抄録
Owing to continuing air pollution problems, stringent regulations are being enforced to reduce unburned hydrocarbon (HC) emissions from spark ignition engines. A number of attempts have been reported on the sources of HC emissions. The crevices are a major source of unburned HC emissions in spark ignition engines. The largest of crevice regions is the piston-ring crevice, however head-gasket, spark plug, and valve seat crevices are not insignificant. After the end of primary engine combustion, some of unburned HCs from crevices are oxidized upon mixing with hot burned gases during the expansion and exhaust processes. The others are emitted to the exhaust port or retained in the cylinder with the residual gases to be recycled in the next cycle. Some of unburned HCs that leave the cylinder are oxidized at the exhaust port. The oxidation process is considered as the dominant HC removal process during engine start-up and warm-up periods. Therefore, quantifying the HC oxidation in the cylinder is potentially important, but a systematic investigation of the HC oxidation has not been conducted due to the difficulties and limitations of engine experiments. A 3-dimensional simulation was developed to predict the oxidation rate of unburned HCs in combustion chamber of a propane-fueled spark ignition engine with consideration of flow, mixing, and heat transfer. The computational moving mesh with the piston and head-gasket crevices was constructed for a commercial 4-valve spark ignition engine. A FAE premixed turbulent combustion model and a flame wall quenching model were applied to simulate flame propagation. In order to predict the unburned HC oxidation, a 4-step oxidation model was used. The mixing, oxidation, and exhausting of unburned HCs are examined, and the relative importance of different crevices was studied. 68.1% of fuel from the crevices are oxidized during the expansion and exhaust processes, and ethylene corresponding 17.8% of oxidized fuel are produced by the fuel oxidation. Head-gasket crevice HCs are exhausted early from blowdown period, on the other side, highly concentrated piston crevice HCs are mainly emitted in the end of the exhaust process. Different locations of crevices influence the oxidation rates, exhaust timings and exhaust degrees. The THC oxidation rate of the piston and the head-gasket crevices are 61.3% and 68.8% respectively. Consequently, the piston crevice contributes to 82.4% and head-gasket crevice does to 17.6% of the engine-out THC emissions relatively. The relative importance of different crevices must be evaluated by considering their shapes, positions, movements, exhaust processes, and engine operating condition as well as volumes, even though crevice size is considered as the most important respect.
