The Chemistry of Fuel Combustion in SI Engines

The Chemistry of Fuel Combustion in SI Engines P M V Subbarao Professor Mechanical Engineering Department Exploit the Chemical Characteristics of Combustion? !? !

Flame Propagation in SI Engines

Model for Laminar Flame Profile in SI Engines

Laminar burning velocity • Nearly all real SI engine designs, require laminar burning velocity data for the air/fuel/residual mixture at the instantaneous in-cylinder pressure and unburned gas temperature. • Models assume the local burning to take place in a laminar fashion (flamelet type models) with constant area. • Data on the stretched laminar burning velocity is to be predicted. • This implies the need for either a library of stretched flamelets or a model for the effect of stretch rate.

Multizone model for Estimation of Laminar Burning Velocity • The burning velocity calculations are made, based on using a spherical closed vessel filled with homogeneous combustible mixture. At time t = 0, the mixture is ignited at the center; a spherical flame front is established instantaneously and begins to propagate outwardly to reach the wall. The following conditions are used in the analysis of the behavior of the system during this interval.

Conditions for Estimation of Laminar Burning Velocity 1. Prior to the onset of ignition, the unburned gas is initially at rest and the gas has a uniform temperature and composition. 2. The thickness of the reaction zone is negligible, and the flame front is smooth and spherical. 3. The burnt gas fraction x is at local thermodynamic and chemical equilibrium. 4. The unburned gas fraction (1 − x) is at local equilibrium but with fixed (original) chemical composition. 5. The pressure within the combustion vessel is a function of time only and is independent of the flame front. 6. The compression path of both the unburned and the burned gas is adiabatic and reversible.

1. The gas behaves as a semi-perfect gas. 2. The effects of body forces and of radiative energy transfer can be neglected. 3. There is no heat transfer between the burned and the unburned gases.

Equation for Lminar burning velocity as

Effect of Equivalence Ration on Laminar Burning Velocity

Effect of Burned Gas Mole Fraction on SL

Effect of Gas Temperature on SL

Effect of Pressure on SL

Empirical Correlations to Select Combustion Parameters T in K & p in atm. f SL 0 f f

Heywood Constants

Correlations for Turbulent Burning Velocity • Groff and Matekunas obtained a relation that takes into account the effect of spark timing on the flame speed ratio. • They obtained the following relation for flame speed ratio • ST/SL = 2. 00 + 1. 21(u´/SL)(p/pm)0. 82 s • Where p is the pressure, k. Pa • pm is the corresponding motoring pressure, k. Pa • s is the spark advance factor given by • s = 1. 0 + 0. 05 0. 4 • where is the spark advance, crank angle degrees before top dead centre.

Selection of Spark Timing • Spark timing relative to TC affects the pressure development and thus the engine imep and power. • Ignite the gas before TC to center the pressure pulse around TC. • The overall burning angle is typically between 40 to 60 o, depending on engine speed. motored