Study on Cyclic Variations of Laminar Flame Speed in Homogeneous Lean charge Spark Ignition Combustion

It is known that lean combustion is effective as one of the ways which improves thermal efficiency of a gasoline engine. In the interest of furthering efficiency, the use of leaner mixtures is desired. However, to realize robust lean combustion it is necessary to reduce combustion cyclic variation while managing the emission nitrogen oxides. In this study, combustion analysis was carried out focusing on cyclic variations of the heat release of lean combustion. Since the initial flame kernel growth speed has a great effect on the indicated mean effective pressure, laminar flame speed (LFS) around the spark plug was analyzed. Infrared absorption spectrophotometry was used for the measurement of a fuel concentration around the spark plug. Moreover, a LFS predicting formula, which can be used in an area leaner than before, was drawn from detailed chemical reaction calculation results, and the LFS around the spark plug was also calculated through the use of this formula. With the presence of high in-cylinder flow or a pre-mixed fuel delivery the LFS variations around the spark plug decreased as TDC of compression was approached. As equivalence ratio was decreased, an increase in heat release variation was observed (COV of LFS). By COV definition the results present that the increase in COV of LFS is predominantly caused by a decrease in average of LFS as opposed to the standard deviation of the data set. This is inherent of lean combustion’s physical nature. In a similar fashion to lean combustion, but in the case of EGR combustion, heat release fluctuated in accordance to dilution ratio. These two scenarios can be linked by common LFS.