An experimental study of exhaust hydrocarbon emissions from a spark ignition engine

The control of hydrocarbon emissions from spark ignition engines is important and there is a need for a better understanding of the mechanisms contributing to this source of emissions. The absorption/desorption mechanism is believed to be a significant contributor to hydrocarbon emissions. The aim of the project has been to validate the absorption/desorption model experimentally. Modelling studies predict that sufficient fuel vapour is absorbed into the cylinder wall oil film on the compression and early firing strokes with subsequent desorption late in the expansion stroke to seriously affect hydrocarbon emissions in the exhaust, as temperatures are too low for adequate oxidation. The experimental investigation which follows develops a method to allow accurate measurement of hydrocarbon specie levels in the exhaust of a spark ignition engine. The object of the experiment was to detect and measure differences in hydrocarbon levels for two fuels, Iso-octane and Iso-pentane, at identical engine conditions and therefore establish whether the absorption/desorption effect is significant to hydrocarbon emissions. The theory behind this relies upon the varying solubilities of both fuels in a given oil with temperature wh ich shoul d refl ect a varyi ng base fuel hydrocarbon emissions level for each fuel if absorption/desorption is in fact taking place. Samples of exhaust were taken from a Ricardo-hydra test engine and analysed chromatographically. Experimental results (which compare favourably with prediction from modell ing studies) indicate about 30 percent of total hydrocarbons are due to the absorption/desorption mechanism. In conclusion, the results suggest that the solubility of the fuel in the engine oil has a significant effect on hydrocarbon levels in the exhaust.