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Control of autoignition timing in a hcci engineControl of autoignition timing in a hcci engine description/claimsThe Patent Description & Claims data below is from USPTO Patent Application 20060288966, Control of autoignition timing in a hcci engine. Brief Patent Description - Full Patent Description - Patent Application Claims
BACKGROUND OF THE INVENTION [0001] 1. Field of the Invention [0002] This invention relates to methods and systems for controlling autoignition timing of an internal combustion engine operated in a homogeneous-charge compression-ignition mode. [0003] 2. Background Information [0004] A conventional gasoline-fueled internal combustion engine employs spark ignition where the fuel and air am premixed and a spark initiates a flame that propagates through the fuel/air mixture in the combustion chamber. The other common type of internal combustion engine employs compression ignition where the fuel and air are purposely kept separate until shortly before top dead center in the engine when the temperature of the air in the combustion chamber is high due to the compression. The fuel then is quickly injected into the combustion chamber as a very fine mist, which partially mixes with the air and autoignites in the combustion chamber. The timing of the fuel injection timing thus controls the autoignition timing. Diesel engines are illustrative of this type of compression ignition engine. [0005] Homogeneous-charge compression-ignition (HCCI) internal combustion engines are known and offer the potential to reduce fuel consumption and NO.sub.x emissions. An HCCI engine employs a premixed fuel/air charge to the combustion chamber as in a spark ignition engine, while the charge is ignited by compression ignition as in a diesel engine when the temperature of the air-fuel charge reaches an autoignition temperature in the combustion chamber. HCCI engines typically are provided with a conventional spark plug for each cylinder and relatively low compression ratios, typically close to those of spark ignition (SI) engines, to permit switching of operation of the engine from the HCCI mode at lower engine torques to the S1 mode at higher engine torques without engine knocking. [0006] Control of autoignition timing in an HCCI engine is more difficult than in a diesel engine, which controls fuel injection timing to control autoignition timing. In an HCCI engine, the composition and temperature of the fuel-gas mixture in the combustion chamber must be controlled to control autoignition timing. [0007] It has been proposed to control HCCI autoignition timing using what has been called a negative valve overlap strategy that provides internal exhaust gas recirculation in the combustion chamber. Negative valve overlap control strategy involves trapping hot residual burned gas in the cylinder to subsequently mix with fresh air inducted into the combustion chamber. The trapped burned gas raises the temperature of the air-burned gas mixture to promote autoignition. Autoignition timing (delay) is represented by the equation: t=A exp(E/RT), where t is the time it takes for the mixture in the combustion chamber to autoignite, often called the ignition delay, A is an empirical constant, E is an activation energy and is a function of the composition of the mixture, such as type of fuel, fuel/air mixture amount of residuals, etc., and R is the universal gas constant. Because the equation expresses an exponential relationship, it is evident that temperature of the mixture plays a key role in determining if and importantly when autoignition will occur. [0008] Pursuant to negative valve overlap control strategy, the exhaust valve doses before top dead center (TDC) and the intake valve opens after TDC such that both valves are closed at TDC of the exhaust stroke. Such strategy controls trapping of hot residual burned gas in the combustion chamber to, in turn control the autoignition timing. FIG. 5 shows a plurality of intake and exhaust valve lift curves versus crank angle for an HCCI engine for purposes of illustrating the negative valve overlap strategy where different negative valve overlaps are shown for use at different engine torques. In particular, for different engine torques, different pairs of intake and exhaust valve lift curves (e.g., curves 1I, 1E; 2I, 2E; 3I, 3E: and so on) are employed in coordination with one another to provide the desired negative overlap for a particular engine torque. That is, intake and exhaust valve lift curves 1I, 1E would be used in coordination for a particular engine torque, different intake and exhaust valve lift curves 2I. 2E would be used in coordination for a different particular engine torque, and so on. The negative valve overlap control strategy is described by Willard et al. in "The knocking syndrome--its cure and its potential". SAE 982483, 1998. [0009] When engine speed or torque changes, the autoignition timing of the HCCI engine tends to change. For example, at higher torque autoignition timing tends to advance, resulting in the increase in hear transfer losses, NO.sub.x emissions, and combustion noise. Therefore, the engine control system should adjust to move the autoignition timing back to the optimum crank angle. At lower engine torque, autoignition timing tends to be retarded resulting in an increase of CO emissions and lower combustion efficiency. The engine control system should adjust to move the autoignition timing back to the optimum crank angle. [0010] Moreover, it is desirable to operate the engine with a stoichiometric air-fuel mixture and with a conventional three-way catalyst for after-treatment of exhaust gases. Control of the mass of trapped hot residual burned gas in the cylinder can provide control of autoignition timing during HCCI engine operation. There