Condensation of Fuel on Combustion Chamber Surfaces as a Mechanism for Increased HC Emissions from SI Engines During Cold Start

Condensation of fuel vapor on the cold surfaces within the combustion chamber is investigated as a possible mechanism for increased HC emissions from SI engines during cold start. A one-dimensional, transient, mass diffusion analysis is used to examine the condensation of single-species fuels on the surfaces of the combustion chamber as the pressure within the cylinder rises during compression and combustion, and re-vaporization during expansion, blowdown, and exhaust. The effects of wall temperature, fuel volatility, and engine load and speed on this mechanism are also discussed. This analysis shows that low-volatility fuel components can condense on the surfaces of the combustion chamber when the surface temperatures are sufficiently low. This condensed fuel may re-vaporize during the power and exhaust strokes, or it may remain in the combustion chamber until surface temperatures rise, perhaps tens of seconds later. It is also shown that a significant portion of the condensed fuel does not re-vaporize until blowdown, at which time bulk gas temperatures are too low for complete in-cylinder oxidation. It is also shown that flame passage may not significantly affect the re-vaporization rate under some circumstances. Although it is difficult to quantify the contribution of this mechanism to the total cold-start HC emissions from a port-fuel injected engine, this analysis may provide a better understanding of the cold-start HC emissions behavior from gasoline-fueled engines. It also appears that the proposed mechanism may be most important for experiments involving pure (single species) fuels, for prevaporized gasoline studies, and for direct injection gasoline engines.