Liquid Fuel Effects on the Unburned Hydrocarbon Emissions of a Small Engine

The effect of the presence of liquid fuel in the intake manifold on unburned hydrocarbon (HC) emissions of a spark-ignited, carbureted, air-cooled V-twin engine was studied. To isolate liquid fuel effects due to the poor atomization and vaporization of the fuel when using a carburetor, a specially conditioned homogeneous, pre-vaporized mixture system was developed. The homogeneous mixture system (HMS) consisted of an air assisted fuel injection system located approximately 1 meter upstream of the intake valves. The results from carburetor and HMS are compared. To verify the existence of liquid fuel in the manifold, and to obtain an estimate of its mass, a carburetor-mounted liquid fuel injection (CMLFI) system was also implemented. The conditions tested were 10% and 25% load at 1750 RPM, and 25%, 50%, and 100% at 3060 RPM. The results of the comparison show that the liquid fuel in the intake manifold does not have a statistically significant influence on the averaged HC emissions. In addition, the cycle-resolved HC emissions for both systems follow the same trends and are comparable in magnitude. Heat release analysis showed little difference between fuel mixture delivery system. These results suggest that under steady state operation the HC emissions for this engine are not sensitive to the presence of liquid fuel in the intake manifold. Stop injection tests performed with the CMFLI system show that 60-80 cycles worth of liquid fuel is held in the intake manifold depending on operating condition.