Method for controlling a prime mover

Abstract

Procedure comprising: when driving a hybrid vehicle only via engine torque, rotating an engine without fuel supply at less than a threshold speed until a piston temperature is higher than a threshold, characterized in that during the rotation, intake and / or exhaust valves of one or more engine cylinders are deactivated so that several cylinders can be heated at a given time via compression.

Description

Area

[0001] The present application relates to methods for controlling engine tow in a hybrid vehicle system. Background and summary

[0002] Engines can be configured with direct fuel injectors that inject fuel directly into a combustion cylinder (direct injection) and / or port fuel injectors that inject fuel into a cylinder port (port fuel injection). Direct injection allows for greater fuel efficiency and higher power output, in addition to better enabling the charge cooling effect of the injected fuel.

[0003] From DE 10 2010 034 443 A1 a method according to the preamble of claim 1 is known.

[0004] However, direct-injection engines also produce more particulate emissions (or soot) due to diffuse flame propagation, where the fuel may not be adequately mixed with air prior to combustion. Because direct injection is inherently a relatively late fuel injection, there may not be enough time for the injected fuel to mix with air in the cylinder. Under some operating conditions, the liquid droplets may directly impinge on combustion surfaces, such as the piston, head, and liner. Likewise, the injected fuel does not experience turbulence as it passes through the valves. Consequently, pockets of rich combustion may exist, which can locally generate soot, affecting exhaust emissions. Emissions may be further exacerbated during engine cold-start operation.In particular, soot is generated due to poor fuel vaporization caused by poor fuel injector spray characteristics at low pressure in the fuel rail and / or fuel impinging on the cold metal surfaces of the combustion chamber until the combustion chamber is fully warmed up.

[0005] Engine test data suggests that particulate emissions can be reduced by increasing fuel rail pressure, as the fuel pump is typically camshaft-driven and the engine must be rotated to pump fuel. An exemplary approach to increasing fuel rail pressure prior to engine start is shown by Birch et al. in WO 2013 / 076217 A2. Therein, during braking of a hybrid electric vehicle operating in an electric mode, at least a portion of the negative torque is used to intermittently coast the engine, thereby improving engine lubrication and fuel rail pressure.

[0006] However, the present inventors have identified potential problems with such an approach. For example, even with adjusted fuel rail pressure, heating of the combustion chambers may not be sufficient to significantly reduce particulate emissions during the subsequent engine restart. For example, the engine may not be able to be rotated to or held in a position where sufficient heat transfer can occur. Thus, engine test data further indicates that particulate emissions can be greatly reduced by engine heating. Thus, even at high fuel pressure and even if the combustion chamber is not sufficiently heated, soot emissions may still occur during engine restart.In addition, due to engine start-up time requirements, the number of engine revolutions allowed before a first fuel injection into the engine may be limited, thereby limiting the increase in fuel rail pressure to below an optimal level.

[0007] In one example, some of the above aspects may be addressed, at least in part, by a method according to claim 1. In this way, slow engine rotation may be used to sufficiently warm an engine prior to a restart, thereby improving particulate emissions from the engine during direct fuel injection upon the subsequent engine restart.

[0008] For example, when operating a hybrid vehicle in an electric mode and while a cylinder piston temperature is below a threshold temperature, the engine may be slowly cranked without fueling via the hybrid vehicle's motor / generator to prepare the engine for an impending engine start. In one example, the slow cranking may be initiated at least 2-3 minutes before an engine start. The engine is slowly rotated so that all cylinders warm up as they undergo respective compression and power strokes. Thus, each compression stroke of the engine causes the compressed air to heat and transfer heat to the cylinder head and pistons. Although the absolute amount of heat transferred to the engine may be small, the heat is transferred directly to a location where heating reduces soot emissions.During slow engine rotation without fuel, each engine cylinder is warmed via compression stroke heating. Thus, the engine may be rotated via the hybrid vehicle's motor / generator at a speed less than the threshold for preheating. In particular, the engine may be rotated more slowly than the engine would be rotated via a starter motor during engine cranking prior to a restart. For example, during a typical starter motor start, the engine may be cranked at 150 RPM, whereas during slow cranking via the hybrid vehicle's engine, the engine may initially be cranked at 10 RPM (to the first position) and then at 30 RPM to subsequent positions.Additionally, during slow tow, an intake throttle may be kept closed so that the compressed air charge is drawn back into the engine with no net flow to the exhaust. Optionally, an EGR valve may be opened to recirculate flow back to the engine and reduce engine vacuum. By slowly rotating each cylinder through a compression stroke, the engine acts as a heat pump, and at the bottom of the exhaust stroke, the cylinder air charge may become cooler than ambient. However, over each cylinder cycle, net cylinder piston heating may result. After sufficient warm-up of the engine cylinders, and when the piston temperature is above the threshold, cylinder fuel injection may resume to restart the engine.In some examples, the engine may be further rotated after the initial slow engine rotation to pre-position the engine for the upcoming engine start. For example, the engine may be rotated to a position that improves engine restartability before cylinder fueling resumes.

[0009] In this way, an engine can be rotated slowly, allowing heat from compression time to warm the cylinder chamber. By rotating an engine slowly for an extended period of time prior to engine start, heat generated during a cylinder compression stroke can be transferred to cylinder walls and used to warm the engine in anticipation of engine start. By preheating the engine, particulate emissions from the engine can be reduced, particularly during engine cold start. In addition, fuel pressure can be increased to an optimal value for start-up, improving fuel injector spray characteristics during restart. Overall, cold-start emissions can be improved.

[0010] Furthermore, a method according to claim 11 and a method according to claim 16 are proposed. Advantageous embodiments are specified in the dependent claims.

