Study on micro-flame ignited (MFI) hybrid combustion characteristics of a dual-fuel optical engine at different lambdas

摘要

Ultra-lean burn combustion can effectively improve fuel economy of gasoline engines. But its application to real gasoline engines is impeded by unstable and slow combustion. Micro-flame ignited (MFI) hybrid combustion mode can be one solution. To understand the relation between heat release processes and flame features of ultra-lean gasoline-air mixture with dimethyl ether (DME) auto-ignition in MFI combustion mode, experiments were conducted in an optical engine at the lambdas from 1.8 to 2.2. The results show that the misfire limit in MFI mode can be extended to lambda 2.2. However, lambda 2.0 is a proper lean-burn limit for MFI combustion. The combustion process includes the oxidation of DME and the hybrid combustion of DME and gasoline at different lambdas while it still experiences the gasoline auto-ignition at lambda 1.8. With the increase of lambda, the ignition timing delays and combustion duration prolongs, leading to decreased indicated mean effective pressure and increased cyclic variation. The decreased stretch-rate and increased speed decay rate of blue and yellow flame can improve combustion stability. Lower stretch-rate and higher speed decay rate derived from yellow flame relative to that from blue flame contributes to the extension of lean-burn limit in MFI combustion mode.