Study for ignition characteristics and potential of gasoline autoignition combustion with spark assist

A spark assist system was installed in a gasoline direct-injection single-cylinder test engine with the aim of controlling the ignition timing and accomplishing combustion of gasoline fuel by auto/compression ignition. A primary reference fuel having an octane number of 90 (PRF 90) was used to evaluate experimentally the spark assist function for gasoline auto/compression ignition and to examine the feasibility of combustion with a short ignition delay equivalent to conventional diesel combustion using the engine system. An optically accessible single-cylinder test engine was also used to evaluate and investigate spark-assisted auto/compression ignition. Ignition timing controllability with combinations of spark and injection timings for gasoline auto/compression ignition was also investigated under different operating load conditions. The results showed that the spark assist system was required to optimize fuel sprays and a high possibility was seen for soot-less and high thermal efficiency gasoline auto/compression ignition combustion. High thermal efficiency and low emissions performance were also obtained. Thermal efficiency and emissions performance were almost the same under the different load conditions. Maximum indicated thermal efficiency of approximately 42% (compared with 43% for diesel combustion) was obtained along with NOx emissions of 800 ppm (approximately the same as for diesel combustion) and near-zero soot (compared with 5-20 mg/m3 for diesel combustion).