Real-time start of combustion detection based on cylinder pressure signals for compression ignition engines

The start of combustion (SOC) has a direct impact on the intensity of heat release and an indirect effect on engine noise and pollutant formation. Because ignition delay can vary during the system's lifetime due to changes in fuel quality or gradual deterioration of the engine and injection system, real-time detection of the SOC is required to optimize engine performance. In this study, engine compression and expansion strokes are treated as polytropic processes based on a thermodynamic analysis of cylinder pressure signals where the polytropic exponent is defined as an equivalent isentropic index. A real-time SOC detection method applying the first derivative of the equivalent isentropic index is proposed. The presented method using two-stage threshold levels of 0.4 and 0.1 is free of exponential and logarithmic computations, significantly reducing calculation time. It is implemented in a real engine control unit (ECU) and validated in both stationary and transient conditions. Test results show that this method is able to detect multiple SOCs caused by multiple injections and has latencies of 0.5 °CA, 0 °CA and 0 °CA to the separation points of cylinder pressure from motoring pressure at three engine operation points.