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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.

High bandwidth transient data from a multi-cylinder diesel engine operating in a low temperature combustion regime was analyzed to identify and characterize the transient response behaviors primarily responsible for transient emissions of NO and UHC. Numerous different speed and load transients as well as different combustion modes and control strategies were studied to determine how these parameters affect transient performance. Limitations in the transient response of the air system were found to be the largest contributor to transient emissions, although the mechanism by which these limitations affect performance can vary greatly depending on conditions. Analysis of the data shows that transient emissions for low temperature combustion strategies are highly dependent on cycle-to-cycle changes in intake charge conditions. No fundamental difference was observed between the transient processes controlling speed and load changes.