Knock characteristics and combustion regime diagrams of multiple combustion modes based on experimental investigations

Spark assisted compression ignition (SACI) is a kind of combustion mode that bridges the gap between homogeneous charge compression ignition (HCCI) under low load and spark ignition (SI) under high load. In a multi-mode combustion engine, different combustion modes are used under different engine loads and the transition between combustion modes is necessary. To realize transitions among the three combustion modes, this work studies combustion and knock characteristics of HCCI, SACI and SI, and proposes region diagrams that divide the operation regions of different combustion modes. The results show that knocking combustion of HCCI and SI is totally different, and SACI knock combines the features of both HCCI knock and SI knock. Further analysis indicates that compression ignition (CI) of SACI relies on both initial thermodynamic state and flame propagation. And the more auto-ignition is dependent on flame propagation, the more uncontrollable CI process becomes. Region diagrams show that HCCI works with high i-EGR ratios, and an HCCI knock region exists with moderate i-EGR ratios. Transition from normal SACI to HCCI knock can be completed even in two cycles. Therefore, air-fuel mixture and i-EGR should be carefully organized to avoid this region. In the region diagram of SACI-SI, there is an instable transition region between SACI and SI, which is caused by instability of flame propagation due to such amount of EGR and the reliability of CI process on flame propagation. Fuel stratification is helpful to suppress knock, advance combustion phase by earlier spark timing, and improve combustion stability. But it cannot eliminate the unstable region. Twin spark adds another flame kernel into combustion chamber and greatly improves combustion stability. With this strategy, stable transition between SACI and SI modes can be achieved.