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Twenty-three hydrocarbon components were analysed in the exhaust emissions from a 2.3 litre gasoline engine. The effect of a three-way catalyst on emission rates was investigated, as was the effect of addition to fuel of specific aromatic and olefinic compounds. The addition of 1-hexene and 1-octene (olefins) caused statistically significant increases in reactive olefins - ethene and propene - in the exhaust. The addition of benzene and toluene led to increases in these compounds in the exhaust, and indicated that whilst fuel-toluene is the main source of toluene emissions, the emission of benzene has sources in addition to fuel-benzene. A three-way catalyst, when operating at > 600°C, eliminated most hydrocarbons except methane and traces of the light aromatics. At idle, however, the catalyst exhibited substantial selectivity towards different hydrocarbons according to their ease-of-oxidation.

Natural gas is one of the alternative fuels to diesel being considered for low emissions heavy-duty applications. The favoured operating strategies for low emissions SI gas engines are identified as those with high levels of dilution - stoichiometric operation with EGR, and lean-burn. A well-matched exhaust catalyst is needed to produce the lowest emissions levels. Increasing the accuracy of transient air-fuel ratio control is shown to improve the emissions still further. The most favourable combinations of engine operating strategy and control accuracy are identified with respect to fuel economy and first cost. The Co-Nordic Natural Gas Bus Project is an example of an engine development programme aimed at achieving the lowest possible exhaust emissions levels, and as such uses the lowest emissions approach of a stoichiometric engine strategy with EGR and high accuracy control.