is a need to also control air-fuel ratio to provide a stoichiometric mixture for engine operation over a wide range of climate and weather conditions without altering the autoignition timing. [0011] However, use of negative valve overlap as a single control variable in HCCI engine control strategy to control both the autoignition timing and the air-fuel ratio at different operating conditions is problematic in that use of a single negative valve overlap variable in the control strategy offers insufficient degrees of freedom to control the air-fuel ratio, in-cylinder gas temperature, and residual fraction of burned gas in the in-cylinder gas in a manner to provide favorable values for all of these parameters at different operating conditions. SUMMARY OF INVENTION [0012] The present invention provides a method and system embodying a particular valve timing strategy to control the autoignition timing of a four stroke internal combustion engine operated in the HCCI mode at different engine operating conditions such as at different operator (driver) demanded engine torques. A-particular valve timing strategy varies lift timing of the intake valve relative to the exhaust valve, or vice versa, and relative to top dead center in response to a change in operator demanded engine torque, for example, to vary amount of trapped residual burned gas in the combustion chamber flowing to an intake or exhaust port and back to the combustion chamber by which the residual gas loses thermal energy and is cooled. Such control of the flow of residual burned gas between the combustion chamber and intake or exhaust port and thus its temperature by the valve timing strategy is used to control the temperature of the fresh air/residual burned gas mixture in the combustion chamber into which fuel is mixed and thus the autoignition timing to suit a given engine torque demand. [0013] In an illustrative embodiment of the invention, the exhaust valve timing is substantially fixed before TDC over successive engine cycles to control the air-fuel ratio in the combustion chamber. The opening time of the intake valve is varied relative to TDC (e.g., advanced toward TDC) over successive intake cycles in a manner that changes the temperature of the fresh air/residual burned gas mixture in the combustion chamber into which the fuel is mixed and thus the autoignition timing. The exhaust valve timing and/or the fuel injection pulse width can be adjusted slightly to compensate for the effect of the temperature change of the mixture on the mass of the inducted fresh air in the combustion chamber. Further, for each intake event, an initial intake valve opening event preferably is provided immediately after the exhaust valve closes and before TDC followed by a main intake valve event occurring after TDC in a manner to reduce or minimize engine pumping losses. [0014] In another illustrative embodiment of the invention, the intake valve lift timing is substantially fixed after TDC over successive engine cycles to control the air-fuel ratio in the combustion chamber. The closing time of the exhaust valve is vaned relative to TDC (e.g., retarded toward TDC) over successive exhaust cycles in a manner that changes the temperature of the fresh air/residual burned gas mixture in the combustion chamber into which fuel is mixed and thus the autoignition timing. The intake valve timing and/or the fuel injection pulse width can be adjusted as needed in order to compensate for the effect of the temperature change of the mixture on the mass of the inducted fresh air in the combustion chamber. For each exhaust event, a first main exhaust valve opening event preferably is provided before TDC followed by a subsequent secondary exhaust valve event occurring after TDC immediately before opening of the intake valve in a manner to reduce or minimize engine pumping losses. [0015] The above advantages of the present invention will become more readily apparent from the following description taken with the following drawings. BRIEF DESCRIPTION OF DRAWINGS [0016] FIG. 1 is a schematic view of an internal combustion engine and an electronic engine control unit for practicing an embodiment of the invention. [0017] FIG. 2 is diagram illustrating intake and exhaust valve lift-curves versus crank angle (where TDC is bottom dead center and TDC is top dead center) at a given engine speed and torque for an embodiment pursuant to the invention. [0018] FIG. 3 is diagram illustrating intake and exhaust valve lift curves versus crank angle at a given engine speed and torque for another embodiment pursuant to the invention having double intake valve events. [0019] FIG. 4 is diagram illustrating intake and exhaust valve lift curves versus crank angle at a given engine speed and torque for another embodiment pursuant to the invention having double exhaust valve events. [0020] FIG. 5 is a diagram illustrating conventional coordinated intake and exhaust valve lift curves versus crank angle (where BDC is bottom dead center and TDC is top dead center) of an HCCI engine at different engine torques to provide different negative valve overlaps wherein intake and exhaust lift curves 1I, 1E are employed at a given torque: curves 2I, 2E are employed at a different torque: and so on. DESCRIPTION OF THE INVENTION Continue reading about Control of autoignition timing in a hcci engine... Full patent description for Control of autoignition timing in a hcci engine Brief Patent Description - Full Patent Description - Patent Application Claims Click on the above for other options relating to this Control of autoignition timing in a hcci engine patent application. ###  How KEYWORD MONITOR works... a FREE service from FreshPatents1. Sign up (takes 30 seconds). 2. Fill in the keywords to be monitored. 3. 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