This work evaluates the effect of charge stratification on combustion phasing, rate of heat release and emissions for HCCI combustion. Engine experiments in both optical and traditional single cylinder engines were carried out with PRF50 as fuel. The amount of stratification as well as injection timing of the stratified charge was varied.
It was found that a stratified charge can influence combustion phasing, increasing the stratification amount or late injection timing of the stratified charge leads to an advanced CA50 timing. The NOx emissions follows the CA50 advancement, advanced CA50 timing leads to higher NOx emissions. Correlation between CA50 can also be seen for HC and CO emissions when the injection timing was varied, late injection and thereby advanced CA50 timing leads to both lower HC and CO emissions. This trend can not be seen when the stratification amount was varied, increased stratification amount leads to higher CO emission and for operating condition with late CA50 timing the HC emissions also increase with increasing stratification amount. Optical studies, with high speed CCD camera, show that an increase in stratification leads to poor combustion quality near the cylinder walls, due to leaner mixtures near the cylinder walls and this results in higher HC and CO emissions.
The maximum rate of heat release depends on stratification amount - a larger amount gives a lower rate of heat release but the main heat release is advanced. Varied injection timing results in different phasing of the main heat releases.
The use of charge stratification for HCCI combustion can lead to a larger operating range, due to its effect on combustion phasing and rate of heat release, since the upper load range is partly restricted by too high rates of heat release leading to high pressure oscillations and the lower load to late combustion phasing leading to high cycle-to-cycle variations.