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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.

In practical lean burn engines used to date, the use of a stratified air-fuel configuration, with a comparatively rich mixture in the vicinity of the spark plugs, has resulted in the stable combustion of an overall lean mixture. However, because a comparatively rich mixture is burned during the first half of combustion, NOx emissions are not reduced sufficiently. This research focused on a form of lean burn with homogeneous premixture that would be able to balance low NOx emissions with combustion controllability. It is widely known that homogeneous lean premixed gas has poor flame propagation characteristics. To determine the dominant cause of this, this study investigated the combustion properties of a single-cylinder engine while changing the compression ratio and intake temperature. As a result, the primary cause of combustion fluctuation, the abnormal cycle has a low TDC temperature compared to that of other cycles.

In order to efficiently enhance engine sound quality under acceleration, the authors have developed an evaluation method for primary judgment of the sound quality of engine combustion noise at the stage of advanced engine development before the prototype vehicle is built. This method is an application of an existing method for evaluating the sound quality of engine combustion noise in vehicle interiors to the evaluation of noise and vibration at an engine acoustic test bench. In this method, it is necessary to consider the air-borne and the structure-borne components separately. The analysis procedure for the air-borne component is as follows. First, the sound pressure at a point 1 m away from the engine and the in-cylinder pressure of each cylinder are measured simultaneously in a semi-anechoic engine dynamometer test chamber. Next, the signal correlated with engine combustion is extracted from the measured sound pressure using the time domain combustion noise separation method.