[0011] It should be understood that the above Summary is provided to introduce, in simplified form, a selection of concepts further described in the Detailed Description. It is not intended to reveal critical or essential features of the claimed subject matter, the scope of which is defined solely by the claims following the Detailed Description. Furthermore, the claimed subject matter is not limited to implementations that solve any of the disadvantages noted above or in any part of this disclosure. Short description of the drawings Fig. 1 shows an example layout of a hybrid vehicle system. Fig. 2 shows a partial engine view. Fig. 3 - 4 show an exemplary method for warming up an engine prior to engine start by slowly rotating the engine without fueling via engine torque. Fig. 5 shows an exemplary engine warm-up operation for reducing particulate emissions from a hybrid vehicle system. Fig. 6 shows an exemplary characteristic map of cylinder heating during a compression stroke. Detailed description

[0012] The following description relates to systems and methods for reducing particulate emissions from an engine, such as the engine system of Fig. 2, which is used in a hybrid vehicle system, such as the plug-in hybrid electric vehicle from Fig. 1, is coupled. A controller may be configured to execute a routine, such as the routines of Fig. 3 - 4 to rotate the engine during vehicle operation using engine torque without fuel supply to ensure compression stroke heat transfer ( Fig. 6) to heat engine combustion chambers while also increasing fuel pressure. Then, the engine may be further rotated to pre-position the engine for engine restart. An example engine rotation operation is shown in Fig. 5. In this way, the quality of exhaust emissions is improved, especially during cold starts.

[0013] Fig. 1 shows a hybrid propulsion system 100 for a vehicle. In the embodiment shown, the vehicle is a hybrid electric vehicle (HEV). The propulsion system 100 includes an internal combustion engine 10 having a plurality of cylinders 30. Fuel may be supplied to each cylinder of the engine 10 from a fuel system (not shown) including one or more fuel tanks, one or more fuel pumps, and fuel injectors 66.

[0014] The engine 10 supplies power to the transmission 44 via the torque input shaft 18. In the example shown, the transmission 44 is a power-split transmission (or transaxle) that includes a planetary gear set 22 and one or more rotating gear members. The transmission 44 also includes an electric generator 25 and an electric motor 26. The electric generator 24 and the electric motor 26 may also be referred to as electric machines because they can each operate as either a motor or a generator. Torque is the output from the transmission 44 to drive the vehicle drive wheels 52 via a power transfer gear 34, a torque output shaft 19, and a differential / axle assembly 36.

[0015] The generator 24 is drivingly connected to the electric motor 26 so that the electric generator 24 and the electric motor 26 can each be operated using electrical energy from an electrical energy storage device, shown herein as battery 54. In some embodiments, a power conversion device, such as an inverter, may be coupled between the battery and the motor to convert the DC output of the battery to an AC output for use by the motor. However, in other embodiments, the inverter may be configured within the electric motor.

[0016] The electric motor 26 can be operated in a regenerative mode, i.e., as a generator, to capture energy from the vehicle's motion and / or the engine and convert the captured kinetic energy into a form suitable for storage in the battery 54. Furthermore, the electric motor 26 can be operated as a motor or generator, as needed, to augment or capture torque provided by the engine.

[0017] The planetary gear set 22 includes a ring gear 42, a sun gear 43, and a planetary carrier assembly 46. The ring gear and the sun gear may be coupled to each other via the carrier. A first input side of the planetary gear set 22 is coupled to the engine 10, while a second input side of the planetary gear set 22 is coupled to the generator 24. An output side of the planetary gear set is coupled to the vehicle drive gears 52 via the power transfer case 34, which includes one or more meshing gear members 60-68. In one example, the meshing gear members 60-68 may be stepped gears, with the carrier assembly 46 capable of distributing torque to the stepped gears. The gear members 62, 64, and 66 are mounted on a countershaft 17, with gear member 64 engaging a gear member 70 driven by the electric motor.The electric motor 26 drives the gear member 70, which acts as a torque input for the applied transmission. In this way, the planetary carrier 46 (and consequently the engine and generator) may be coupled to the vehicle wheels and the engine via one or more gear members. The hybrid drive system 100 may operate in various embodiments, including a full hybrid system where the vehicle is powered solely by the engine and generator together, or solely by the electric motor, or a combination thereof. Alternatively, assist or mild hybrid embodiments may be used, where the internal combustion engine is the primary torque source and the electric motor selectively adds torque under certain conditions, such as during a tip-in event.

[0018] For example, the vehicle may be driven in an engine mode, wherein the engine 10 is operated and used as the primary torque source for driving the wheels 52. During the engine mode, fuel may be supplied to the engine 10 from a fuel tank via the fuel injector 66, allowing the engine to rotate under fuel supply to provide torque for propelling the vehicle. In particular, engine power is supplied to the ring gear of the planetary gear set. Simultaneously, the generator provides torque to the sun gear 43, thereby creating a reaction torque to the engine. Consequently, torque is output through the planetary carrier to the gears 62, 64, 66 on the countershaft 17, which in turn delivers power to the wheels 52.In addition, the engine may be operated to output more torque than required for propulsion, with the additional power then being absorbed by the generator (in generator mode) to charge the battery 54 or to supply electrical power for other vehicle loads.

[0019] In another example, the vehicle may be driven in an assist mode in which the engine 10 is operated and used as the primary torque source for driving the wheels 52, and the electric motor is used as a supplemental torque source to cooperate with and supplement the torque provided by the engine 10. During the assist mode, fuel is supplied to the engine 10 as in the engine mode to fuel the engine and provide torque to the vehicle wheels.

[0020] In yet another example, the vehicle may be propelled in an engine-off or electric mode in which the battery-powered electric motor 26 is operated and used as the sole torque source for driving the wheels 52. Thus, during the electric mode, no fuel may be injected into the engine 10, regardless of whether the engine is rotating or not, and the vehicle is propelled solely by engine torque. The electric mode may be used, for example, during braking, at low speeds, at low loads, when stopped at traffic lights, etc. In particular, engine power is supplied to the gear member 70, which in turn drives the gear members on the countershaft 17 and thus the wheels 52.

[0021] The drive system 100 may further include a control system including a controller 12 configured to receive information from a plurality of sensors 16 (various examples of which are described herein) and send control signals to a plurality of actuators 81 (various examples of which are described herein). As one example, the sensors 16 may include various pressure and temperature sensors, a fuel level sensor, various exhaust gas sensors, etc. The various actuators may include, for example, the gear set, cylinder fuel injectors (not shown), an air intake throttle coupled to the engine intake manifold (not shown), etc. Additional sensors and actuators are described in Fig. 2. The controller 12 may receive input data from the various sensors, process the input data, and trigger the actuators in response to the processed input data based on instructions or code programmed therein according to one or more routines. Example control routines are described herein with reference to Fig. 3-4 described.

