Initial Flame Development under Fuel Stratified Conditions

Initial flame development and propagation were visualized under different stratified conditions to estimate the effects of stratification on the engine performance in a port injection SI engine. Experiments were performed in an optical single cylinder engine modified from a production engine and images were captured through the quartz window mounted in the piston by an intensified CCD camera. Four cylinder heads, which have different intake ports, were assembled on the cylinder block to vary port swirl ratios. Stratification was controlled by the combination of the port swirl ratio and injection timing. Under each condition, flame images were captured at the pre-set crank angles. These were averaged and processed to characterize the flames. The flame stability was estimated by the weighted average of flame area and luminosity.

It is known that initial flame stability and flame development were strongly dependent on the stratified conditions and the initial flame stability governed the engine stability and lean misfire limits. Port-induced air motion was decayed in the combustion chamber at the end of the compression stroke and the propagating direction of initial flame was affected by the tumble flow induced in the squish area.