additionally or optionally, the controller includes further instructions to infer an actual ash load of the BPF based on a duration of actuation of the switch; and subsequent to filter regeneration, maintain an aggregate of the actual soot load and the actual ash load at the target load by advancing a time of actuation of the fuel injector. In any or all of the preceding examples, additionally or optionally, the controller includes further instructions to: when the actual soot load exceeds the target load, regenerate the filter to reduce the soot load at the filter, wherein a rate of regeneration is limited by retarding spark timing to increase engine soot output until the actual soot load is at the target load.It should be noted that the example control and estimation routines included herein may be used with various engine and / or vehicle system configurations. The control methods and routines disclosed herein may be stored as executable instructions in non-transitory memory and executed by the control system including the controller in combination with the various sensors, actuators, and other engine hardware. The specific routines described herein may represent one or more of any number of processing strategies such as event-driven, interrupt-driven, multi-tasking, multi-threading, and the like. Thus, various illustrated acts, acts, and / or functions may be performed in the order illustrated, in parallel, or in some cases omitted. Likewise, the order of processing is not necessarily required to achieve the features and advantages of the example embodiments described herein, but rather is provided for ease of illustration and description. One or more of the illustrated acts, operations, and / or functions may be repeatedly performed depending on the particular strategy being employed. Further, the described acts, operations, and / or functions may graphically represent code to be programmed into non-transitory memory of the computer readable storage medium in the engine control system, in which the described acts are carried out by executing the instructions in a system including the various engine hardware components in combination with the electronic controller.It should be understood that the configurations and routines disclosed herein are exemplary in nature and that these specific embodiments are not to be considered in a limiting sense, as numerous variations are possible. For example, the above technology may be applied to V6, I4, I6, V12, 4-cylinder horizontally opposed, and other engine types. The subject matter of the present disclosure includes all novel and non-obvious combinations and sub-combinations of the various systems and configurations, and other features, functions, and / or characteristics disclosed herein.The following claims particularly set forth certain combinations and sub-combinations which are considered novel and non-obvious. These claims may refer to "a" element or "a first" element or the equivalent thereof. Such claims are to be understood to include inclusion of one or more such elements and neither require nor exclude two or more such elements.

Claims

A method for an engine (10), comprising: responsive to a current soot level (306) of an exhaust particulate filter (72) being less than a target soot level (307) and an ash level (308) at an exhaust particulate filter (72) being less than a threshold (309) for the ash level (308), adjusting one or more of a fuel injection timing and a fuel injection pressure to increase soot output of the engine (10) until the current soot level (306) is at the target soot level (307), wherein the target soot level (307) varies based on the engine temperature (304) and the engine load.The method of claim 1, wherein the target soot level (307) is varied to maintain a trapping rate of soot at the filter above a threshold trapping rate, and the target soot level (307) is set to a first target soot level (307) during engine start conditions when the engine temperature (304) is below a threshold temperature (305) and the engine speed (302) is below a threshold speed (303), the target soot level (307) is set to a second target soot level (307) during engine idle conditions when the engine temperature (304) is above the threshold temperature (305) and the engine speed (302) is below the threshold speed (303), and the target soot level (307) is set to a third target soot level (307), when the engine temperature (304) is above the threshold temperature (305) and the engine speed (302) is above the threshold speed (303), wherein the first target soot level (307), the second target soot level (307), and the third target soot level (307) are different from each other.The method of claim 2, wherein the first target soot level (307) is based on a fuel injection schedule during engine start conditions, the second target soot level (307) is based on a fuel injection schedule during engine idle conditions, and the third target soot level (307) is based on a fuel injection schedule when the engine temperature (304) is above the threshold temperature (305) and the engine speed (302) is above the threshold speed (303), wherein the fuel injection schedules are one or more of a split ratio of fuel injected via a port fuel injector relative to fuel injected via a direct fuel injector, a split ratio of fuel injected via the direct fuel injector during a compression stroke relative to fuel injected via the direct fuel injector during an intake stroke and a number of fuel injections via the direct fuel injector per fueling event during the compression stroke.The method of claim 1, further comprising estimating an ash level of the filter based on a regeneration duration and a regeneration temperature for one or more previous regeneration events of the filter, and further varying the target soot level (307) based on the estimated ash level to maintain an aggregated ash and soot load of the filter within a threshold load.The method of claim 1, wherein adjusting one or more of the fuel injection timing and the fuel injection pressure includes advancing the fuel injection timing (312) and reducing the fuel injection pressure to actively increase the soot output of the engine (10).The method of claim 5, wherein advancing the fuel injection timing (312) includes one or more of a beginning of the injection timing, an end of the injection timing, and an average injection time.The method of claim 6, wherein a degree of advancing the start of the injection timing and a degree of reducing the fuel injection pressure are increased as a difference between the target soot level (307) and the current soot level (306) increases.The method of claim 1, further comprising, in response to the current soot level (306) at the exhaust particulate filter (72) being higher than the target soot level (307), initiating filter regeneration, and in response to a rate of filter regeneration being higher than a threshold rate, retarding spark timing to generate soot at the engine (10) while continuing to regenerate the filter.The method of claim 8, wherein an applied amount of spark retard is increased when the rate of filter regeneration exceeds the threshold rate.The method of claim 1, wherein the engine (10) is supplied with gasoline and wherein the filter is a gasoline particulate filter.A method for an engine (10), comprising: in response to the soot load of an exhaust particulate filter (72) being less than a load threshold (309) and the ash level of the exhaust particulate filter (72) being less than an ash level, increasing the soot output of the engine (10) until the soot load reaches the load threshold (309); and in response to the soot load being greater than the threshold load, independent of the ash level (308) of the exhaust particulate filter (72), regenerating the filter while retarding spark timings until the soot load is at the threshold load.The method of claim 11, wherein increasing the soot output of the engine (10) comprises one or more of advancing the injection timing of a cylinder fuel injection and decreasing the pressure in the fuel line to actively generate soot in the engine (10) and increase the soot load of the exhaust particulate filter (72).The method of claim 12, wherein an amount for advancing the injection timing and an amount for decreasing the fuel rail pressure is increased when the soot load falls below the threshold (309).The method of claim 11, wherein regenerating the filter while retarding spark timing includes increasing the amount of spark retard with an increase in the regeneration rate of the filter while continuing to regenerate the filter.The method of claim 11, wherein the ash load is estimated based on the duration of the filter regeneration and the soot load is estimated based on a pressure differential across the particulate filter.

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