This work experimentally investigates how the dwell time between
pilot injection and main injection influences combustion
characteristics and emissions (NOx, CO, THC and Smoke) in a
single-cylinder DI diesel engine. Additionally, results from diesel
injection are compared with those shown in dimethyl ether (DME)
injection under the identical injection strategy to demonstrate the
sensitivity of the combustion characteristics and emissions to
changes of the fuel type. Two fuel injection systems are applied
for this experiment due to the differences of fuel characteristic
with regard to physical and chemical properties. The injection
strategy is accomplished by varying the dwell time (10°CA, 16°CA
and 22°CA) between injections at five main injection timings (-4°CA
aTDC, -2°CA aTDC, TDC, 2°CA aTDC and 4°CA aTDC).
It was found that pilot injection offers good potential to lower
the heat-release rate with reduced pressure traces regardless of
the dwell time between injections and fuel type. Pilot injection
also allows stable combustion even for extensive combustion phasing
retard, which reduces the NOx emissions. This means that it is
indeed possible, and higher BMEP could be achieved for pilot-main
injection operation. Contrary to the expectations, pilot injection
is found to play a lesser role in reducing the NOx emissions, but
NOx formation is mainly suppressed by the combustion phasing retard
effect. A comparison of the emissions between diesel injection and
DME injection under identical injection strategy shows that even
DME injection exhibits higher NOx emissions than diesel injection,
pilot injection is found to be more effective on DME for reducing
the amount of NOx with combustion phasing retard, which indicates a
level of NOx emissions similar to that of diesel injection. The CO
and THC emission levels vary significantly with injection
strategies in diesel injection, but they are typically found to be
lower than 100 ppm in DME injections. Although the diesel
pilot-main injection conditions show higher PM emissions than
single-injection condition, DME has little PM regardless of
injection strategy.