Effect of engine variables on Ignition lag
1) Fuel- it is depend on chemical nature of fuel. The higher, the self-ignition temp of fuel, the longer will be the ignition lag.
2) Mixture Ratio-The ignition lag is smallest for the mixture ratio which gives the maximum temperature this mixture ratio is somewhat richer than the stoichiometric ratio.
3) Initial pressure and temperature –increasing the intake temp, pressure, compression ratio and retarding spark, all reduce the ignition lag.
4) Electrode gap-It affects establishment of the nucleus of flame. If the gap is too small, quenching of the flame nucleus may occur & rang of fuel –air ratio for the development of a flame nucleus is reduced.
5) Compression ratio-An increase in the compression ratio can reduce the ignition lag.
6) Turbulence-Ignition lag is not much affected by turbulence intensity. Turbulence is directly proportional to engine speed.
Effects of engine variables of Flame propagation
1) Fuel-Air ratio-Maximum flame velocities occur when mixture strength is 10% of stoichiometric. Lean mixtures release less thermal energy resulting in lower flame temperature & flame speed. Very rich mixtures have incomplete combustion (Some carbon only burns to CO & not to CO2), which results in production of less thermal energy & hence flame speed is again low.
2) Compression ratio-A higher compression ratio increases the pressure & temperature of the working mixture & decrease the concentration of residual gases. High pressures & temperature of the compressed mixture also speed up the flame propagation.
3) Intake temp & pressure-Increase in the intake temperature & pressure increases the flame speed.
4) Engine load-With increases in the engine load the cycle pressures increase. Hence the flame speed increases.
5) Turbulence-The flame speed is very low in non-turbulent mixtures. A turbulent motion of the mixture intensifies the processes of heat transfer & mixing of the burned & unburned portions in the flame front (diffusion). These two factors cause the velocity of turbulent flame to increase practically in proportion to the turbulence velocity. However, excessive turbulence is also undesirable.
6) Engine speed - The higher the engine speed the greater the turbulence inside the cylinder. For this reason the flame speed increases almost linearly with engine speed. The crank angle required for the flame propagation, which is the main phase of combustion, will remain almost constant at all speed.
7) Engine size- Engine of similar design generally run at the same piston speed. This is achieved by smaller engine having larger RPM & larger engines having smaller RPM. Due to the same piston speed. The inlet velocity, the degree of turbulence & the 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.