Liquid Fuel Impingement on In-Cylinder Surfaces as a Source of Hydrocarbon Emissions From Direct Injection Gasoline Engines

Abstract

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 compression stroke) is used to produce a fuel economy benefit via unthrottled lean operation. The sources of engine-out hydrocarbon emissions for late direct injection are bulk flame quench, low temperatures for post-combustion oxidation, and fuel impingement on in-cylinder walls. An experimental technique has been developed that isolates the wall impingement source from the other sources of HC emissions from DIG engines. A series of steady-state and transient experiments is reported for which the HC emissions due to operation with a premixed charge using a gaseous fuel are compared to those when a small amount of liquid fuel is injected onto an in-cylinder surface and the gaseous fuel flow rate is decreased correspondingly. The steady-state experiments show that wetting any in-cylinder surface dramatically increases HC emissions compared to homogeneous charge operation with a gaseous fuel. The results of the transient fuel injection interrupt tests indicate that liquid-phase gasoline can survive within the cylinder of a fully warmed-up firing engine and that liquid fuel vaporization is slower than current computational models predict. This work supports the argument that HC emissions from DIG engines can be decreased by reducing the amount of liquid fuel that impinges on the cylinder liner and piston, and by improving the vaporization rate of the fuel that is deposited on these surfaces.