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.