Design and Simulation of Two Stroke Engines

Design and Simulation of Two-Stroke Engines
tended electrode spark plugs to ensure that ignition reaches into the vaporizing fuel plume,
and that the ignition period is reasonably extensive in time terms. For that reason the rapid
rise, but short duration, spark from a capacitor discharge ignition system has to be replaced by
the inherently longer lasting spark of an inductive ignition system. Actually, the "bowl-inpiston"
chamber is even more effective than the "total-offset" design but, for crankcase compression
engines, the piston becomes much too hot at high load; this design could operate
well in an externally scavenged engine where it can receive the conventional under-piston
coolant flow from an oil jet.
For the QUB deflector piston engine, such as the QUB500rv shown in Fig. 7.51, the
deflector very conveniently provides the "pocket" into which fuel is to be injected. The proximity
of the spark plug electrodes to the injected fuel plume is evident and the necessity of
using extended electrode spark plugs is virtually eliminated; which means that an important
item of concern regarding engine durability has been removed. It does not mean that further
detailed experimental optimization of this chamber to enable good stratified burning is eliminated,
but it is more readily optimized than the more open geometry of a loop-scavenged
design. The test results given above for both the RTL and ABI injection were acquired without
the extensive optimization of combustion chamber geometry which characterizes similar
R&D efforts for loop-scavenged engines [7.38-7.40].
All of the above comments on design for stratified combustion doubtless seem instantly
logical to you, but be assured that it was learned more slowly and with greater difficulty than
that.
Therefore, from the light load test results at 1600 rpm, the ram-tuned, liquid-injection
system performed in a superior manner to the QUB air-assisted injection system, but this does
not necessarily imply that one is universally superior to the other for stratified combustion. It
simply means that for the single geometry experiment conducted it was superior. In another
physical configuration that might not prove to be the case.
Stratified combustion at idle conditions in the QUB500rv engine
During the conduct of experiments down to the idle condition, the ram-tuned liquid system
alone accomplished this ultimate test of combustion stratification. A stable idle condition
was achieved with the RTL system at 900 rpm with the following characteristics: DR is
measured at 0.37; SOF is 50° btdc; ignition timing is 35° btdc; AFRQ is 42.7; exhaust gas
oxygen content is 19.9% by volume; exhaust gas carbon monoxide content is 1.8% by volume;
exhaust gas hydrocarbon content is 1714 ppm (FID); and the exhaust gas temperature is
88°C. This latter value is of some concern from an emissions standpoint as an oxidation
catalyst would not "light-off' at this low temperature.
A multi-cylinder engine based on the QUB500rv
The test results presented above show that the QUB500rv engine design is well suited to
the direct air-assisted injection or ram-tuned injection system. The excellent fuel economy
and emissions, allied with the inherent manufacturing advantages of QUB cross-scavenging,
make this engine design a serious contender for automotive two-stroke engines. At QUB, a
three-cylinder engine has been constructed and tested based on the above knowledge. Due to
confidentiality, the test results cannot be debated, except to say that in the light load and low
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