Design and Simulation of Two Stroke Engines

The other equations reveal for air at 293 K:
Chapter 2 - Gas Flow through Two-Stroke Engines
CP= 1022 J/kgK Cv = 734J/kgK y = 1.393
It will be seen that the value of the ratio of specific heats, y, is not precisely 1.4 at standard
atmospheric conditions as stated earlier in Sec. 2.1.3. The reason is mostly due to the fact that
air contains argon, which is not included in the above analysis and, as argon has a value of y
of 1.667, the value deduced above is weighted downward arithmetically.
The most important point to make is that these properties of air are a function of temperature,
so if the above analysis is repeated at 500 and 1000 K the following answers are found:
for air:
T = 500K CP = 1061 J/kgK Cv = 773 J/kgK y = 1.373
T=1000K CP= 1143 J/kgK Cv = 855 J/kgK y= 1.337
As air can be found within an engine at these state conditions it is vital that any simulation
takes these changes of property into account as they have a profound influence on the
characteristics of unsteady gas flow.
Exhaust gas
Clearly exhaust gas has a quite different composition as a mixture of gases by comparison
with air. Although this matter is discussed in much greater detail in Chapter 4, consider the
simple and ideal case of stoichiometric combustion of octane with air. The chemical equation,
which has a mass-based air-fuel ratio, AFR, of 15, is as follows:
2CoHig + 25
79
02 + — No
21
= 16C02 + 18H20 + 94.05N2
The volumetric concentrations of the exhaust gas can be found by noting that if the total
moles are 128.05, then:
16 9 94 05
X>co = = 0.125 uH 0 = = 0.141 t>N = —•— = 0.734
2 128.05 2 128.05 2 128.05
This is precisely the same starting point as for the above analysis for air so the procedure
is the same for the determination of all of the properties of exhaust gas which ensue from an
ideal stoichiometric combustion. A full discussion of the composition of exhaust gas as a
function of air-to-fuel ratio is in Chapter 4, Sec. 4.3.2, and an even more detailed debate is in
the Appendices A4.1 and A4.2, on the changes to that composition, at any fueling level, as a
function of temperature and pressure.
In reality, even at stoichiometric combustion there would be some carbon monoxide in
existence and minor traces of oxygen and hydrogen. If the mixture were progressively richer
than stoichiometric, the exhaust gas would contain greater amounts of CO and a trace of H2
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