Gas chromatography tests have been applied to the exhaust gases of a spark ignition engine to determine the concentration of unburned fuel among the total hydrocarbons. The contribution of unburned fuel was determined with variation in several engine parameters. The fuel tested was isooctane. The varied parameters were the air-fuel ratio, engine speed, ignition timing, compression ratio and coolant and lubricant temperature. The results have shown that the unburned fuel is responsible for most of the HC emitted, 50-73 per cent, depending on the engine working conditions. Methane, acetylene, ethylene, ethane, propylene and isobutylene were also analysed, as well as isooctane. The total contribution of the lighter species remained practically unaltered when the parameters were varied, though their individual concentrations did change. Thus, the unburned HC was seen to determine all trends of exhaust hydrocarbons.
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