System and method for controlling intake valve timing in homogeneous charge compression ignition engines

A control system for a homogeneous charge compression ignition (HCCI) engine includes first and second modules. The first module determines an adjusted intake valve opening (IVO) timing based on a base IVO timing and an IVO timing adjustment, wherein the IVO timing adjustment is based on one or more of a plurality of operating parameters. The second module controls intake valves of the HCCI engine based on the adjusted IVO timing.

FIELD

The present disclosure relates to internal combustion engines and more particularly to a system and method for controlling intake valve timing in homogeneous charge compression ignition (HCCI) engines.

BACKGROUND

Internal combustion engines draw air into an intake manifold through an induction system that may be regulated by a throttle. The air in the intake manifold is distributed to a plurality of cylinders and combined with fuel from a plurality of fuel injectors creating an air/fuel (A/F) mixture. The A/F mixture is combusted within the cylinders to drive pistons that rotatably turn a crankshaft and generate drive torque.

Homogeneous charge compression ignition (HCCI) engines may operate in a plurality of modes. In a first combustion mode, HCCI engines compress the A/F mixture using the pistons until the A/F mixture auto-ignites. Alternatively, in a second combustion mode (also known as mixed-mode), HCCI engines may compress the A/F mixture using the pistons and may provide spark combust the compressed A/F mixture.

SUMMARY

A control system for a homogeneous charge compression ignition (HCCI) engine includes first and second modules. The first module determines an adjusted intake valve opening (IVO) timing based on a base IVO timing and an IVO timing adjustment, wherein the IVO timing adjustment is based on one or more of a plurality of operating parameters. The second module controls intake valves of the HCCI engine based on the adjusted IVO timing.

A method for controlling a homogeneous charge compression ignition (HCCI) engine includes determining an adjusted intake valve opening (IVO) timing based on a base IVO timing and an IVO timing adjustment, wherein the IVO timing adjustment is based on one or more of a plurality of operating parameters, and controlling intake valves of the HCCI engine based on the adjusted IVO timing.

DETAILED DESCRIPTION

A homogeneous charge compression ignition (HCCI) engine achieves the necessary critical pressure and temperature for combustion by controlling negative valve overlap (NVO). NVO represents camshaft or crankshaft degrees between exhaust valve closing (EVC) and intake valve opening (IVO). NVO is determined based on requested airflow and could be symmetric about top dead center (TDC) of gas exchange. The EVC essentially determines the amount of trapped “internal” residual which remains in the cylinder (i.e. the exhaust valve closes before TDC, preventing the remaining burned gas from being expelled). This trapped residual that remains in the cylinder is re-compressed till the piston reaches TDC and is expanded by the downward motion of the piston until IVO.

For optimization of pumping losses, the trapped residual is essentially a spring during this process. If the IVO is too early, not all of the energy stored in the “spring” is extracted during the expansion. Burned gas is thereby forced back into the intake manifold and pumping losses increase. If the IVO is too late, however, the pumping losses increase due to the pressure in the cylinder being far below the intake manifold pressure at IVO. Factors that change the effective energy of the spring during this compression and expansion process alter the IVO at which the minimum pumping loss is obtained. In general, the optimum IVO is earlier if energy is removed and the optimum IVO is later if energy is added. For example, factors which add energy include, but are not limited to, spark events with fuel present in the charge (mixed mode). Additionally, for example, factors which remove energy include, but are not limited to, charge loss due to leakage past the piston rings, heat transfer to cylinder liners, and/or charge cooling due to early fuel injection.

Adding spark events and injection events in the re-compression phase can cause positive pumping mean effective pressure (PMEP). Positive PMEP may cause a commanded IVO timing to differ from a symmetric IVO timing with respect to EVC timing. Rather, a predetermined IVO timing commanded by a control system may be earlier than or later than the symmetric IVO timing. As previously described, an earlier than desired IVO timing may allow air to escape from a cylinder whereas a later than desired IVO timing may prevent a desired amount of air from entering the cylinder. Early or late IVO timing may increase pumping losses thereby decreasing fuel economy and/or performance.

Accordingly, a system and method for improved control of intake valve timing in an HCCI engine is presented. The system and method may determine an IVO timing adjustment based on a PMEP of cylinders of the engine. Specifically, the system and method may determine the IVO timing adjustment based on one or more of a plurality of operating parameters that affect combustion energy. For example, the plurality of operating parameters may include a number of sparks per cycle, air charge loss, air charge cooling. The number of sparks per cycle may increase combustion energy whereas the air charge loss and air charge cooling may decrease combustion energy.

