Design and Simulation of Two Stroke Engines

Design and Simulation of Two-Stroke Engines
Fig. 7.26 Variable exhaust port area and port timing control devices.
The exhaust timing edge control valve
It is clear from Fig. 7.27(a) that the butterfly valve controls only the area at the exhaust
port rather than the port opening and closing timing edges as well. On Fig. 7.27 is sketched
the timing control valve originally shown in Fig. 5.2, which fits closely around the exhaust
port and can simultaneously change both the port timing and the port area. The effectiveness
of changing exhaust port timing is demonstrated in the simulations discussed in Sees. 5.2.2
and 7.3.4. There are many innovative designs of exhaust timing edge control valve ranging
from the oscillating barrel type to the oscillating shutter shown in Fig. 7.27. The word "oscillating"
may be somewhat confusing, so it needs to be explained that it is stationary at any one
load or speed condition but it can be changed to another setting to optimize an alternative
engine load or speed condition. While the net effect on engine performance of the butterfly
valve and the timing edge control valve is somewhat similar, the timing edge control valve
carries out the function more accurately and effectively. Of course, the butterfly valve is a
device which is cheaper to manufacture and install than the timing edge control device.
Concluding remarks on AR combustion and port timing control
The fundamental message to the designer is that control over the exhaust port timing and
area has a dramatic influence on the combustion characteristics, the power output, the fuel
economy and the exhaust emissions at light load and low engine speeds.
7.3.5 Conclusions regarding the simple two-stroke engine
The main emphasis in the discussion above is that the simple two-stroke engine is capable
of a considerable level of optimization by design attention to scavenging, combustion,
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