[0022] Fig. 2 shows an exemplary embodiment of a combustion chamber or cylinder of an engine 10 (of Fig. 1). The engine 10 may receive control parameters from a control system including a controller 12 and 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. The cylinder (also referred to herein as "the combustion chamber") 30 of the engine 10 may include combustion chamber walls 136 with a piston 138 positioned therein. The piston 138 may be coupled to the 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 gear system. Furthermore, a starter motor may be coupled to the crankshaft 140 via a flywheel to enable starting operation of the engine 10. In particular, the generator 24 (of Fig. 1) and the engine 26 (from Fig. 1) must be coupled to the crankshaft and provide torque for towing the engine.

[0023] Cylinder 30 may receive intake air via a series of intake air passages 142, 144, and 146. Intake air passage 146 may communicate with other cylinders of engine 10 in addition to cylinder 30. In some embodiments, one or more of the intake passages may include a boosting device, such as a turbocharger or a supercharger. For example, Fig. 2 illustrates engine 10 configured with a turbocharger including a compressor 174 disposed between intake ports 142 and 144 and an exhaust turbine 176 disposed along exhaust port 148. Compressor 174 may be at least partially driven by exhaust turbine 176 via a shaft 180, wherein the boosting device is configured as a turbocharger. However, in other examples, such as when engine 10 is provided with a supercharger, exhaust turbine 176 may optionally be omitted, wherein compressor 174 may be driven by a mechanical input from an engine or the engine. A throttle valve 20 including a throttle plate 164 may be provided along an intake port of the engine to vary the flow rate and / or pressure of intake air supplied to the engine cylinders.For example, the throttle valve 20 may be located downstream of the compressor 174, as shown in FIG. Fig. 2, or alternatively, it may be provided upstream of the compressor 174. In some embodiments, as described with reference to Fig. 3, a charge air cooler (CAC) may be positioned downstream of the compressor 174 and upstream of the throttle valve 20 for cooling a boosted air charge supplied to the engine. Alternatively, the CAC may be positioned downstream of the throttle valve integrated into the intake manifold 146.

[0024] Exhaust passage 148 may receive exhaust gases from other cylinders of engine 10 in addition to cylinder 30. An exhaust gas sensor 128 is shown coupled to exhaust passage 148 upstream of an emission control device 178. Sensor 128 may be selected from various suitable sensors for providing an indication of exhaust air / fuel ratio, such as a linear oxygen sensor or UEGO (universal or wide-range exhaust gas oxygen), a dual-state oxygen sensor or EGO sensor (as shown), a HEGO (heated EGO), a NOx, an HC, or a CO sensor. Emission control device 178 may be a three-way catalyst (TWC), a NOx trap, various other emission control devices, or combinations thereof.

[0025] The exhaust temperature may be estimated by one or more temperature sensors (not shown) positioned in the exhaust conduit 148. Alternatively, the exhaust temperature may be inferred based on engine operating conditions, such as engine speed, load, air-fuel ratio (AFR), spark retard, etc. Furthermore, the exhaust temperature may be calculated by one or more exhaust sensors 128. It should be understood that the exhaust temperature may alternatively be estimated by any combination of temperature estimation methods recited herein.

[0026] Each cylinder of engine 10 may include one or more intake valves and one or more exhaust valves. For example, cylinder 30 is shown including at least one intake poppet valve 150 and at least one exhaust poppet valve 156 positioned in an upper region of cylinder 30. In some embodiments, each cylinder of engine 10, including cylinder 30, may include at least two intake poppet valves and at least two exhaust poppet valves positioned in an upper region of the cylinder.

[0027] The intake valve 150 may be controlled by the controller 12 through cam actuation via the cam actuation system 151. Likewise, the exhaust valve 156 may be controlled by the controller 12 via the cam actuation system 153. The cam actuation systems 151 and 153 may each include one or more cams and may utilize one or more cam profile switching (CPS), variable cam timing (VCT), variable valve timing (VVS), and / or variable valve lift (VVL) systems that may be actuated by the controller 12 to alter valve operation. The position of the intake valve 150 and the exhaust valve 156 may be determined by valve position sensors 155 and 157, respectively. In alternative embodiments, the intake and / or exhaust valves may be controlled by electrical valve actuation.For example, cylinder 30 may alternatively include an intake valve controlled by electric valve actuation and an exhaust valve controlled via cam actuation, including CPS and / or VCT systems. In still other embodiments, the intake and exhaust valves may be controlled by a common valve actuator or a common valve actuation system, or a VAV actuator or a WS actuation system.

[0028] Cylinder 30 may have a compression ratio that is the ratio of volume when piston 138 is at bottom dead center or top dead center. Conventionally, the compression ratio ranges from 9:1 to 13:1. However, in some examples where different fuels are used, the compression ratio may be increased. This may occur, for example, when using higher octane fuels or fuels with higher latent heat of vaporization. The compression ratio may also be increased when using direct injection due to its effect on engine knock.

[0029] In some embodiments, each cylinder of engine 10 may include a spark plug 192 for initiating combustion. Under certain operating modes, ignition system 190 may provide an ignition spark to combustion chamber 30 via spark plug 192 in response to an ignition advance signal SA from controller 12. However, in some embodiments, spark plug 192 may be omitted, for example, when engine 10 is capable of initiating combustion through auto-ignition or by injecting fuel, as may be the case with some diesel engines.

[0030] In some embodiments, each cylinder of engine 10 may be configured with one or more fuel injectors for supplying fluid that suppresses knock or pre-ignition. In some embodiments, the fluid may be a fuel, in which case the injector is also referred to as a fuel injector. As a non-limiting example, cylinder 30 is shown to include a fuel injector 166. Fuel injector 166 is shown to be directly coupled to cylinder 30 to inject fuel directly therein in proportion to the pulse width of a signal FPW received from controller 12 via an electronic driver 168. In this manner, fuel injector 166 provides so-called direct injection (hereinafter referred to as "DI") of fuel into combustion cylinder 30. Although Fig. While Figure 2 shows the injector 166 as a side injector, it may also be positioned above the piston, for example, near the position of the spark plug 192. Such a position may improve mixing and combustion when the engine is running on an alcohol-based fuel, due to the lower volatility of some alcohol-based fuels. Alternatively, the injector may be positioned above and near the intake valve to improve mixing.

