A new combustion mode namely multiple premixed compression
ignition (MPCI) for gasoline engines was proposed. The MPCI mode
can be realized by two or more times gasoline injections into
cylinder with a high pressure around the compression TDC and
featured with a premixed combustion after each injection in the
cylinder, which is different from the existed gasoline direct
injection compression ignition (GDICI) modes such as homogeneous
charge compression ignition (HCCI) mode with gasoline injection
occurred in intake stroke, and partially premixed compression
ignition (PPCI) mode with multiple gasoline injections in intake
and compression strokes before the start of combustion (SOC).
Therefore the spray and combustion of the MPCI mode are
alternatively occurred as
"spray-combustion-spray-combustion" near the TDC, rather
than "spray-spray-combustion" sequence as traditional
PPCI gasoline engines. Since the combustion process of the MPCI
mode is mainly controlled by the multiple gasoline injections, the
SOC and pressure rise rate (PRR) can be controlled easily and
effectively through injection timing, injection split ratio and
number of the injections. It is expected that each premixed
combustion process in the MPCI engine can be realized in the center
of combustion chamber with pure air filling the gap between the
combustion area and the cylinder wall, resulting in not only the
low PM and NOx emissions, but also the low THC and CO emissions
while remaining the high thermal efficiency.
The experimental study for the MPCI gasoline engine is carried
out on a single-cylinder compression ignition engine retrofitted
from a four-cylinder light-duty diesel engine with the compression
ratio of 18.5. A stable and moderate two-stage gasoline MPCI mode
is achieved under the running conditions of 0.8 MPa IMEP load and
1400 r/min speed at 70°C intake temperature without EGR and intake
boosting using the gasoline fuel with the research octane number of
94.4. The NO and soot emissions as well as THC and CO are
relatively low, while keeping the indicated thermal efficiency
around 40%, which cannot be easily reached with even diesel fuel
under the same conditions. However, optimization for the gasoline
MPCI combustion mode is still needed in order to improve the fuel
efficiency further. The test results show that the MPCI mode is a
promising and feasible combustion mode for gasoline engines
achieving both of the high efficiency and low emissions especially
at high load.