Design and Simulation of Two Stroke Engines

Design and Simulation of Two-Stroke Engines
Appendix A7.1 The effect of compression ratio on performance characteristics and exhaust
emissions
Appendices A4.1 and A4.2 describe a two-zone combustion model, with equilibrium
thermodynamics giving dissociation for the burn zone constituents and a formation model for
nitric oxide based on reaction kinetics. This model is sufficiently fundamentally based as to
be able to calculate the local thermodynamic conditions during combustion and, when combined
with a GPB simulation of the open cycle behavior of an engine and its ducting, is able
to provide design guidance for the effect of compression ratio on the engine performance
characteristics of power output, fuel consumption, the intake and exhaust noise emissions,
and the gaseous exhaust emissions. In short, as mentioned at the beginning of Sec. 7.3.3, a
full debate can be conducted on the design compromises which inevitably have to be made
for any given engine to satisfy the market and legislative requirements for it.
The engine used as the design example is the standard chainsaw engine, used frequently
throughout the book, but first introduced with geometrical input data in Sec. 5.5.1. The data
are as presented there, but with the trapped compression ratio, CRt, changed successively
from 6.5 to 7.0 (the "standard" value), 7.5 and 8.0. The engine will assuredly give some
detonation at a CRt value of 8.0. For each of the simulations the engine speed employed is
9600 rpm, together with an air-to-fuel ratio of 13.0 on unleaded gasoline, which translates to
a X value of 0.9. The combustion model is exactly as described in Sec. 5.5.1, i.e., as it is
shown in Fig. 4.7(d), with an ignition timing at 24° btdc used within each simulation.
The results of the modeling are shown in Figs. A7.1-A7.4. It should be noted that the
delivery ratio and charging efficiency are ostensibly constant for all of these simulations, thus
any changes in performance characteristics are almost entirely due to combustion variations.
The effect of compression ratio on power and fuel consumption
Eq. 1.5.22 predicts higher power and thermal efficiency for increasing compression ratio.
Thermal efficiency is inversely related to brake specific fuel consumption, which can be seen
4.2 ->
CHAINSAW ENGINE 9600 rpm
r 470 5.8 -i 26
8 9 6 7
TRAPPED COMPRESSION RATIO
Fig. A7.1 Effect of compression ratio on torque, fuel consumption and NOx emissions.
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