[0031] Fuel may be delivered to fuel injector 166 via a high-pressure fuel system 8, which includes fuel tanks, fuel pumps, and a fuel rail. Alternatively, fuel may be delivered at a lower pressure by a single-stage fuel pump, wherein the timing of direct fuel injection during the compression stroke may be more limited than when using a high-pressure fuel system. Although not shown, the fuel tanks may further include a pressure transducer that provides a signal to controller 12. It should be understood that in an alternative embodiment, injector 166 may be a port injector that delivers fuel to the intake port upstream of cylinder 30.

[0032] As described above, Fig. 2 only a single cylinder of a multi-cylinder engine. Thus, each cylinder can analogously contain its own set of intake / exhaust valves, fuel injector(s), spark plug, etc.

[0033] Fuel tanks in fuel system 8 may hold fuel of different qualities, such as different compositions. These differences may include different alcohol content, different octane rating, different latent heat of vaporization, different fuel blends, and / or combinations thereof, etc. In one example, fuels of different alcohol content could include one fuel that is gasoline and another that is ethanol or methanol. In another example, the engine may use gasoline as a first substance and an alcohol-containing fuel blend such as E85 (about 85% ethanol and 15% gasoline) or M85 (about 85% methanol and 15% gasoline) as a second substance. Other alcohol-containing fuels could be a mixture of alcohol and water, a mixture of alcohol, water, and gasoline, etc.

[0034] Further, in the disclosed embodiments, an EGR system may direct a desired portion of air charge or exhaust gas from the exhaust passage 148 to the air intake passage 142. Fig. 2 shows a LP EGR system in which LP EGR is routed through LP EGR passage 240 from downstream of turbine 176 to upstream of compressor 174. The amount of EGR supplied to intake passage 142 can be varied by controller 12 via LP EGR valve 242. Likewise, an HP EGR system (not shown) may be present in which HP EGR is routed through an HP EGR passage from upstream of turbine 176 to downstream of compressor 174. The amount of HP EGR supplied to intake passage 146 can be varied by controller 12 via a dedicated HP EGR valve. The HP EGR system may include a HP EGR cooler, and the LP EGR system may include a LP EGR cooler 246, for example, to transfer heat from the EGR gases to the engine coolant.

[0035] EGR sensors may be located in the EGR channels and may provide an indication of mass flow and / or pressure and / or temperature and / or O 2 concentration and / or exhaust gas concentration. In some embodiments, one or more sensors may be positioned in the HP EGR passage 240 to provide an indication of a pressure, temperature, and / or air-fuel ratio of exhaust gas recirculated through the HP EGR passage. Exhaust gas redirected through the LP EGR passage 240 may be diluted with fresh intake air at a mixing point at the junction of the LP EGR passage 240 and the intake passage 142. In particular, dilution of the EGR flow may be adjusted by adjusting the LP EGR valve 242 in cooperation with a low pressure air intake (LP LAS) throttle valve 230. A percentage dilution of the LP EGR flow may be derived from the output of a sensor 245 in the EGR gas flow.

[0036] In the presentation of Fig. 2, the controller 12 is a microcomputer including a microprocessor unit 106, input / output (I / O) ports 108, an electronic storage medium for executable programs and calibration values, shown in this particular example as a read-only memory (ROM) chip 110, a random access memory (RAM) 112, a keep-alive memory (KAM) 114, and a data bus.In addition to the signals previously discussed, the controller 12 may receive various signals from sensors coupled to the engine 10, including inducted mass air flow (MAF) measurements from the mass air flow sensor 122; engine coolant temperature (ECT) from the temperature sensor 116 coupled to the cooling sleeve 118; a profile ignition pickup (PIP) signal from the Hall sensor 120 (or other type of sensor) coupled to the crankshaft 140; throttle position (TP) from a throttle position sensor; an absolute manifold pressure (MAP) signal from the sensor 124, cylinder fuel-air ratio from the EGO sensor 128, and abnormal combustion from a knock sensor. From the PIP signal, the controller 12 may generate an engine speed signal RPM (revolutions per minute). The manifold pressure signal MAP from a manifold pressure sensor may be used to provide an indication of vacuum or pressure in the intake manifold.

[0037] The read-only memory storage medium 110 may be programmed with computer-readable data representing instructions executable by the processor 106 for performing the methods described below, as well as other variations that are expected but not specifically recited. Example routines are described herein at the Fig. 3 - 4 described.

[0038] Thus, direct-injection engines can generate a large amount of particulate matter (or soot), particularly during cold-start operation. This is partly due to poor fuel injector spray characteristics at the low fuel pressures of an engine start. In addition, soot is generated due to fuel impingement on the cold metal surfaces of the combustion chamber during start-up. Soot generation can be substantially reduced by warming the engine and pressurizing the fuel. However, accomplishing this before an engine start can be difficult due to the large mass of the engine and the limited time and power available before an engine start. Likewise, one engine revolution is required to build fuel pressure because the fuel pump is typically camshaft-driven.However, start-up time requirements may limit the number of engine revolutions allowed before the first fuel injection, resulting in suboptimal fuel pressures at engine start.

[0039] In hybrid vehicle systems, the engine is omitted until power is required for acceleration (in addition to that provided by the vehicle's engine). The present inventors have recognized that the delay experienced when transitioning from an engine-off mode (e.g., electric mode) to an engine-on mode (e.g., assist mode) in a hybrid vehicle may be sufficient to opportunistically prepare the engine for the impending engine restart. In particular, the engine may be rotated slowly during the time delay, such as at a speed less than the speed at which the engine is rotated via an engine starter motor during tow-start (at an engine restart).The engine may be slowly rotated by the motor (such as motor 20) using energy from a system energy storage device (such as storage device 50) that includes system batteries. Alternatively, the engine may be rotated during vehicle deceleration or deceleration events to recover energy that would otherwise be lost through wheel braking.

