An Investigation of the Effects of Fuel Inhomogeneity on the Pressure Rise Rate in HCCI engine using Chemiluminescence Imaging 2010-32-0097

Theoretically, homogeneous charge compression engines (HCCI) are able to grant a high thermal efficiency, as well as a low NOx and particulate emissions. This ability is mainly due to the combustion process, which, contrary to both Diesel and Gasoline engine, is homogeneous in time and space within the combustion chamber. But despite these advantages, the engine operating condition is limited by the narrow boundaries of misfire at low load and knocking at high load. For that matter, one of the numerous ways of overcoming knocking is to deliberately create fuel inhomogeneities within the combustion chamber, since it has proved to lengthen combustion duration and to drastically reduce maximum pressure rise rate (PRR). Nevertheless, though the global effects of fuel inhomogeneities on PRR have been studied, we lack information that explains this phenomenon.

Thus, the goal of this paper is to investigate in details the reason why the whole combustion duration tends to enlarge, according to the fuel inhomogeneities. To make this analysis, a 4-stroke optically accessible engine provided with two different chemiluminescence measuring systems have been used. The first system is a chemiluminescence intensifier, mostly to investigate the fuel properties, and the second one is a chemiluminescence imaging system, with a high-speed camera rather for close and precise observations.

Once demonstrated that chemiluminescence examinations are a good reflection of the rate of heat release in terms of timing and intensity, chemiluminescence imaging is used to analyze those outputs for each individual small area of the combustion chamber. As a result, each of these zones proved to have approximately the same combustion duration but wide spread combustion start. Thus, against all odds, chemical reaction speeds have only a small influence, even though the equivalence ratio fluctuates from one zone to another.