Influence of alternative pre-chamber fuels on engine performance

The application of active pre-chamber (PC) combustion systems offers potential increases of the indicated efficiency of spark-ignition (SI) engines by increasing the lean operating limit. In conventional SI operation, methanol revealed promising engine performance without substantial knock limitation. In PC operation with methanol under lean conditions, the PC jets can fail to ignite the charge in the main combustion chamber (MC) due to methanol’s high enthalpy of vaporization. The PC jet characteristics in turn depend on the applied PC fuel. In this study, it was assumed that the application of alternative liquid PC fuels might extend the lean operating limit of an engine that was operated with methanol as MC fuel. To this end, methanol, ethanol, 2-butanone, ethyl acetate, 1,3-dioxolane, and diethoxymethane were investigated as PC fuels in this study, while the MC was fueled with methanol. The fuels were experimentally investigated on a thermodynamic SI single-cylinder engine equipped with an active PC and a compression ratio of 16.4. Variations of the relative air/fuel ratio in the exhaust gas at net indicated mean effective pressures of 6 bar and 15 bar were performed. The observed differences in the engine performance between the investigated PC fuels were minor because a large fraction of the PC mixture consisted of the MC fuel methanol that was forced into the PC during the compression stroke. However, measured PC-driven HC emissions depended on the applied PC fuel and increased toward the engine’s lean operating limit at low engine load. At high engine load, this behavior was not observed. In PC operation, a maximum net indicated efficiency of 47.3% was observed, which was 0.8%-points higher than in conventional SI operation.