Liquid Film Evaporation Off the Piston of a Direct Injection Gasoline Engine

An optical access engine was used to image the liquid film evaporation off the piston of a simulated direct injected gasoline engine. A directional injector probe was used to inject liquid fuel (gasoline, i-octane and n-pentane) directly onto the piston of an engine primarily fueled on propane. The engine was run at idle conditions (750 RPM and closed throttle) and at the Ford World Wide Mapping Point (1500 RPM and 262 kPa BMEP). Mie scattering images show the liquid exiting the injector probe as a stream and directly impacting the piston top. Schlieren imaging was used to show the fuel vaporizing off the piston top late in the expansion stroke and during the exhaust stroke. Previous emissions tests showed that the presence of liquid fuel on in-cylinder surfaces increases engine-out hydrocarbon emissions. Fuel injector shutoff tests using a wide-range lambda sensor and the Fast-Spec system demonstrated that the effect of the liquid fuel was seen for several cycles after liquid fuel injection was terminated. Combining these hydrocarbon emissions data with the images from a geometrically similar optical engine showed that the evaporation of liquid fuel from in-cylinder surfaces is a slow process, much of which occurs after the in-cylinder gas temperature has dropped below that required for post-flame oxidation.