Low-Load Limit in a Diesel-Ignited Gas Engine

The lean-burn capability of the Diesel-ignited gas engine combined with its potential for high efficiency and low CO2 emissions makes this engine concept one of the most promising alternative fuel converters for passenger cars. Instead of using a spark plug, the ignition relies on the compression-ignited Diesel fuel providing ignition centers for the homogeneous air-gas mixture. In this study the amount of Diesel is reduced to the minimum amount required for the desired ignition. The low-load operation of such an engine is known to be challenging, as hydrocarbon (HC) emissions rise. The objective of this study is to develop optimal low-load operation strategies for the input variables equivalence ratio and exhaust gas recirculation (EGR) rate. A physical engine model helps to investigate three important limitations, namely maximum acceptable HC emissions, minimal CO2 reduction, and minimal exhaust gas temperature. An important finding is the fact that the high HC emissions under low-load and lean conditions are a consequence of the inability to raise the gas equivalence ratio resulting in a poor flame propagation. The simulations on the various low-load strategies reveal the conflicting demand of lean combustion with low CO2 emissions and stoichiometric operation with low HC emissions, as well as the minimal feasible dual-fuel load of 3.2 bar brake mean effective pressure.