Design and Simulation of Two Stroke Engines

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Chapter 7 - Reduction of Fuel Consumption and Exhaust Emissions
AIR-FUEL RATIO
Fig. 7.8 Air-fuel ratio effect on CO and O2 emissions at 10% throttle.
air lost during the scavenge process with about 1 % coming from the inefficiency of the combustion
process. These are probably the best fuel consumption and emission values recorded
on a simple engine at QUB, particularly for an engine capable of 6.2 bar bmep without exhaust
tuning. By comparing notes with colleagues in industrial circles, it is probable that
these figures represent a "state-of-the-art" position for a simple, single-cylinder, two-stroke
engine at this point in history. The levels of fuel economy and exhaust emissions at the lighter
load point of 2.65 bar bmep would be regarded as particularly impressive, and there would be
four-stroke cycle engines which could not improve on these numbers. It is noticeable that the
carbon monoxide levels are at least as low as those from four-stroke cycle engines, but this is
the one data value which truly emanates from the combustion process alone and is not confused
by intervening scavenging losses.
Nevertheless, the values of hydrocarbon emission are, by automotive standards, very
high. The raw value of HC emission caused by combustion inefficiency alone should not
exceed 400 ppm, yet it is 4200 ppm at full load. This is a measure of the ineffectiveness of
homogeneous charging, i.e., scavenging, of the simple two-stroke engine. It forever rules out
the use of simple two-stroke engines in automotive applications against a background of
legislated emissions levels.
To reinforce these comments, recall from Sec. 1.6.3 that exhaust oxygen concentration
can be used to compute trapping efficiency, TE. The data sets shown in Figs. 7.5 and 7.8
would yield a TE value of 0.6 at full throttle and i 0.86 at one-tenth throttle. This latter is a
remarkably high figure and attests to the excellence of the scavenge design. What these data
imply, however, is that if the fuel were not short-circuited with the air, and all other engine
behavioral factors remained as they were, the exhaust HC level could possibly be about 350
ppm, but the full throttle bsfc would actually become 240 g/kWh and the one-tenth throttle
bsfc would be at 260 g/kWh. These would be exceptional bsfc values by any standards and
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