Controlling ignition delay is the key to successfully enable
partially premixed combustion in diesel engines. This paper
presents experimental results of partially premixed combustion in
an optically accessible engine, using primary reference fuels in
combination with artificial exhaust gas recirculation. By changing
the fuel composition and oxygen concentration, the ignition delay
is changed.
To determine the position of the flame front, high-speed
visualization of OH-chemiluminescence is used, enabling a
cycle-resolved analysis of OH formation. A clear correlation is
observed between ignition delay and flame location.
The mixing of fuel and air during the ignition delay period
defines the local equivalence ratio, which is estimated based on a
spherical combustion volume for each spray. The corresponding
emission measurements using fast-response analyzers of CO, HC and
NOX confirm the decrease in local equivalence ratio as a function
of ignition delay.
Furthermore multiple injection strategies are investigated,
applying pilot as well as post injections, in combination with a
main injection at constant load. From these results it is concluded
that both pilot and post injections result in an increase of
unburned hydrocarbon and CO emission and a slight decrease of
nitric oxide emissions.