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Post-flame HC Oxidation
During expansion stroke, the hydrocarbons from the quench layer, oil film and crevices diffuse back into the bulk combustion gases. These hydrocarbons that diffuse back into the burned gases oxidize inside the cylinder and in the exhaust depending upon the burned gas temperature and the availability of oxygen. HC levels in the cylinder prior to opening of exhaust valve are 1.5 to 2 times higher than the concentrations in the exhaust. Empirical correlations have been obtained from the experimental data to estimate HC oxidation in the cylinder and exhaust as given below:
where [ ] denotes concentration of reactants in moles per cm3, XHC and XO2 are the mole fractions of HC and O2, respectively, t is time in seconds, temperature T in K and the density (p/RT) is in moles per cm3. From the oxidation rate given by the above expression, oxidation time, tox for a given concentration [HC] can be estimated as below:
As the quench layers on the cylinder walls are thin, HC from these diffuse rapidly into burned gases and get oxidized. When the bulk gas temperatures are higher than 1300-1400 K, the HC oxidize rapidly. The unburned hydrocarbons from the crevices between piston and cylinder expand back into the bulk gases later in the expansion and exhaust strokes when temperatures have fallen below 1300 K. Just before exhaust valve opens, the gas temperatures are generally around 1250 K, but decrease below 1000K after exhaust blows down. Thus, a significantly large fraction of HC emerging from crevices and oil layers may not be oxidized.
It is estimated that about two third of the hydrocarbons stored in the crevices and absorbed in oil film get oxidized inside the cylinder of the conventional gasoline engines under steady state, mid-load and mid-speed engine operation. At lower loads, extent of postflame HC oxidation would be lower.
During postflame oxidation, partial combustion products and intermediate hydrocarbons due to decomposition of fuel molecules are also produced. These compounds are not present in the original fuel and constitute about 50% percent of the exhaust HC.
Oxidation to an extent of up to 45% of HC which leave the cylinder is possible if high enough temperatures are maintained and the oxidation reactions are not quenched by sudden cooling. In the exhaust port and manifold if residence time is of the order of 50 ms or longer and temperatures greater than 1000 K are maintained, significant oxidation
of HC is possible. The engine could be run at stoichiometric or richer mixtures and with retarded spark timing to obtain a high exhaust gas temperature, and calibrated amount of secondary air is injected at the exhaust port to oxidize and reduce HC emissions. This technique was employed on production engines prior to catalytic emission control.
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