The purpose of this study was to gain a better understanding of
the effects of port fuel injection strategies and thermal
stratification on the HCCI combustion processes. Experiments were
conducted in a single-cylinder HCCI engine modified with windows in
the combustion chamber for optical access. Two-dimensional images
of the chemiluminescence were captured using an intensified CCD
camera in order to understand the spatial distribution of the
combustion. N-heptane was used as the test fuel.
The experimental data consisting of the in-cylinder pressure,
heat release rate, chemiluminescence images all indicate that the
different port fuel injection strategies result in different charge
distributions in the combustion chamber, and thus affect the
auto-ignition timing, chemiluminescence intensity, and combustion
processes. Under higher intake temperature conditions, the
injection strategies have less effect on the combustion processes
due to improved mixing. The intake temperature is a more important
factor affect in a mixture and combustion process. Fuel injection
duration the intake stroke and the intake valve opening phase (-300
°ATDC) results in a more homogeneous charge mixture, which is
beneficial to combustion and fuel economy. With changing coolant
temperature and intake temperature, the in-cylinder temperature
distribution will be altered leading to different combustion
processes. Enhanced thermal stratification has the potential for
smoothing pressure-rise rates. At low loads, increased thermal
stratification is worse because more quenching maybe occur at the
cooler regions in the cylinder. Therefore, less thermal
stratification and higher in-cylinder temperature are better for
low load operation.