Combustion Characterization in a Direct-Injection Stratified-Charge Engine and Implications on Hydrocarbon Emissions

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Combustion Characterization in a Direct-Injection Stratified-Charge Engine and Implications on Hydrocarbon Emissions

Heywood, John B. / Frank, Richard M.

An experimental study was conducted on a direct-injection stratified-charge (DISC) engine incorporating a combustion process similar to the Texaco Controlled Combustion System and operated with gasoline. Analysis of the injected fuel flow and the heat release showed that the combustion process was characterized by three distinct phases: fuel injection and distribution around the piston bowl, flame propagation through the stratified fuel-air mixture, and mixing-controlled burn-out with the heat-release rate proportional to the amount of unburned fuel in the combustion chamber. This characterization was consistent with previous visualization studies conducted on rapid-compression machines with similar configurations.Experiments with varied injection timing, spark plug location, and spark timing showed that the combustion timing relative to injection was critical to the hydrocarbon emissions from the engine. Thus, overleaning of the fuel mixture before burning was identified as a major source of hydrocarbon emissions from the engine at low equivalence ratios. The inflammation phase was found to be particularly delicate at low loads, resulting in increased hydrocarbon emissions due to either delayed flame-kernel development near the spark plug, or slow and/or incomplete flame propagation through the fuel-air mixture. Cycle-resolved HC emissions measurements were used to show that late burn cycles had significantly higher HC emissions levels than cycles with normal combustion durations. The results showed that for a stratified-charge engine to have low hydrocarbon emissions at low equivalence ratios, the residence time of the fuel in the combustion chamber before burning must be minimized.