A recently developed in-cylinder fuel injection probe was used to deposit a small amount of liquid fuel on various surfaces within the combustion chamber of a 4-valve engine that was operating predominately on liquefied petroleum gas (LPG). A fast flame ionization detector (FFID) was used to examine the engine-out emissions of unburned and partially-burned hydrocarbons (HCs). Injector shut-off was used to examine the rate of liquid fuel evaporation. The purpose of these experiments was to provide insights into the HC formation mechanism due to in-cylinder wall wetting. The variables investigated were the effects of engine operating conditions, coolant temperature, in-cylinder wetting location, and the amount of liquid wall wetting.
The results of the steady state tests show that in-cylinder wall wetting is an important source of HC emissions both at idle and at a part load, cruise-type condition. The effects of wetting location present the same trend for idle and part load conditions. Wetting the cylinder liner under the exhaust port yields the highest increase in HC emissions, wetting the cylinder liner under the intake port yields the smallest HC emissions increase, and wetting the top of the piston yields an increase in HC emissions that lies between these two extremes. The coolant temperature has a stronger effect on HCs due to liner wetting for part load than for idle, but little effect for piston wetting for either operating condition. Depositing 10% of the total fuel mass as a liquid on the top of the piston results in approximately 30% and 70% increases in HC emissions for idle and part load conditions, respectively. The percentage increase in HC emissions relative to the LPG-only baseline initially increases rapidly with increasing liquid percentage of the total fuel mass flow but eventually appears to approach a constant value.
The FFID results show that liquid fuel on in-cylinder surfaces produced increased HC concentrations during the entire exhaust process. Wetting the liner under the exhaust valves had a very pronounced effect on the middle peak relative to operation at the same equivalence ratio with a gaseous fuel. Wetting the piston most strongly affected the final peak and also produced a small peak just before the final peak. Wetting the liner under the intake valves had the least effect on the FFID results, increasing the HC concentration of the first peak most, but yielding a somewhat larger percentage increase in the middle peak. Physical explanations for these characteristics are proposed.
Injector shut-off tests indicate that the process of evaporation of liquid fuel from in-cylinder surfaces is slow in comparison to engine cycle times. For the four combinations of operating condition and coolant temperature examined, the 1/e time constant for fuel evaporation from the piston was about 3-6 engine cycles, with the engine continuing to operate on the gaseous fuel after injector shut-off.