factors affecting the flame propagation in S.I. engines

1. Fuel-air ratio : The composition of the working mixture influences the rate of combustion and the amount of heat evolved. With hidrocarbon fuels the maximum flame velocities occur when mixture strength is about 10% rich than stoichiometric. The velocity of flame decreases when the mixture is made leaner or is enriched still more than 10%.

2.  Compression ratio : A higher compression ratio increases the pressure and temperature of the working mixture and decreases the concentration of residual gases. These favourable conditions reduce the ignition lag of combustion. The increase in compression ratio results in increase in temperature which increases the tendency of the engine to detonate.

3.  Intake temperature and pressure: Increase in intake temperature and pressure increases the flame speed.

4.  Engine load : With increase in engine load the cycle pressures increase. Hence the flame speed increases.

5.  Turbulence : The turbulent motion of the mixture intensifies the processes of heat transfer and mixing of the burned and unburned portions in the flame front. These two factors cause the velocity of turbulent flame to increase.

6.  Engine speed : The higher engine speed increases turbulence inside the cylinder. This increases the speed of flame propagation.

7.  Engine size: The speed of flame propagation is smaller in small engines as compared to large size engines.