Storage and Partial Oxidation of Unburned Hydrocarbons in Spark-Ignited Engines - Effect of Compression Ratio and Spark Timing

The concentrations of the individual hydrocarbon species in the exhaust gases from a single-cylinder CFR engine have been measured. The effects of compression ratio and spark timing on these emissions have been observed using propane, isooctane (2,2,4-trimethylpentanerpar; and indolene clear fuels. The results show that total hydrocarbon emissions increase rapidly as the compression ratio increases and decrease sharply as the spark timing is retarded relative to MBT at a constant compression ratio. Changes in the concentration of unburned fuel in the exhaust contribute the most to these variations. These observations are consistent with the predicted increase in crevice and oil film storage of unburned hydrocarbons at higher compression ratio because of the higher peak pressures. They also agree with the predicted increase in post-flame burnup of stored hydrocarbons in the hotter cylinder gases encountered during low compression ratio or retarded spark operation.