A new criterion and method for classifying combustion modes in a high compression ratio spark ignition engine

Internal combustion engines are evolving to attain advanced combustion modes with higher percentage of compression ignition combustion. Homogeneous charge compression ignition is limited by load range and combustion stability, so hybrid combustion modes have emerged as a key research hotspot. In order to optimize hybrid combustion modes, combustion modes should be accurately identified. Therefore, this study introduces a new criterion and method to identify combustion modes. The results show that high compression ratio spark ignition engines are prone to autoignition, however, a trade-off relationship exists between high percentage of compression ignition combustion and the optimal combustion phase. As low percentage of compression ignition is favorable for engine efficiency, reduction of the combustion intensity should be considered. Under spark ignition assistance, both single-fuel and dual-fuel combustion exhibit a combustion mode where spark ignition and compression ignition occur simultaneously, which is defined as spark-assisted compression ignition combustion mode. The peak value (HP) for the second derivative of heat release rate at the onset of a combustion mode is strongly correlated with burning intensity and can be used to identify different combustion modes. The HP of spark ignition combustion is less than 1.5 J/deg3. Spark-assisted compression ignition combustion has an HP between 1.5 J/deg3 and 5.0 J/deg3, while the HP of premixed compression ignition combustion is at least 5.0 J/deg3.