Design and Simulation of Two Stroke Engines

Chapter 7 - Reduction of Fuel Consumption and Exhaust Emissions
QUB 500 RESEARCH ENGINE, 1600 rpm
100 -] ram-tuned liquid and air-blast injection
80 -
^ 60 -
8 40 -
20 -
RTL SOF90
> 1 ' 1
0 1 2
bmep, bar
Fig. 7.63 Light load carbon monoxide emissions.
the "knee" of the curve suggests that combustion may not be complete and that the trapped
air-to-fuel ratio is approximately stoichiometric; in Fig. 7.61, the bsHC figures of 9 g/kWh, at
the same experimental point, would support this view.
The liquid injection system permits stratified combustion down to much leaner overall
air-fuel ratios than the air-assisted injection system. The minimum value of 0.35 bar achieved
at 1600 rpm is not far off an idle condition for a "real" engine, as this QUB 500rv test engine
drives no ancillaries, such as an oil pump, an alternator, an ABI air pump, nor the RTL fuel
pump, etc. The ability of the RTL system to be fueled down to 0.35 bar at the same air throttle
setting implies an overall air-to-fuel ratio of some 50:1 while stratified burning locally at or
about stoichiometric.
Stratified burning in general and in the QUB500rv in particular
Stratified combustion for a spark-ignition engine, as pointed out in Sec. 7.1.2, is a relatively
difficult phenomenon to orchestrate successfully [7.37]. The sketch in Fig. 7.30 is
deliberately drawn to show the basic principle. At light load the injection is timed late and the
ignition early. The fuel spray is aimed so that it remains relatively cohesive and within the
vicinity of the spark plug at ignition. As the piston is approaching tdc it is advantageous to
provide a pocket in the piston crown into which the fuel is sprayed, which is located so as to
move ever closer to the environs of the spark plug electrodes by ignition. The scavenge process
is designed to not have localized high-speed air jets which would widely distribute the
fuel spray over the combustion chamber.
The combustion chamber of the loop-scavenged engine, shown in Fig. 7.30, is specifically
designed to accommodate these needs. It is a "total-offset" chamber of the type shown in
Fig. 4.13 and designed by Prog.4.5. The jargon for the chamber shown in Fig. 7.30 is a "jockey
cap," for obvious shape reasons. Most of the loop-scavenged engines have to employ ex-
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