Effects of Fuel Volatility, Load, and Speed on HC Emissions Due to Piston Wetting 2001-01-2024

Piston wetting can be isolated from the other sources of HC emissions from DISI engines by operating the engine predominantly on a gaseous fuel and using an injector probe to impact a small amount of liquid fuel on the piston top. This results in a marked increase in HC emissions. In a previous study, we used a variety of pure liquid hydrocarbon fuels to examine the influence of fuel volatility and structure on the HC emissions due to piston wetting. It was shown that the HC emissions correspond to the Leidenfrost effect: fuels with very low boiling points yield high HCs and those with a boiling point near or above the piston temperature produce much lower HCs. All of these prior tests of fuel effects were performed at a single operating condition: the Ford World Wide Mapping Point (WWMP). In the present study, the effects of load and engine speed are examined. Four different normal alkanes were used, including one that appears to be near the Leidenfrost point for operation at the WWMP, one that is near the Nukiyama point, and one that appears to be in the transition region. The exhaust hydrocarbons were speciated to differentiate between the emissions resulting from the gaseous fuel and those resulting from the liquid fuel. It is shown that the “Piston Wetting Emissions Index” for engine-out THCs increases with both decreasing speed and decreasing load, and that this is primarily an effect of oxidation kinetics. Speed and load have opposite effects on unburned fuel emissions, and this appears to be a pressure effect. For all speeds and loads the Leidenfrost effect appears to be important: the most volatile fuel has the highest THC and UBF emissions whereas the two least volatile fuels have lower emissions and the fuel that is within the transition regime yields intermediate emissions.