The paper describes the optimization of a 50 cc crankcase
scavenged two-stroke diesel engine operating on dimethyl ether
(DME). The optimization is primarily done with respect to engine
efficiency. The underlying idea behind the work is that the low
weight, low internal friction and low engine-out NOx of
such an engine could make it ideal for future vehicles operating on
second-generation biofuels.
Data is presented for the performance and emissions at the
current state of development of the engine. Brake efficiencies
above 30% were obtained despite the small size of the engine. In
addition, efficiencies near the maximum were found over a wide
operating range of speeds and loads. Maximum bmep is 500 kPa.
Results are shown for engine speeds ranging from 2000 to 5000 rpm
and loads from idle to full load. At all speeds and loads
NOx emissions are below 200 ppm and smokeless operation
is achieved.
Design improvements relative to an earlier prototype are
described. The major alterations are related to air intake
arrangement, exhaust tuning and the fuel injector. Comparison is
made to the first prototype engine and the effects of fuel
injection rate, injection pressure, cylinder head geometry and
injection timing are evaluated at selected engine operating
conditions.
Cylinder pressure, crankcase pressure and rate of heat release
were determined. The engine uses an oversize fuel injector so that
fuel delivery happens within a few crank angle degrees. Since DME
is very volatile a large degree of premixing occurs before
auto-ignition of the fuel. This results in approximately 65% of the
fuel being burnt rapidly in the premixed phase of combustion. The
engine mode of operation can be characterized as premixed
compression ignition (PCI).