[0040] The slow rotation may allow each engine cylinder to be rotated sequentially through a cylinder compression stroke. Consequently, heat generated by air compressed in each cylinder during its respective compression stroke (as in Fig. 6) can be effectively transferred to the cylinder walls. This allows for rapid equilibration of the cylinder wall temperature and the cylinder charge temperature and heating of the cylinder piston. In another example, the engine may be slowly rocked back and forth (i.e., a direction of rotation may be changed frequently while the engine is slowly rotating) so that each cylinder can undergo the compression stroke. In this way, the engine can be rotated slowly so that all of the engine's cylinders can be warmed up before an engine restart. During the subsequent engine restart, when fuel injection resumes, the fuel impinging on the warmer cylinder walls can result in lower soot emissions.In addition, the multiple slow revolutions can allow a sufficient increase in pressure in the fuel rail, thereby improving fuel injector spray characteristics.

[0041] Now on Fig. Referring to FIG. 3, an exemplary routine 300 is shown for slowly cranking an engine prior to an engine restart to enable compression warm-up of the engine. In this way, direct fuel injection soot emissions may be reduced during the subsequent engine restart.

[0042] At 302, vehicle and engine operating conditions may be estimated and / or measured. These may include, for example, brake pedal position, accelerator pedal position, operator torque request, battery state of charge (SOC), engine temperature (Teng), ambient temperature and humidity, barometric pressure (BP), etc. In one example, the hybrid vehicle system is a power-split hybrid vehicle system.

[0043] At 304, a vehicle operating mode may be determined based on the estimated operating conditions. For example, based at least on the estimated driver torque request and the battery state of charge, it may be determined whether the vehicle should operate in an engine-only mode (where the engine drives the vehicle wheels), an assist mode (where the battery assists the engine in driving the vehicle), or an electric-only mode (where only the battery drives the vehicle). If the requested torque can be provided only by the battery, in one example, the vehicle may operate in the electric-only mode, where the vehicle is driven using only engine torque.In another example, if the requested torque cannot be provided by the battery, the vehicle may be operated in engine mode or assist mode, in which the vehicle is propelled with at least some engine torque. The vehicle may accordingly be operated in the particular operating mode.

[0044] At 306, it may be confirmed that the vehicle is in electric mode. If electric mode is not confirmed, the hybrid vehicle may be propelled with at least some engine torque at 308. For example, the vehicle may be propelled with only engine torque (for example, in engine mode) or a combination of engine torque and motor torque (for example, in assist mode). If electric mode is confirmed, the routine includes propelling the hybrid vehicle with only motor torque at 310.

[0045] At 312, engine cylinder temperatures may be estimated, inferred, or modeled, and it may be determined whether engine heating is required to reduce emissions upon subsequent engine restart. In one example, cylinder piston temperatures may be assessed, and it may be determined that heating is required if the piston temperature is below a threshold temperature. In another example, cylinder wall temperatures may be compared to a cylinder charge temperature, and it may be determined that heating is required if the difference between the cylinder wall temperature and the cylinder charge temperature is greater than a threshold amount. In still further examples, while propelling the vehicle with engine torque, it may be determined whether an engine start is imminent.For example, based on operating conditions such as operator pedal position, battery charge level, etc., it may be determined whether the engine needs to be restarted to meet the operator torque request. If engine cylinder temperatures indicate that no further warming is required and / or if no impending engine restart is confirmed, the routine may end.

[0046] If warming is required, the controller at 314 may spin the engine without fuel in anticipation of the impending engine restart to increase piston temperatures prior to the restart. As in Fig. 4, the controller may slowly rotate the engine without fueling via engine torque (at less than a threshold speed, such as less than an engine cranking speed) so that each cylinder is heated via compression heating during a compression stroke of the cylinder. Thus, during the slow rotation, each cylinder of the engine may be gradually rotated to a first position in which the cylinder is in a compression stroke and temporarily held in the first position so that a temperature of the cylinder walls and charge can be equilibrated. The slow rotation allows each cylinder to also be rotated and temporarily held in the compression stroke where the cylinder is heated. Thus, the cylinder may then be cooled as the cylinder then continues to rotate into the subsequent power stroke.However, the cylinder may be heated more during the compression stroke than the cylinder is cooled during the power stroke, allowing a net heating of the cylinder across the slow rotation. In this way, the slow rotation enables a heat pump effect within the cylinder. The slow rotation of the engine can advantageously use heat generated in the compression stroke of the selected cylinders to heat the combustion chamber and thereby preheat the engine prior to engine restart. By warming the engine prior to restart, cold-start particulate emissions resulting from direct injection of fuel onto cold combustion chamber surfaces can be reduced. In addition, fuel pressure can be increased, improving fuel spray characteristics and thus further reducing soot production.

[0047] In another example, engine rotation may be performed such that a first engine cylinder is undergoing the compression stroke, with the first cylinder having a piston positioned closest to the compression stroke BDC. For example, the first cylinder just compression warmed may have a piston positioned before or immediately after the compression stroke BDC of an engine cylinder. The first cylinder may then be rotated too close to TDC of the compression stroke. It may be desirable to temporarily remain at TDC to reduce the torque required to hold the engine in a fixed position.

[0048] At 316, after sufficient engine preheating, the engine may be selectively pre-positioned across the engine. In particular, the engine may be rotated without fueling to a position from which engine restartability is enhanced. For example, the engine may be rotated so that an engine cylinder is at or near intake valve closing (IVC). This allows compression of the cylinder during engine cranking prior to an engine restart, and cylinder firing may occur in less than 180 degrees. In another example, the cylinder may be rotated so that the piston is at or near TDC of fueling and compression. The cylinder may then wait for spark. In the latter example, some pressure loss may be present.In yet another example, the engine may be rotated so that an engine cylinder is at the beginning of a compression stroke, where the cylinder is ready to receive fuel via direct injection, or partway through an exhaust stroke, where the cylinder is ready to receive fuel via port injection. With the engine pre-positioned, the engine may be restarted, if necessary. For example, the engine may be restarted due to an increase in operator torque demand that cannot be met by a motor or battery of the hybrid vehicle system. Alternatively, the engine may be restarted due to a drop in battery charge. Additionally, the engine may be restarted to run an air compressor to meet HVAC needs.After the engine restart conditions are met, the engine can be towed and supplied with fuel so that engine combustion can be restarted.

