Extending the High Load Operating Limit of a Naturally-Aspirated Gasoline HCCI Combustion Engine

Homogenous Charge Compression Ignition ( HCCI) combustion offers efficiency improvements compared to conventional gasoline engines. However, due to the nature of HCCI, traditional HCCI combustion can be realized only in a limited operating range. The HCCI operation at high load is limited by excessive combustion noise. In order to maximize the fuel economy benefits of HCCI its operating range needs to be extended to higher loads. In particular, one immediate benefit of an increased load range on the NEDC driving cycle is the avoidance of transitions between SI operation and HCCI operation. In this research a detailed investigation of the fundamental reasons for high combustion noise was performed. Spark-assisted HCCI combustion was found to be a key factor to reduce combustion noise at high load condition. Injection timing for charge cooling, spark timing for the onset of combustion through flame propagation, smaller NVO and higher EGR are the enablers to achieve the appropriate spark-assisted HCCI combustion at high load operation. The high load limit was successfully extended while maintaining good efficiency and complying with emissions regulations. The effect of double injection strategy was also investigated. It is found that even though slight improvement in the tradeoff between combustion noise and combustion stability can be achieved through double injection strategy, fuel consumption is deteriorated due to incomplete combustion.