Measurements of HC Concentration near Spark Plug and Its Effects on Combustion

Mixture preparation is a key contributor to both the combustion and emissions in automotive gasoline engines. The air-fuel ratio near the spark plug may have an effect on combustion characteristics since it is related to early flame development. Therefore, cycle resolved measurements of equivalence ratio near the spark plug is particularly important for better understanding of its contribution on combustion and emissions.

This paper describes how we determined the in-cylinder equivalence ratio from the measured hydrocarbon concentration near the spark plug using a Fast Response Flame Ionization Detector (FRFID). The procedures established were then applied to a limited range of engine operating conditions, and the cycle resolved equivalence ratio near the spark plug was determined from the measured hydrocarbon concentration. The results showed that with premixed propane fuel the cyclic variation of equivalence ratio near the spark plug was quite small and correspond with overall equivalence ratio from the exhaust UEGO sensor signal. But on the contrary, with gasoline fuel the equivalence ratio near the spark plug varied by up to 10% and did not coincide with the UEGO sensor signal on a cyclic basis. The variation of equivalence ratio near the spark plug affected the heat release rate, which was then reflected on the indicated mean effective pressure. It was also observed that cyclic variation of equivalence ratio near the spark plug might be one origin of cycle-by-cycle variation.