[0049] Now on Fig. Referring to FIG. 4, an example routine 400 is shown for slowly spinning an engine without fueling to better prepare an engine for an upcoming engine restart by warming the engine and increasing pressure in the fuel rail. This allows for the reduction of particulate emissions from the engine when the engine is subsequently restarted.

[0050] At 402, the method includes slowly rotating the engine via a motor of the hybrid vehicle prior to an impending engine restart and while propelling the vehicle with engine torque. Here, the engine may propel the vehicle and rotate the engine. As previously stated, the engine may be rotated at less than a threshold speed. The threshold speed, in one example, may be an engine cranking speed. That is, the engine may be rotated at a lower speed than the speed at which the engine would be rotated by a starter motor during an engine cranking and restart. For example, during engine cranking without fueling, the engine may be rotated via a starter motor at 150 RPM.In comparison, during slow spin for cylinder warm-up, the engine may be rotated at 10-30 RPM via the hybrid vehicle's electric motor / generator. In other examples, the threshold speed at or below which the engine is slowly rotated may be higher or lower based on operating parameters such as oil temperature, ambient temperature, or noise, vibration, and harshness.

[0051] In one example, slow engine rotation may be initiated in a cylinder (e.g., a first cylinder) selected based on a proximity of a cylinder piston position relative to a compression stroke TDC. For example, a controller may identify a cylinder whose piston is closest to a compression stroke TDC or in a position experiencing at least a threshold compression magnitude. At 404, the routine includes rotating the engine such that each cylinder is sequentially heated during a compression stroke of the cylinder. As rotation continues, each cylinder may be cooled during a power stroke of the cylinder immediately following the compression stroke. However, the cylinder may be heated more during the compression stroke than the cylinder is cooled during the power stroke, allowing a net heating of each cylinder via a heat pump effect.Thus, during a compression stroke, the air charge of each cylinder is compressed, generating heat. Rotating an engine such that a cylinder is kept on its compression stroke allows heat to be transferred from the compressed air to the cylinder walls, cylinder head, and piston, thereby increasing engine temperature.

[0052] At 406, the routine includes maintaining an engine intake throttle closed during rotation. Closing the intake throttle allows the compressor air charge to be retracted into the engine without net flow from the exhaust. This thus reduces the potential for emissions to become trapped in the crankcase. In still further examples, during slow engine rotation, an EGR valve of the engine's EGR system (such as the LP EGR valve 242 of Fig. 2) be opened at least partially. By opening the EGR valve, exhaust flow from the cylinder is recirculated to the engine, reducing engine vacuum. In this process, air is pumped in a closed circuit. By allowing the heat of compression to be removed at TDC during operation, the charge is made cooler than at the start of compression. The charge can then be forced out the rear exhaust pipe of the engine or recirculated through the EGR system so that the same charge is used over and over again. This limits the potential for hydrocarbons in the exhaust. On a naturally aspirated engine equipped with a conventional EGR system, the EGR valve could be kept open during the warm-up cycle to reduce or even eliminate the net airflow through the engine.

[0053] In some embodiments, intake and / or exhaust valves of one or more engine cylinders may be deactivated during rotation so that multiple cylinders may be heated via compression at a given time. Thus, compression strokes of consecutively firing cylinders in an inline four-cylinder engine are spaced 180 crankshaft degrees (degrees CA). If the engine is equipped with a deactivation mechanism that allows one or more engine cylinders to be selectively deactivated (for example, via deactivatable cylinder valves), more than one cylinder may be placed on a "compression" stroke at the same time. In other words, in a four-cylinder engine, it may be possible to heat two cylinders in the same steady state.For example, in the four-cylinder in-line engine where the firing order is 1-3-4-2, cylinders 1 and 4 can be heated together, while cylinders 2 and 3 are heated together.

[0054] At 408, it may be determined whether a reduction in vehicle speed is occurring. For example, it may be determined whether a vehicle braking or deceleration event is occurring. If so, at 410, the engine may be rotated via the wheels during vehicle braking or deceleration. In doing so, wheel torque that would otherwise be dissipated as heat or used for regenerative braking may be advantageously used for engine rotation. During the vehicle braking or deceleration event, the routine further includes temporarily increasing engine speed (the unfueled engine rotation) by rotating the engine via the wheels of the hybrid vehicle.Although the routine suggests rotating the engine via engine torque during vehicle braking or deceleration and continuing to opportunistically rotate the engine via the wheels, in other examples, the engine may only be selectively rotated at less than the threshold speed without fueling during vehicle braking and deceleration events that occur while the vehicle is being propelled via engine torque. The routine then proceeds to 412. If no decrease in vehicle speed is confirmed at 408, the routine proceeds directly to 412. At 412, it may be determined whether the engine cylinders have been sufficiently warmed. For example, it may be determined whether a piston temperature is greater than a threshold or whether a temperature difference between cylinder walls and cylinder charge is less than a threshold.The temperature difference may be based on temperatures that are estimated or inferred. Further, the temperature difference may be implemented via a simple timer. For example, sufficient cylinder heating may be determined to have occurred when each cylinder spends more than a threshold time in a compression stroke. In another example, the engine temperature or an average cylinder piston temperature may be evaluated (e.g., compared to a threshold temperature). If the piston temperature is higher than the threshold (or if the temperature difference across the cylinder is less than the threshold), the routine includes terminating the slow engine rotation at 416.Otherwise, at 414, the routine includes continuing to rotate the engine without fuel at less than the threshold speed until the temperature difference between cylinder walls and cylinder charge is less than the threshold (or until the engine temperature or average cylinder piston temperature is greater than the threshold temperature).

[0055] In some examples, control may also determine if a pressure in an engine fuel rail is greater than a threshold pressure. If not, the routine may maintain engine rotation without fueling at less than the threshold speed until the fuel rail pressure is above the threshold pressure. However, because the fuel rail pressure may build within a number (e.g., five to ten) of engine pump strokes, and because a threshold number of pump strokes (e.g., two) is reached every engine rotation, the fuel rail pressure may reach the threshold pressure by the time the cylinder temperature is sufficiently elevated.

