A high-swirl, low Compression Ratio (CR), optically accessed engine that was able to produce a stratified charge was used to investigate the differences in HCCI combustion and in the propagation of the autoignition front between a non-stratified and a stratified charge. Furthermore the relevance of charge stratifying an engine using variable injection timing with large temperature inhomogeneities was investigated.
The CHEMKIN code and a detailed reaction mechanism were used to simulate the fuel chemistry of ignition and combustion in a low CR engine. The aim of the simulation was to quantify the effect of initial mixture temperature, Ti and A/F ratio on cool flame and main ignition timing and to evaluate the possibility of charge stratifying our engine.
To visualise HCCI combustion, the propagation of the autoignition front and variations in the location of autoignition between closed-valve injection timing (leading to a weakly stratified charge) and open-valve injection timing (leading to a strongly stratified charge), natural-light images were acquired using a fast camera. Temperature inhomogeneities were introduced by utilising a camshaft which allowed 40% Internal Exhaust Gas Recirculation (IEGR) by residual gas trapping.
Computational results show that both temperature and A/F ratio affect the timing of cool flame and main ignition and the peak temperature and pressure of HCCI combustion. However, in the presence of temperature inhomogeneities, the effect of A/F ratio variations is minimal. However, when preheating the inlet air (i.e. uniform temperature before fuel injection), fuel stratification can offer the possibility of controlling HCCI combustion.
Experimental results show, that with closed-valve injection timing, HCCI combustion led to a higher peak pressure and lesser variability in the maximum peak pressure, than with open-valve injection timing under the same conditions. Furthermore, in the case without EGR and closed-valve injection timing, autoignition started under the primary intake valve near the cylinder wall, while in the case without EGR and open-valve injection timing, autoignition started between the exhaust-valve and the secondary intake valve, closer to the centre of the piston. With 40% IEGR and closed-valve injection timing, autoignition started between the exhaust-valve and the primary intake valve near the cylinder wall. These differences can be explained by the difference in location of hot gases due to the injection timing or due to IEGR. Finally, without EGR, a “uniform” autoignition front of HCCI combustion from the original sites of autoignition was observed compared to a more “random development” of the autoignition front with 40% IEGR. Strong local inhomogeneities - possibly a very rich mixture at a low-temperature - could be observed with 40% IEGR.