Effect of in-cylinder liquid fuel films on engine-out unburned hydrocarbon emissions for SI engines

Abstract

Nearly all of the hydrocarbon emissions from a modern gasoline-fueled vehicle occur when the engine is first started. One important contributing factor to this is the fact that, during this time, temperatures throughout the engine are low - below the point at which all of the components of the gasoline can readily vaporize. Consequently, any fuel that enters the combustion chamber in liquid form can escape combustion and subsequently be exhausted as hydrocarbon emissions. An experimental study was performed in a firing engine in which liquid gasoline films were established at various locations in the combustion chamber and the resulting impact on hydrocarbon emissions was assessed. Unique about this setup was that it combined direct visual observation of the liquid fuel films, measurements of the temperatures these films were subjected to, and the determination from gas analyzers of burned and unburned fuel quantities - all with cycle-level or better resolution. An increase in the hydrocarbon emissions was observed with liquid gasoline films present in the combustion chamber. This increase depended upon both the location of the film and the temperature of that location, and correlated with estimates of the mass of fuel in the film. The largest impact was observed when the head near the exhaust valve was wetted; the smallest impact was observed when the piston on the intake side of the engine was wetted. In general, as engine temperatures increased the hydrocarbon emissions due to the liquid fuel films decreased. It was also identified when, in the exhaust event, fuel from the films was actually exhausted. The effect of the location of the liquid fuel film can best be understood in terms of the time before flame arrival at that location, the local flow over the film, and the extent to which the overall flow in the combustion chamber carries fuel from the film to the exhaust valve. The primary effect of wall temperature is to affect the amount of vaporization from the film: as temperature increases more vaporization occurs before flame arrival, resulting in less fuel that can vaporize post-flame as unburned fuel emissions.