Effects of Injection Timing on the Lean Misfire Limit in an SI Engine 970028
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.
Fuel has been changed to CNG(Compressed Natural Gas) to see the presumed effect of the presence of liquid droplet and 2-phase flow. Visualization has been made for gaseous and liquid fuel injection. Lean misfire limit variation of gas and liquid fuels are similar in macro view scope, but shows difference in detailed patterns and shifted tendencies because of different penetration velocity and probably the two-phase flow effects.
To understand the stratification process, Laser Rayleigh Scattering(LRS) measurement of fuel concentration in the steady flow rig was made. Mixing process of fuel and air depends mainly on macro intake air flow motion i.e. convection. Stratification is governed by swirl ratio and injection timing, and the peripherical distribution of fuel owing to strong swirl is very important for axial stratification.