• Energy Research

Abstract 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.

For CNG/diesel dual fuel engines, the effects of pilot fuel quantity and injection timing are noticeable and significant. In this study, the emission characteristics of a CNG–diesel dual fuel engine with different pilot diesel fuel quantity and optimized pilot injection timing were investigated. The CO emission levels under dual fuel mode are considerably higher than that under normal diesel operation modes even at high load, which indicated that there exist some flame extinction regions. Dual fuel mode reduces NOx emissions by 30% averagely in comparison to diesel mode. That is because most of the fuel is burned under lean premixed conditions which result in lower local temperature. The unburned HC emissions under dual-fuel mode are obviously higher than that of the normal diesel mode, especially at low to medium loads. And around 90% of the THC emissions were unburned methane, which means the flame does not propagate throughout the charge. THC emissions reduce significantly with the increase of the pilot diesel quantity. Thanks to the premixed nature of the combustion mode and the methane molecular structure, the PM emission is reduced obviously under dual fueling condition. The PM emission is increased with the increase of the pilot fuel quantity.