Design and Simulation of Two Stroke Engines

Chapter 7 • Reduction of Fuel Consumption and Exhaust Emissions
Fig. 7.37 Hydrocarbon emission levels from the Piaggio stratified charging
engine (from Ref. [7.1]).
ditions where the machine is accelerated and driven in the 15-50 km/h zone. The proposed
European ECE-R40 cycle is such a driving cycle [7.21].
The reduction of hydrocarbon emissions is particularly impressive, as can be seen from a
direct comparison of the original engine in Fig. 7.11 with the stratified charging engine in Fig.
7.37. The standard engine, already discussed in Sec. 7.2.1, showed a minimum contour of
1500 ppm HC (C6, NDIR), at a light load but high speed point. In the center of the load-speed
map in Fig. 7.11 the figures are in the 2500 ppm region, and at the light load point of 1 bar and
1500 rpm, the figure is somewhat problematic but 5000 ppm would be typical. For the stratified
charging engine the minimum contour is reduced to 200 ppm HC, the center of the loadspeed
picture is about 500 ppm, and the all-important light load and speed level is somewhat
in excess of 1000 ppm. This is a very significant reduction and is the level of diminution
required for a successful automotive engine before the application of catalytic after-treatment.
A comparison of the carbon monoxide emission levels of the standard engine in Fig. 7.10
and of the stratified charging engine in Fig. 7.36 shows significant improvements in the two
areas where it really matters, i.e., at light loads and speeds and at high loads and speeds. In
both cases the CO emission is reduced from 2-3% to 0.2-0.3%, i.e., a factor of 10. The absolute
value of the best CO emission at 0.2% is quite good, remembering that these experimental
data were acquired in 1978.
Also note that the peak bmep of the engine is slightly reduced from 4.8 bar to 4.1 bar due
to the stratified charging process, and there is some evidence that there may be some diminution
in the air utilization rate of the engine. This is supplied by the high oxygen emission
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