The role of charge stratification for reducing ringing in gasoline engine homogeneous charge compression ignition combustion investigated by optical imaging

Homogeneous charge compression ignition offers the possibility to reduce the fuel consumption of gasoline passenger car engines. However, the combustion strategy is limited to low loads due to pressure oscillations at higher loads. A strategy for extending the homogeneous charge compression ignition load range is charge stratification, using, for example, late direct injection to prolong the combustion duration and reduce the rate of pressure rises and pressure oscillations. In this study, local temperatures and fuel concentrations near top dead centre in a gasoline engine operating in homogeneous charge compression ignition mode were measured using two-wavelength planar laser-induced fluorescence, and the following combustion was analysed using high-speed video to investigate the effects of fuel and temperature stratification on combustion in order to explain the ringing inhibiting effect of charge stratification for fuels displaying single-stage ignition. The extent of spatial distribution of combustion timing correlated well with the extent of fuel and temperature stratification. Furthermore, the gas was leaner and hotter in early igniting regions, while it was richer and colder in late igniting regions. The dampening effects of charge stratification on the combustion speed and pressure oscillations are probably due to rich conditions in the latest burning regions (where combustion is usually most intense) slowing down combustion, which explains why the strategy only works when the global air-to-fuel ratio is not excessively lean