The variation of in-Cylinder Mixture Ratios during Engine Cranking at Low Ambient Temperatures

Abstract
The development of mixture conditions in the cylinder of a fuel-injected spark ignition engine during engine cranking has been investigated at ambient temperatures down to —20°C. Mixtures near to the spark plug location were sampled and analysed to determine the air-fuel ratio and the relative proportions of light, medium and heavy components in the fuel. At low temperatures, the local air-fuel ratio varies substantially during the compression stroke, as does mixture composition. The change in mixture ratio over successive cycles of cranking depends on the fuel injected per cycle and the fuel-transfer characteristics of the intake port. The success or failure of combustion initiation is observed to depend only on the mixture air-fuel ratio at the spark plug. The upper limit on this mixture ratio for successful first-fire appears to be near to 35 : 1 by mass. Air-borne fuel in the cylinder accounts for only a small percentage of that supplied by the injector.

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