[0056] After all engine cylinders have been warmed up, if restart conditions are met, the routine includes resuming cylinder fuel injection to restart the engine. For example, at 418, the routine includes spinning the engine without fueling at or above the threshold speed. The engine may be spun to the engine cranking speed, for example, via an engine starter motor. At 420, the routine may optionally include selecting an engine cylinder in which to resume cylinder fueling. The cylinder may be selected based on a piston position. For example, a cylinder that is at or near IVC may be selected. At 422, fuel may be injected into the selected cylinder during engine cranking for engine restart.It should be understood that in another example, the engine may not select an engine cylinder for resuming cylinder fueling, but may resume fueling when needed.

[0057] It should be understood that in still further examples, after slowly rotating the engine for cylinder warm-up, the routine may include further rotating the engine without fueling via the vehicle engine to a position optimal for engine restartability. For example, the engine may be rotated to a position from which the engine may be quickly restarted if engine restart conditions are not met immediately after cylinder warm-up. In one example, the further rotating may include rotating to a position where an engine cylinder is at or near IVC.In this way, a method is provided for an engine, comprising: when propelling a hybrid vehicle only via engine torque and in anticipation of an impending engine restart, spinning an engine without fuel at less than an engine cranking speed until the piston temperature is greater than a threshold; and after the piston temperature is greater than the threshold, restarting the engine by resuming cylinder fueling. Wherein spinning the engine without fuel at less than an engine cranking speed comprises spinning the engine between 10 and 30 RPM. The threshold is based on the intake air charge temperature. Spinning the engine comprises spinning the engine via a motor of the hybrid vehicle.The method further comprises maintaining an engine intake throttle closed while the engine is rotated at less than the engine cranking speed. In embodiments where the engine includes an EGR passage for recirculating air charge from an engine exhaust to an engine intake, the method further comprises maintaining an EGR valve of the EGR passage open while the engine is rotated at less than the engine cranking speed. In this way, by accelerating cylinder warm-up, particulate emissions from an engine may be reduced during an engine restart with at least some direct injection of fuel into an engine cylinder.

[0058] Fig. Figure 6 graphically shows the effect of compression stroke heating in map 600. In particular, a first set of plots 602-604 are shown, representing the change in temperature in the cylinder as the cylinder is rotated through the compression stroke. A second set of plots 612-614 represent the change in pressure in the cylinder as the cylinder is rotated through the compression stroke. In the example shown, the engine is slowly rotated at 30 RPM. In each set, plots 602 and 612 (solid lines) show the calculated data, while plots 604 and 614 (dashed lines) show the simulated data. The calculated data represents a scenario in which there is no heat flow and no heat is transferred to the cylinder walls and piston. In comparison, the simulated data represents a scenario in which there is heat flow and heat is transferred to the cylinder walls and piston.The plotted ideal characteristics begin at intake valve closure (IVC; at approximately 625 degrees CA) and end at exhaust valve closure (EVO; at approximately 832 degrees CA). The calculated characteristics (plots 604, 614) were based on the isentropic process and volume ratio. The ideal characteristics (plots 602, 612) are then recalculated using P1 and V1 of the isentropic process and volume ratio at IVC. As can be seen, large amounts of heat are transferred to the cylinder walls and pistons during the compression stroke, although some cooling occurs after the compression stroke. In particular, the simulated data shows how heat is lost from the compressed air as it flows to the cylinder walls and pistons, with the in-cylinder temperature consequently dropping. Furthermore, the heat is transferred directly to the location where heat transfer has a major impact on particulate emissions.Heat transfer is used to advantageously increase the piston temperature. Specifically, compression stroke heating is repeated over several cycles until the piston temperature is above a threshold temperature. During slow engine rotation, each cylinder can therefore be heated during that cylinder's compression stroke. Consequently, when the engine is restarted and fuel is delivered to the preselected cylinder, the liquid fuel droplets can directly impinge on hot combustion surfaces, resulting in enhanced fuel vaporization.

[0059] An exemplary engine revolution is now shown in map 500 by Fig. 5. Map 500 shows vehicle speed at plot 502, engine speed at plot 504, a battery state of charge (SOC) at plot 506, a cylinder piston temperature at plot 508, and fuel rail pressure at plot 510. All plots are shown as a function of time along the x-axis. Vehicle propulsion may be started at t1. At the time of vehicle propulsion, engine start conditions may not be met and the vehicle may be propelled only via motor torque. For example, the vehicle may be a hybrid vehicle operating in an electric mode. Between time t1 and t2, the battery SOC may vary with correspondingly varying operator demand and vehicle speed, with the battery SOC being reduced at a faster rate as the vehicle speed increases.Thus, while the vehicle is propelled using engine torque between t1 and t2, the piston temperature may be below threshold temperature 509. To enable cylinder heating and thus reduce particulate emissions and improve engine performance when the engine is subsequently operated, the engine may also be slowly rotated without fuel supply via the engine starting at t1. Slowly rotating the engine via the engine results in two effects. First, the piston temperature is gradually increased. Second, the pressure in the fuel rail is increased.

[0060] After t2, engine rotation continues, but the operator torque demand and vehicle speed decrease. As a result, battery SOC may gradually decrease (at a slower rate) because only engine rotation continues over motor torque. Shortly after t2, a vehicle deceleration event occurs. During this event, instead of dissipating wheel torque as heat, the engine is rotated via the wheels. Based on the decrease in wheel torque that occurs during the vehicle deceleration event, at least a portion of the wheel torque is applied to engine rotation, with a temporary increase in engine rotation speed.

[0061] At t3, the vehicle speed increases again, but the engine restart conditions are not met. Furthermore, an engine restart is not desired because the piston temperature, although warmer than the piston temperature at t1, is still below threshold 509. Consequently, a high level of particulate emissions may be generated during direct injection of fuel into the cold cylinder. Thus, the engine restart is delayed, and the vehicle continues to be driven using only engine torque.

