HCCI combustion timing control with Variable Valve Timing

Homogeneous Charge Compression Ignition (HCCI) is a promising concept for combustion engines to reduce both emissions and fuel consumption. In HCCI engines, a homogeneous air-fuel mixture auto-ignites due to compression, which is unlike traditional spark ignition or diesel engines where ignition is started with either a spark or fuel injection. HCCI combustion control is a challenging issue because there is no direct initiator of combustion in these engines. Variable Valve Timing (VVT) is one effective way to control the combustion timing in HCCI engines. VVT changes the amount of trapped residual gas and the effective compression ratio both of which have a strong effect on combustion timing. In order to control HCCI combustion, a physics based control oriented model is developed that includes the effect of trapped residual gas on combustion timing. The control oriented model is obtained by model order reduction of complex chemical kinetic reaction mechanisms. This method allows different fuels to be incorporated using a standard methodology and fills the gap between complex models with highly detailed chemical kinetics and simple black box models that have been used in model based control. The control oriented model is used to develop ignition timing PI control using simulation. The PI control modulates the trapped residual gas using variable valve timing as the actuator. The results indicate that the controller can track step changes in HCCI combustion timing.