The air charge loss may be due to leakage past the piston rings entering the crankcase from the cylinder. The air charge cooling may be due to decrease in temperature due to fueling events. Specifically, the air charge cooling may be based on fuel injection timing, number of fuel injections per cycle, air/fuel (A/F) ratio, vaporization efficiency, and/or heat transfer (e.g., to the cylinder walls). Furthermore, the vaporization efficiency and heat transfer may vary as a function of other operating parameters such as engine speed, engine load, and/or engine wear.

The system and method may then control intake valves of the HCCI engine based on an adjusted IVO timing. The adjusted IVO timing may be based on a base IVO timing and the IVO timing adjustment. For example, the base IVO timing may be symmetric to an EVC timing about the gas exchange TDC. The desired NVO (in camshaft or crankshaft degrees) is the sum of the EVC before TDC and the IVO after TDC. The desired NVO may be based on driver input such as a desired/requested mass air flow (MAF). Additionally, in some implementations, MAF into the HCCI engine and/or oxygen-level in exhaust gas produced by the HCCI engine may be used as feedback to further improve intake valve timing control (i.e., faster and/or more accurate adjustments to IVO timing).

Referring now toFIG. 1, an engine system10includes an engine12. The engine12may be an HCCI engine. The engine12draws air into an intake manifold14through an induction system16that may be regulated by a throttle18. For example, the throttle18may be electrically controlled using electronic throttle control (ETC). A MAF sensor20may measure a MAF rate into the intake manifold14. For example, the measured MAF rate may indicate an engine load. The air in the intake manifold14is distributed to a plurality of cylinders22through a plurality of intake valves24, respectively.

The air is combined with fuel from a plurality of fuel injectors26, respectively, to create an A/F mixture. For example, the fuel injectors26may be configured for single point injection (SPI). The A/F mixture is compressed by pistons (not shown) within the cylinders22. The A/F mixture may be combusted either by compression or by compression and spark from spark plugs28. In HCCI mode, the A/F mixture may auto-ignite when the temperature and/or pressure of the compressed A/F mixture exceeds a critical threshold. In mixed-mode, on the other hand, the temperature and/or pressure of A/F mixture may be increased in the re-compression phase after EVC and then ignited by spark from the spark plugs28.

The combustion of the A/F mixture drives the pistons (not shown) which rotatably turn a crankshaft30and generate drive torque. An engine speed sensor32measures a rotational speed of the crankshaft30(e.g., in revolutions per minute, or RPM). The crankshaft30may be housed in a crankcase (not shown). A flow model may be used to estimate or a blowby or crankcase pressure sensor34may be used to indirectly measure a volume of blowby gases that are entering the crankcase (not shown) past piston rings in the cylinders22. For example, the measured blowby may be used to determine the air charge loss within a cylinder22. The drive torque may be transferred to a driveline36of a vehicle via a transmission (not shown). For example, a sensor may measure a rotational speed of the driveline36, the measured rotational speed indicating vehicle speed.

Exhaust gas produced during combustion may be expelled from the cylinders22through a plurality of exhaust valves38, respectively, and into an exhaust manifold40. An oxygen sensor42measures an amount of oxygen in the exhaust gas. The exhaust gas may be treated by an exhaust treatment system44before being released into the atmosphere. For example, the exhaust treatment system44may include, but is not limited to, at least one of an oxidation catalyst (OC), a nitrogen oxide (NOx) adsorber/absorber, a selective catalytic reduction (SCR) catalyst, a particulate matter (PM) filter, and a three-way catalytic converter. Driver input46represents input from a driver of the vehicle. For example, the driver input46may be a position of an accelerator (e.g., a pedal).

A control module50controls operation of the engine system10. The control module50may receive signals from throttle18, the MAF sensor20, the intake valves24, the fuel injectors26, the spark plugs28, the engine speed sensor32, the flow model, the blowby or crankcase pressure sensor34, the driveline36, the exhaust valves38, the oxygen sensor42, the exhaust treatment system44, and/or driver input46. The control module50may control the throttle18, the intake valves24, the fuel injectors26, the spark plugs28, the exhaust valves38, and/or the exhaust treatment system44. The control module50may also implement the system or method of the present disclosure.

