Abstract
The mixture concentration in the vicinity of the spark plug at the time of spark ignition is a critical parameter for the performance of SI engines. Infrared absorption is often used to measure this parameter in firing cycles. It was already made clear that the mixture concentration near the spark plug is a primary factor explaining combustion stability in direct injection engines, although the effect of the local mixture concentration has not been confirmed for a nearly homogeneous mixture. In this research, the mixture concentration in the vicinity of the spark plug was quantitatively measured by using infrared absorption with an optical fiber sensor built into the spark plug of a port injection engine. Measurements were made for two cases, by changing the injected fuel amount from a rich to a lean air-fuel ratio between the cycles and by keeping the fuel amount constant between the cycles at three different overall air-fuel ratios. It was experimentally confirmed that the mixture concentration near the spark plug has little impact on combustion stability with a homogenous mixture distribution. It has also been confirmed that fiber-sensor-based infrared absorption can be used for cycle-by-cycle analysis of the local mixture concentration even for a homogeneous mixture distribution.
