Hydrocarbon Oxidation in the Exhaust Port of a Spark Ignition Engine

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Hydrocarbon Oxidation in the Exhaust Port of a Spark Ignition Engine

Heywood, John B. / Mendillo, John V.

An experimental study was performed to determine the fraction of unburned hydrocarbon emissions which are oxidized in the exhaust port of a spark ignition engine. The technique used was injection of a CO2 quench gas into the exhaust port, at various planes along the port centerline, to cool the exhaust gas and “freeze” the hydrocarbon oxidation reactions. By quenching the reaction of hydrocarbons at the cylinder exit plane, cylinder exit hydrocarbon emissions levels were determined. By differencing the concentration of hydrocarbons observed during quenching and nonquenching operation, the fraction of hydrocarbons reacting in each section of the exhaust was determined.Experiments were carried out to define the effects of engine operating conditions on the fraction of hydrocarbons oxidized. The variables studied were fuel-air equivalence ratio, speed, load, spark advance and compression ratio. The results of the experimental parametric study were correlated with those predicted by an existing exhaust port oxidation model.The results show that reductions in hydrocarbon level due to oxidation in the exhaust port range between 2 and 37%. Those variables that cause substantial changes in gas temperature, port residence time, or oxygen concentration were most significant. Cylinder exit hydrocarbon emissions levels and the extent of reaction within the exhaust port for a range of fuel composition were also determined.A set of experiments with air injection into the exhaust, at the cylinder exit plane, showed that substantial additional oxidation of hydrocarbons can be obtained with injection into the exhaust gases before significant cooling occurs.