Liquid fuel impingement on in-cylinder surfaces as a source of hydrocarbonemissions from direct injection gasoline engines

Hydrocarbon (HC) emissions from direct injection gasoline (DIG) engines are significantly higher than those from comparable port fuel injected engines , especially when ''late'' direct injection (injection during the compressi on stroke) is used to produce a fuel economy benefit via unthrottled lean o peration. The sources of engine-out hydrocarbon emissions for late direct i njection are bulk flame quench, low temperatures for postcombustion oxidati on, and fuel impingement on in-cylinder walls. An experimental technique ha s been developed that isolates the wall impingement source from the other s ources of HC emissions from DIG engines. A series of steady-state and trans ient experiments is reported for which the HC emissions due to operation wi th a premixed charge using a gaseous fuel are compared to those when a smal l amount of liquid fuel is injected onto an in-cylinder surface and the gas eous fuel flow rate is decreased correspondingly The steady-state experimen ts show that wetting any in-cylinder surface dramatically increases HC emis sions compared to homogeneous charge operation with a gaseous fuel. The res ults of the transient fuel injection interrupt tests indicate that liquid-p hase gasoline can survive within the cylinder of a fully warmed-up firing e ngine and that liquid fuel vaporization is slower than current computationa l models predict. This work supports the argument that HC emissions from DI G engines cart be decreased by reducing the amount of liquid fuel that impi nges on the cylinder liner and piston, and by improving the vaporization ra te of the fuel that is deposited on these surfaces.