Referring now toFIG. 2, an example of the control module50is shown. The control module50may include an adjusted IVO timing module60and a valve control module64. The control module50may also include a desired NVO module68, a base IVO timing module72, and an adjustment determination module76. The adjusted IVO timing module60may also be referred to as a first module. The valve control module64may also be referred to as a second module. The desired NVO module68may also be referred to as a third module. The base IVO timing module72may also be referred to as a fourth module. The adjustment determination module76may also be referred to as a fifth module.

The adjusted IVO timing module60determines an adjusted IVO timing. The adjusted IVO timing module60may determine the adjusted IVO timing based on a base IVO timing and an IVO timing adjustment. For example, the base IVO timing may be symmetric to a base EVC timing about a midpoint of a desired NVO (i.e., point of TDC gas exchange). The valve control module64may control the intake valves24based on the adjusted IVO timing. The adjusted IVO timing may increase fuel economy and/or performance.

The desired NVO module68determines the desired NVO based on driver input46. The driver input46may include a desired MAF. For example, the driver input46may be based on a position of an accelerator (e.g., a pedal). The base IVO timing module72determines the base IVO timing based on the desired NVO. As previously described, the base IVO timing module72may determine the base IVO timing so as to be symmetric to the base EVC timing about the midpoint of the desired NVO or point of TDC gas exchange.

The adjustment determination module76determines the IVO timing adjustment. The IVO timing adjustment may be based on a PMEP of cylinders22of the HCCI engine12. The PMEP of the cylinders22, however, may be unknown. Rather, implementing pressure sensors in each of the cylinders22may increase costs. Therefore, the adjustment determination module76may determine the IVO timing adjustment based on one or more of a plurality of operating parameters that affect combustion energy. For example, the plurality of operating parameters may include a number of sparks per cycle, air charge loss, and air charge cooling. The number of sparks per cycle may increase combustion energy whereas the air charge loss and air charge cooling may decrease combustion energy.

The air charge may cool due to fueling. Specifically, the air charge temperature may decrease based on fuel injection timing, number of fuel injections per cycle, fuel type, A/F ratio, vaporization efficiency, and/or heat transfer. Vaporization efficiency may represent a percentage of the A/F mixture that is combusted during an engine cycle. Heat transfer may represent a decrease in air charge temperature due to the transfer of heat to the cylinder walls. The vaporization efficiency, heat transfer, and a charge loss correction are may be functions of other operating parameters such as engine speed, engine load, and/or engine wear.

In some implementations, the fuel injection timing and A/F ratio may be predetermined. The number of fuel injections and/or number of sparks per cycle, on the other hand, may vary depending on the operating mode of the HCCI engine12. The air charge loss, the engine speed, and the engine load may be estimated by the flow model or measured indirectly by the blowby or crankcase pressure sensor34, the engine speed sensor32, and the MAF sensor20, respectively. In addition, the heat transfer may be based on various temperatures such as intake air temperature (IAT), engine coolant temperature (ECT), and/or exhaust gas temperature (EGT).

Specifically, the IVO timing adjustment may advance the adjusted IVO timing when the following parameters increase: (i) air charge loss due to blowby and/or (ii) air charge cooling due to fuel injection, which is further based on fuel injection timing, number of fuel injections per cycle, fuel type, A/F ratio, vaporization efficiency, and/or heat transfer. The vaporization efficiency and heat transfer may be functions of engine speed, engine load, engine wear, etc. In addition, the IVO timing adjustment may retard the adjusted IVO timing when the following parameter increases: number of spark events per cycle during mixed mode combustion. In other words, depending on the implementation, the IVO timing adjustment may be (i) a positive quantity and therefore added to or subtracted from the base IVO timing according to the parameters, or (ii) a positive or negative quantity added to the base IVO timing.

Referring now toFIG. 3, an example method for controlling intake valve timing in an HCCI engine begins at100. At100, the control module50determines a desired NVO based on driver input46(e.g., desired MAF). At104, the control module50determines a base IVO timing to be symmetric to a base EVC timing about a midpoint of the desired NVO. At108, the control module50determines the IVO timing adjustment based on one or more of the plurality of operating parameters. At112, the control module50determines the adjusted IVO timing based on the base IVO timing and an IVO timing adjustment. At116, the control module50controls intake valves of the engine based on the adjusted IVO timing. Control may then return to100.