Module 2:Genesis and Mechanism of Formation of Engine Emissions
Lecture 8:Mechanisms of HC Formation in SI Engines
HC Emissions from Wall Quenching
Single wall quench layer thickness typically varies from 0.05 to 0.1 mm. It decreases with increase of engine load as higher wall temperature results at higher engine loads, which reduces heat loss to the walls from the reaction zone, and consequently a smaller quench layer thickness is obtained. However, at top dead centre the surface to volume ratio of the combustion chamber is at its maximum and at this point the wall quench layer may comprise of 0.1 to 0.2 percent of the total charge inducted into the cylinder.
Studies on combustion of pre-mixed fuel air mixtures in combustion bombs show that when all the crevices in the bomb are eliminated by filling with solid material, unburned HC concentrations were just about 10 ppmC only. Such low concentrations result as after flame quenching the hydrocarbons in the quench layer thickness on the single walls diffuse in the hot burned gas quite early and get oxidized. Typically, most hydrocarbons would get oxidized on diffusion in the high temperature burned gases within 2-3 milliseconds of the flame quench. These studies showed that the contribution of single wall quench layers to the total unburned HC emission is quite small.
Crevice HC
Crevices in the combustion chamber are narrow regions into which fuel-air mixture can flow but flame cannot propagate due to their high surface to volume ratio causing high heat transfer rates to walls.
The largest crevice in the combustion chamber is between cylinder wall and piston top land, and second land.
Other crevices present are along the gasket between cylinder head and block, around intake and exhaust valve seats, threads around spark plug and space around the central electrode of the spark plug.
Piston – ring - cylinder crevice is shown schematically in Fig. 2.12.Table 2.3 gives typical volumes contained in different the crevice regions in the cylinder of a production engine. Total crevice volume is about 3 to 5 percent of the clearance volume and the piston and cylinder crevice constitutes around 70 to 80 percent of the total crevice volume.
