New exhaust emission laws require significant reduction of tailpipe hydocarbon emissions. The cold start phase of engine operation is a critical period when HC emissions must be minimized. High driveability index (DI) or low volatility fuel causes the open loop air/fuel (A/F) ratio during an initial cold start of the engine to shift lean which in turn contributes to unstable combustion. To compensate for this lean shift, the open loop A/F ratio must be commanded richer than necessary to allow acceptable driveability with high DI fuels which consequently increases tailpipe HC emissions. With engine cold start conditions being equal, (coolant temperature, engine speed, engine load, ignition timing, and commanded A/F ratio), the difference in cold start engine-out A/F ratio can only be attributed to the volatility characteristics of the fuel. The A/F ratio in a combustion chamber and hence, the temperature of the exhaust gas, is a function of the volatility of the fuel.
The recent development of fast response, wide range platinum RTD type sensors for exhaust gas temperature measurement makes possible a method for indirectly measuring differences in A/F ratio during the initial cold start phase of engine operation.
This paper reports the results of an evaluation comparing the engine out exhaust gas temperatures during cold starts to actual engine out A/F ratios using low and high DI fuels and shows a method which may be used to detect the cold start volatility characteristic of the fuel.