The effect of pilot injection and exhaust gas recirculation
(EGR) on premixed charge compression ignition (PCCI) combustion was
investigated in a single-cylinder direct-injection diesel engine
with low engine speed and low load. The injection timing of PCCI
combustion was fixed at 25 ~ 30 crank angle degree before top dead
center (°CA BTDC) based on the ignition delay and power output. The
level of oxides of nitrogen (NOx) emissions of PCCI combustion was
68% lower than that of conventional diesel combustion owing to the
reduction of near-stoichiometric region which is well known as the
main source of NOx formation. However, the indicated mean effective
pressure (IMEP), hydrocarbon (HC), particulate matter (PM) and
carbon monoxide (CO) emissions deteriorated compared with
conventional diesel combustion because of early injection, advanced
combustion phase and lowered combustion temperature.
EGR has been applied to PCCI combustion. As EGR rate increased,
IMEP was increased due to the decrease of negative work during the
compression stroke and NOx emissions was decreased because of
dilution and thermal effects of EGR.
The pilot injection with a variation of split ratio and dwell
time was also explored to enhance the mixing of fuel and fresh air.
Certain conditions of pilot injection were effective for increase
of IMEP and decrease of CO, HC and PM emissions because in-cylinder
temperature was raised due to pilot injection, and this temperature
rise enhanced the evaporation of the main injected fuel. The level
of NOx emissions was higher than that of single injection because
of raised in-cylinder temperature.
Consequently, the power output and emissions could be improved
by applying pilot injection and EGR simultaneously in PCCI
combustion.