1995-10-01

Crevice Flow and Combustion Visualization in a Direct-Injection Spark-Ignition Engine Using Laser Imaging Techniques 952454

Crevice flows of hydrocarbon fuel (both liquid and vapor) have been observed directly from fuel-injector mounting and nozzle-exit crevices in an optically-accessible single-cylinder direct-injection two-stroke engine burning commercial gasoline. Fuel trapped in crevices escapes combustion during the high-pressure portions of the engine cycle, exits the crevice as the cylinder pressure decreases, partially reacts when mixed with hot combustion gases in the cylinder, and contributes to unburned hydrocarbon emissions. High-speed laser Mie-scattering imaging reveals substantial liquid crevice flow in a cold engine at light load, decreasing as the engine warms up and as load is increased. Single-shot laser induced fluorescence imaging of fuel (both vapor and liquid) shows that substantial fuel vapor emanates from fuel injector crevices during every engine cycle and for all operating conditions. Early in the crevice-flow process, some of the emerging fuel vapor (imaged by laser-induced fluorescence) burns as a rich diffusion flame (imaged by flame luminosity), but most of the crevice flow fails to burn as the cylinder pressure and temperature fall. Crevice HC's are a significant (but not the predominant) source of hydrocarbon emissions in this two-stroke engine, since most of the crevice flow hydrocarbons are retained as residual fuel in the combustion chamber. Similar laser-imaging techniques are applicable to four-stroke spark-ignition engines, where crevice flows are believed to be the dominant hydrocarbon-emissions source.

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