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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.

A commercial DOHC 4-Cylinder sequential MPI SI engine was modified as a research single cylinder engine. And four kinds of cylinder head with the same combustion chamber geometry have been used to induce in-cylinder flow of different swirl ratio. To investigate the effect of injection timing on the lean misfire limit (LML), experiments have been made at selected engine speeds for each cylinder head. Fuel injection timing was varied while running the engine at a constant speed. And the LML was defined as the mixture ratio at which engine speed deviates more than 10 rpm from the present speed resulting in the engine instability which might be causing from misfire or partial combustion. Results show that LML or stability of engine is not affected by engine speed because early flame stability is dominated not by turbulence but by AFR around the spark plug at spark timing. Stratification of mixture resulting from port swirl and injection timing were shown to govern the LML.