The Effect of In-Cylinder Wall Wetting Location on the HC Emissions from SI Engines

The effect of combustion chamber wall-wetting on the emissions of unburned and partially-burned hydrocarbons (HCs) from gasoline-fueled SI engines was investigated experimentally. A spark-plug mounted directional injection probe was developed to study the fate of liquid fuel which impinges on different surfaces of the combustion chamber, and to quantify its contribution to the HC emissions from direct-injected (DI) and port-fuel injected (PFI) engines. With this probe, a controlled amount of liquid fuel was deposited on a given location within the combustion chamber at a desired crank angle while the engine was operated on pre-mixed LPG. Thus, with this technique, the HC emissions due to in-cylinder wall wetting were studied independently of all other HC sources.

Results from these tests show that the location where liquid fuel impinges on the combustion chamber has a very important effect on the resulting HC emissions. The highest HC emissions resulted from fuel impingement on the exhaust-side of the cylinder liner, and wetting on the piston-top was second in importance. The HC emissions due to this mechanism were found to be insensitive to the timing of the injection and to the coolant temperature. This surprising result is indicative of low vaporization rates for the liquid gasoline on the combustion chamber surfaces. The results presented here indicate that the impingement of liquid fuel on the piston top and cylinder liner is an important source of HC emissions from direct-injection spark-ignition (DISI) engines, and that this mechanism is at least partially responsible for their high HC emissions.