The effect of the liquid fuel in the intake manifold, the ring pack crevices and the oil film on the unburned hydrocarbon (HC) emissions of a spark-ignited, carbureted, air-cooled V-twin engine was studied. Tests were performed for a range of engine load, two engine speeds, various air-fuel ratio and with a fixed ignition timing. For the liquid fuel study, stop fuel injection tests were performed to estimate the mass of liquid fuel in the intake manifold. The results indicated the presence of up to 60-80 cycles worth of fuel. The steady state HC emissions comparison of a pre-vaporized homogeneous mixture system (no liquid fuel) with the carburetor (with presence of liquid fuel) showed that the HC emissions were not affected by the presence of liquid fuel. These results were supported by the cycle-resolved HC emissions and the heat release analysis comparison that showed little difference between both fuel systems, as well. The ring pack contribution to the engine-out HC emissions was estimated using a simplified ring pack gas flow model. The results show that at high load the ring pack crevices are the major contributor to the HC emissions, however at low load the ring pack is not the dominant source of hydrocarbons. It was found that at light load the incomplete combustion is likely the main source of HC emissions. The contribution of fuel adsorption in engine oil and its subsequent desorption following combustion to the engine-out hydrocarbon (HC) emissions was studied by comparing steady and cycle-resolved HC emission measurements from operation with a standard full-blend gasoline, and with propane, which has a low solubility in oil. The results indicated that the HC emissions were not altered by the different fuels. In addition, steady state and dynamic simulations of the liquid-gas phase equilibrium and the adsorption-desorption respectively of the fuel-oil system showed the solubility of propane in the oil was approximately an order of magnitude lower than for gasoline, and that the oil layer contribution is very small compared with the ring pack contribution.
Salazar, V. M. Unburned Hydrocarbon Emission Mechanisms in Small Engines. University of Wisconsin-Madison, 2008.