(i) Cylinder size - Engine of similar design generally run at the same speed. This is achieved by smaller engine having larger RPM and larger engines having smaller RPM. Due to the same piston speed, the inlet velocity, the degree of turbulence and flame speed are nearly same in similar engines regardless of the size. i.e. the number of crank degrees required for flame travel will be about the same irrespective of engine size, provided the engine are similar.
(ii) Cylinder wall temperature- As the temperature of cylinder wall increases it reduces ignition lag & flame propagation will increases.
iii) Compression ratioA higher compression ratio increases the pressure and temperature of the working mixture and decreases the concentration of the residual gases. These favorable conditions reduce the ignition lag of combustion. High pressure and temperature of the compressed mixture speeds up the flame propagation.
(iv) Inlet air temperature- Increasing the intake temperature and pressure increasing the compression ratio and retarding the spark, all reduces the ignition lag. Pre-increase in the intake temperature and pressure increases the flame speed.