Investigation of Post-Flame Oxidation of Unburned Hydrocarbons in Small Engines 2011-01-0141

The post-flame oxidation of unburned hydrocarbons released from the ring-pack crevice was investigated for a small, air-cooled, spark-ignition utility engine. Spark timing sweeps were performed at 50, 75 and 100% load and speeds of 1800, 2400 and 3060 RPM while operating at a 12:1 air-fuel ratio, which is typical for these engines. A global HC consumption rate (GCR) was introduced based on the temporal profile of the mass released from the ring pack; the mass release after CA90 and up to the point where the remainder of the ring pack HC mass is equal to the exhaust HC level was taken as the mass oxidized, and a rate was defined based on this mass and the corresponding crank angle period over which this took place. For all conditions tested, the GCR varied with the spark timing; advanced spark timing gave higher GCR. A threshold temperature, which corresponds to the bulk gas temperature at the end of the post-oxidation process, was found to vary with operating condition, and thus a simple model based on a kinetically defined critical oxidation temperature is insufficient to describe the process. A mixing-controlled model that assumes that the post oxidation is controlled by a mixing rate given by the maximum mass flow rate of the crevice mixture returning to the cylinder was found to describe the data well - a single linear correlation was able to fit the data. A kinetically controlled model that assumes that finite-rate chemistry controls the oxidation process also describe the data well, giving a good linear fit in an Arrhenius-type plot. The relative merits and weaknesses of both post-oxidation models were discussed.