An Experimental Study of a Spark-assisted

Licentiatavhandling, 2006

Future requirements for reductions in fuel consumption and emissions from ground vehicles might be met by using the HCCI combustion concept, in which a more or less homogenous air fuel mixture is compressed to auto-ignition. This consumes less fuel than normal SI combustion and its ability to burn lean mixtures at low temperatures has a positive impact on exhaust emissions. However, there are challenges associated with this concept, for instance its limited operating range and combustion control. This work evaluates the effect of a SI or CI stratified charge on HCCI combustion. The basic idea is to initiate HCCI combustion with a spark ignited stratified lean charge or by staged combustion mediated by charge stratification. To verify that the combustion sequence in experimental systems consisted of flame front combustion followed by HCCI combustion, photographs of OH chemiluminescence or direct images of the combustion were taken. Engine experiments in both optical and traditional single-cylinder engines were carried out with mixtures of n-heptane and iso-octane for the main fuel and mixtures of n-heptane and iso-octane or hydrogen for the stratified charge. It was optically verified that it is possible to combine SI and HCCI combustion, that a stratified charge can result in a staged combustion, and that either an SI or a CI stratified charge can be used to induce HCCI combustion. By varying the injection timing and the amount of charge stratification (either SI or CI) the combustion phasing can be controlled. Staged combustion due to charge stratification can result in reductions in the maximum rate of heat release. The use of charge stratification to induce HCCI combustion can expand the HCCI operating range, due to its effects on combustion phasing and rate of heat release (when the stratified charge is CI), since the upper load range is restricted by excessive rates of heat release, which lead to high pressure oscillations and the lower load to late combustion phasing, which leads to high cycle-to-cycle variations.