Design and Simulation of Two Stroke Engines

It will be seen that expansion of Eq. 1.5.12 gives:
Chapter 1 - Introduction to the Two-Stroke Engine
TE= m tr SE (1.5.13)
msrefSR
It will also be seen under ideal conditions, in Chapter 3, that mtr can be considered to be
equal to msref and that Eq. 1.5.13 can be simplified in an interesting manner, i.e.,
^ SE
TE = —
SR
In Sec. 1.6.3, a means of measuring trapping efficiency in a firing engine from exhaust
gas analysis will be described.
1.5.4 Charging efficiency
Charging efficiency, CE, expresses the ratio of the filling of the cylinder with air, by
comparison with filling that same cylinder perfectly with air at the onset of the compression
stroke. After all, the object of the design exercise is to fill the cylinder with the maximum
quantity of air in order to burn a maximum quantity of fuel with that same air. Hence, charging
efficiency, CE, is given by:
CE = jni§i_ (1514)
m sref
It is also the product of trapping efficiency and scavenge ratio, as shown here:
CE = B*L x _i^as_ = TE x SR (1 -5.15)
m as
m sref
It should be made quite clear that this definition is not precisely as defined in S AE J604d
[1.24]. In that SAE nomenclature Standard, the reference mass is declared to be m^f from
Eq. 1.5.2, and not msref as used from Eq. 1.5.4. My defense for this is "custom and practice in
two-stroke engines," the fact that it is all of the cylinder space that is being filled and not just
the swept volume, the convenience of charging efficiency assessment by the relatively straightforward
experimental acquisition of trapping efficiency and scavenge ratio, and the opinion
of Benson [1.4, Vol. 2].
7.5.5 Air-to-fuel ratio
It is important to realize that there are narrow limits of acceptability for the combustion of
air and fuel, such as gasoline or diesel. In the case of gasoline, the ideal fuel is octane, C8Hlg,
which burns "perfectly" with air in a balanced equation called the stoichiometric equation.
Most students will recall that air is composed, volumetrically and molecularly, of 21 parts
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