Design and Simulation of Two Stroke Engines

Design and Simulation of Two-Stroke Engines
More recently, and aimed specifically at the simple two-stroke engine used in mopeds,
chainsaws and small motorcycles, Laimbock [7.21] presents experimental data on the effect
of using the advice given in this chapter. He shows the results for a 125 cm 3 high-performance
motorcycle engine when the scavenging and carburetion have been optimized and an exhaust
timing edge control valve is used. For such small motorcycles there are emission control laws
in Switzerland, Austria and Taiwan. The most severe of these is in Switzerland, where the
machine must execute a driving cycle and emit no more than 8 g/km of CO, 3 g/km of HC and
0.1 g/km of NOx. Laimbock shows that a production 125 cm 3 motorcycle engine, which has
a peak bmep of 8 bar at 9000 rpm and is clearly a high specific output power unit, has emissions
on this cycle of 21.7 g/km of CO, 16.9 g/km of HC and 0.01 g/km of NOx. Clearly this
motorcycle is unsuitable for sale within such regulations. By optimizing the scavenging and
carburetion, the same machine will have emission characteristics on the same cycle of 1.7
g/km of CO, 10.4 g/km of HC and 0.03 g/km of NOx. Thus, the optimization procedures
already discussed in the chapter lowered the CO and HC significantly, but raised the NOx
levels. The HC level is still unacceptable from a Swiss legal standpoint. By introducing an
oxidation catalyst into the tuned exhaust pipe of this engine in the manner shown in Fig. 7.28,
Laimbock provides experimental evidence that the peak power performance of the motorcycle
is barely affected, but the emissions are dramatically reduced. In this case, where the
catalyst is of the oxidizing type, the test results on the Swiss driving cycle gave emission
levels of 0.8 g/km of CO, 1.9 g/km of HC and 0.02 g/km of NOx; such a machine is now well
within the limits pending or proposed by many legislative bodies worldwide.
As far as fuel consumption is concerned, Laimbock [7.21] shows that the original 125 cc
production motorcycle on the test driving cycle had a fuel consumption level of 20.8 km/liter,
the model with improved scavenging and carburetion did 29.5 km/liter, while the final version
with the exhaust catalysts fitted traveled 31.2 km/liter of gasoline.
There is no logical reason why a similar approach cannot be successful for any type of
simple two-stroke cycle engine.
Finally, it is possible that the contribution of internal combustion engines to the atmospheric
pollution by nitrous oxide, N2O, may be a very important factor in the future [7.24].
There is every indication that a two-stroke engine produces this particular nitrogen oxide
component in very small quantities by comparison with its four-stroke engine counterpart.
7.4 The more complex two-stroke engine
If the two-stroke engine is to have relevance in the wider automotive application, the raw
level of unburned hydrocarbons in the exhaust system before catalytic after-treatment will
have to be further reduced while the engine retains its high specific power output. Equally,
levels of specific fuel consumption in the range of 250 to 300 g/kWh will be required over
much of the speed and load range. In this case, it is essential that no fuel is ever lost to the
exhaust system during scavenging, thereby deteriorating the thermal efficiency of the engine.
It is clear from the earlier discussion in this chapter that the most miniscule quantity lost in
this manner gives unacceptably high levels of HC emission. Clearly, an engine designed to
accomplish these criteria is going to be much more mechanically or electronically complex.
The Achilles' heel of the simple two-stroke engine is the loss of fuel when it is supplied
in conjunction with the scavenge air. Remove this problem, albeit with added complexity, and
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