Design and Simulation of Two Stroke Engines

Design and Simulation of Two-Stroke Engines
but would show little free oxygen. If the mixture were progressively leaner than stoichiometric,
the exhaust gas would contain lesser amounts of CO and no H2 but would show higher
concentrations of oxygen. The most important, perhaps obvious, issue is that the properties of
exhaust gas depend not only on temperature but also on the combustion process that created
them. Tables 2.1.2 and 2.1.3 show the ratio of specific heats, y, and gas constant, R, of exhaust
gas at various temperatures emanating from the combustion of octane at various air-fuel ratios.
The air-fuel ratio of 13 represents rich combustion, 15 is stoichiometric and an AFR of
17 is approaching the normal lean limit of gasoline burning. The composition of the exhaust
gas is shown in Table 2.1.2 at a low temperature of 293 K and its influence on the value of gas
constant and the ratio of specific heats is quite evident. While the tabular values are quite
typical of combustion products at these air-fuel ratios, naturally they are approximate as they
are affected by more than the air-fuel ratio, for the local chemistry of the burning process and
the chamber geometry, among many factors, will also have a profound influence on the final
composition of any exhaust gas. At higher temperatures, to compare with the data for air and
exhaust gas at 293 K in Table 2.1.2, this same gaseous composition shows markedly different
properties in Table 2.1.3, when analyzed by the same theoretical approach.
T=293 K
AFR %CO
13 5.85
15 0.00
17 0.00
AFR
13
15
17
Table 2.1.2 Properties of exhaust gas at low temperature
% by Volume
%co2
8.02
12.50
11.14
%H20
15.6
14.1
12.53
%o2
0.00
0.00
2.28
%N2
70.52
73.45
74.05
R
299.8
290.7
290.4
Table 2.1.3 Properties of exhaust gas at elevated temperatures
T=500 K
R
299.8
290.7
290.4
Y
1.362
1.350
1.352
AFR
13
15
17
T=1000 K
R
299.8
290.8
290.4
Y
1.388
1.375
1.376
Y
1.317
1.307
1.310
From this it is evident that the properties of exhaust gas are quite different from air, and
while they are as temperature dependent as air, they are not influenced by air-fuel ratio, particularly
with respect to the ratio of specific heats, as greatly as might be imagined. The gas
constant for rich mixture combustion of gasoline is some 3% higher than that at stoichiometric
and at lean mixture burning.
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