A Numerical Investigation on Effects of Charge Stratification on HCCI Combustion

A fully coupled multi-dimensional CFD and reduced chemical kinetics model is adopted to investigate the effects of charge stratification on HCCI combustion and emissions. Seven different kinds of imposed stratification have been introduced according to the position of the maximal local fuel/air equivalence ratio in the cylinder at intake valve close. The results show that: The charge stratification results in stratification of the in-cylinder temperature. The former four kinds of stratification, whose maximal local equivalence ratios at intake valve close locate between the cylinder center and half of the cylinder radius, advance ignition timing, reduce the pressure-rise rate, and retard combustion-phasing. But the following three kinds of stratification, whose maximal local equivalence ratios at intake valve close appear between half of the cylinder radius and the cylinder wall, have little effect on the cylinder pressure. For the two discussed equivalence ratios, all kinds of stratification can reduce unburned fuel emissions; all kinds of stratification can reduce formaldehyde emissions when the mixture is lean and only the last two kinds of stratification can reduce formaldehyde emissions when the mixture is comparatively rich; the former four kinds of stratification deteriorate the CO and NOx emissions and the last two kinds of stratification can reduce unburned fuel, formaldehyde and CO emissions and maintain low NOx emissions simultaneously.

Key words: n-heptane, multi-dimensional, stratification, emissions