Design and Simulation of Two Stroke Engines

Design and Simulation of Two-Stroke Engines
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dramatic consequences described.
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Fig. A7.3 Effect of compression ratio on combustion rates.
The effect of compression ratio on combustion
Fig. A7.2 summarizes the combustion information. With increasing compression ratio,
the peak cylinder pressure is shown to rise from 25 bar to 31 bar and the location of that peak
is shown to occur ever earlier from 13.7°atdc to 11.4°atdc. The burned and unburned zone
peak temperatures are shown on the same figure. The profiles of these changes are shown in
Fig. A7.3, and the rate of pressure rise in the cylinder, in terms of pressure change per unit
time, is observed to increase. In actuality, the combustion pressure wave created will be sharper
as the compression ratio rises and will compress the gas in the unburned zone more noticeably.
This model, which does not include a combustion pressure wave, nevertheless shows
that the unburned zone peak temperature increases from 858 to 882°C.
While considering this information, it is important to remember that the simulation employs
the same Vibe function, and the same ignition timing and ignition delay, for each simulation
at differing compression ratios. Clearly, the use of a retarded ignition timing will allay
some of the thermal stress and loading seen at the higher compression ratios, albeit at the
expense of power and thermal efficiency.
The effect of compression ratio on detonation
The profile with respect to time of the compression and, more important, the heating of
the unburned zone is shown in Fig. A7.4. It can be seen that there is considerable similarity of
temperature between compression ratios of 6.5 and 7, although the NO formation rate given
in Fig. A7.1 is affected much more severely. At higher compression ratios the unburned zone
temperatures are significantly higher.
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