Controlled autoignition inside an active pre-chamber using alternative fuels

The efficiency of spark-ignition engines can be increased by increasing the excess air dilution using active pre-chambers (PCs), which are characterized by larger inner PC volumes than passive ones. Increases of both the inner PC volume and the PC surface area can yield increases of the wall heat transfer and thus the efficiency losses. Hence, removing the spark plug to reduce the inner PC volume seems to be a potential solution to decrease the efficiency losses. However, this approach would require a PC fuel that yields an autoignition of the PC charge. In addition, the PC fuel should contain oxygen to reduce the PC-driven HC, CO, and soot emissions. The potential of such a combustion concept was not investigated in literature yet. To this end, the application of controlled autoignition (CAI) inside an active PC was investigated in this study to assess whether the spark plug could potentially be removed. Experimental investigations on a thermodynamic spark-ignition single-cylinder engine with an active PC and a compression ratio of 18.1 were performed. The main combustion chamber was fueled with methanol, whereas diethoxymethane was injected into the PC. In contrast to methanol, diethoxymethane is not knock resistant, enabling the CAI of the PC charge. The applied fuels yielded a reliable CAI of the PC charge at a net indicated mean effective pressure of 12 bar, indicating no significant disadvantages in the combustion process compared to a methanol-fueled PC, which was conventionally ignited by a spark plug. In particular, the same maximum net indicated efficiency of 47% was achieved at a relative air/fuel ratio in the exhaust gas of 2.7. The findings of this study point out the applicability of CAI in an active PC, which provide a basis for redesigning PCs with a specific focus on reducing both the inner PC volume and the surface area. • Applicability proof of controlled autoignition inside an active pre-chamber. • Application of methanol and diethoxymethane as pre-chamber fuels. • Spark-ignition engine with compression ratio of 18.1. • Using end of injection into pre-chamber to control combustion phasing. • No significant disadvantages of controlled autoignition over spark ignition observed.