[0062] Between t3 and t4, the engine torque is used to slowly rotate the engine and drive the vehicle.

[0063] Between t3 and t4, the piston temperature is increased above the threshold temperature 509. At t4, the vehicle speed increases again, and the engine restart conditions are considered met. In particular, an engine restart is enabled because the piston is warm enough. Accordingly, after t4, the engine is cranked faster via a starter motor, and cylinder fueling is resumed. Then, the engine speed increases while driving the vehicle with at least engine torque. Furthermore, the engine torque may be used to charge the battery.

[0064] In this way, the example of Fig.5, a method for an engine, comprising: delaying an engine restart until a piston temperature is brought above a threshold or until a difference between the engine temperature and the engine air charge temperature (or the cylinder piston temperature and the cylinder charge temperature) over engine revolution is brought below a threshold, wherein the engine is rotated without fuel at less than an engine cranking speed. Further, the method may include rotating the engine without fuel at the engine cranking speed during the engine restart and then resuming engine fueling.

[0065] In this manner, an engine of a hybrid vehicle may be slowly cranked using a motor during a transition from operation in an electric mode to an engine mode to warm the engine prior to an engine start. By slowly rotating the engine without fueling for a duration prior to an impending engine restart, heat generated from air compressed in a cylinder during a compression stroke may be transferred to cylinder walls and pistons and advantageously used to warm the engine. By warming the engine prior to an engine start, engine particulate emissions resulting from direct-injected fuel may be reduced, particularly during an engine cold start. Furthermore, fuel pressure may be sufficiently increased.The resulting improvement in fuel injector spray characteristics during restart further reduces engine particulate emissions. Overall, engine cold-start exhaust emissions and engine performance can be improved.

[0066] 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. The specific routines described herein may represent one or more of any number of processing strategies, such as event-driven, interrupt-driven, multitasking, multithreading, and the like. As such, various illustrated acts, operations, and / or functions may be performed in the illustrated order, in parallel, or in some cases omitted.Likewise, the order of processing is not required to achieve the features and advantages of the embodiments described herein, but is provided for ease of illustration and description. One or more of the illustrated acts, operations, and / or functions may be performed repeatedly depending on the particular strategy employed. Furthermore, 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.

[0067] It should be understood that the configurations and routines disclosed herein are exemplary in nature, and that these specific embodiments are not to be interpreted in a limiting sense, as numerous variations are possible. For example, the above technology may be applied to V-6, I-4, I-6, V-12, horizontally opposed four, 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.

[0068] The following claims particularly point out certain combinations and subcombinations that are considered novel and non-obvious. These claims may refer to "an" element or "a first" element, or the equivalent thereof. Such claims should be understood to encompass the inclusion of one or more such elements, neither requiring nor excluding two or more such elements. Other combinations and subcombinations of the disclosed features, functions, elements, and / or properties may be claimed by amending the present claims or by presenting new claims in this or a related application. Such claims, whether their scope is broader, narrower, the same, or different with respect to the original claims, are also considered to be included within the subject matter of the present disclosure.

Claims

[1] Procedure comprising: when driving a hybrid vehicle only via engine torque, rotating an engine without fuel supply at less than a threshold speed until a piston temperature is higher than a threshold value, characterized by that during rotation, intake and / or exhaust valves of one or more engine cylinders are deactivated so that several cylinders can be heated via compression at a given time. [2] The method of claim 1, wherein the threshold speed comprises an engine cranking speed when the engine is rotated without fuel supply via a starter motor. [3] The method of claim 2, further comprising resuming cylinder fuel injection to restart the engine after the piston temperature is less than the threshold. [4] The method of claim 3, wherein resuming cylinder fuel injection comprises selecting a cylinder for resuming fuel injection based on proximity of a cylinder piston position relative to intake valve closure. [5] The method of claim 1, wherein turning the engine without fueling comprises turning the engine via a motor of the hybrid vehicle prior to an impending engine restart. [6] The method of claim 1, wherein rotating the engine without fueling comprises rotating the engine via wheels of the hybrid vehicle during vehicle braking or vehicle deceleration. [7] The method of claim 1, further comprising, during vehicle braking or deceleration, temporarily increasing an engine speed by rotating the engine via wheels of the hybrid vehicle. [8] The method of claim 1, further comprising maintaining rotation of the engine without fueling at less than the threshold speed until the pressure in the fuel rail is above a threshold pressure. [9] The method of claim 1, further comprising maintaining an engine intake throttle valve closed during rotation. [10] The method of claim 1, wherein rotating the engine comprises rotating the engine such that each cylinder is heated during a compression stroke of the cylinder and cooled during an expansion stroke of the cylinder, wherein the cylinder is heated more during the compression stroke than the cylinder is cooled during the expansion stroke. [11] Method for an internal combustion engine, comprising: when driving a hybrid vehicle using only engine torque and in anticipation of an impending engine restart, rotating an engine without fuel at less than engine cranking speed until the piston temperature is higher than a threshold value, and after the piston temperature is higher than the threshold, Restarting the engine by resuming cylinder fuel supply, characterized by that the threshold is based on an intake air charge temperature. [12] The method of claim 11, wherein rotating the engine without fuel at less than an engine cranking speed comprises rotating the engine between 10 and 30 RPM. [13] The method of claim 11, wherein rotating the engine comprises rotating the engine via a motor of the hybrid vehicle. [14] The method of claim 11, further comprising maintaining an engine intake throttle closed while rotating the engine at less than the engine cranking speed. [15] The method of claim 11, wherein the engine includes an EGR passage for recirculating air charge from an engine exhaust to an engine intake, the method further comprising maintaining an EGR valve of the EGR passage open while the engine is rotated at less than the engine cranking speed. [16] Method for an internal combustion engine, comprising: Delaying an engine restart until a piston temperature is brought above a threshold via engine rotation, wherein the engine is rotated without fuel at less than an engine cranking speed, characterized bythat delaying until the piston temperature is brought above a threshold comprises delaying until a difference between the cylinder piston temperature and the cylinder air charge temperature is brought below a threshold difference. [17] The method of claim 16, further comprising rotating the engine without fueling at the engine cranking speed during the engine restart and then resuming engine fueling.

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