Effect of Spark Assistance on Autoignition Combustion in a Small Two-Stroke Engine

Abstract

The local thermal energy of the in-cylinder mixture before combustion is critical to enable autoignition (AI) to occur in a gasoline engine. When the thermal energy is insufficient, AI combustion may become very unstable and misfire may occur. Spark assistance has been proposed as a cost-effective and convenient way to enhance the local thermal energy to reach the level required for AI. It also has the potential to make AI temporally controllable. In the work reported in this paper, spark-assisted AI was experimentally investigated on a 160 cm3 two-stroke engine. The spark assistance and internal exhaust gas recirculation were jointly applied to achieve and control AI. The results showed that, at higher engine loads, the spark timing had a significant effect on AI combustion. The timing of the AI was advanced with advanced spark timing. The peak cylinder pressure and heat release rate increased with advanced spark timing. However, the combustion duration was quite independent of the spark timing. At lower engine loads, the spark assistance significantly reduced the cyclic variation in AI combustion. At a fixed engine operating condition with a light load, the spark assistance was only effective when the spark timing is sufficiently advanced. The advancement of the spark timing required to make the spark assistance effective may vary with the engine load and the air-to-fuel ratio.