Design and Simulation of Two Stroke Engines

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Chapter 4 - Combustion in Two-Stroke Engines
simulation using a flame propagation model. To execute this accurately, so as to phase the
peak power point at the correct "rich" mixture level and the peak thermal efficiency at the
correct "weak" mixture level, it is necessary to employ reaction kinetics within the combustion
theory. This is a complex topic well beyond the scope of this design-based text and
within the province of scientific research into combustion at a fundamental level.
The experimental evidence, based on many fueling loops carried out on several research
engines at QUB, reveals that the relative combustion efficiency with respect to equivalence
ratio, T|af, is measured as:
Tiaf = -1.6082 + 4.6509A - 2.0746A- 2
(4.3.26)
Analysis of this function reveals that T)af has a maximum of unity at a A, value of about
1.12, i.e., 12% "weak" of stoichiometric, and produces a maximum total heat release at a A,
value of about 0.875, i.e., at 14% "rich" of stoichiometric.
Effect of equivalence ratio for compression-ignition engines
The effect of fueling level, in terms of the trapped equivalence ratio, A, is best incorporated
from experimental evidence, as with the spark-ignition case above. The word trapped
equivalence ratio is used to make the point that the exhaust residual within a diesel engine
cylinder contains significant quantities of oxygen. On the other hand, for a correctly designed
two-stroke diesel engine the scavenging efficiency should be above 90% and there should not
be much exhaust gas residual present in any case. The experimental evidence reveals that the
relative combustion efficiency with respect to equivalence ratio, r|af, is measured as:
naf = -4.18 + 8.87A. - 5.14A 2 + X 3
(4.3.27)
Analysis of this function reveals that r|af has a maximum of unity at a A. value of 2.0, i.e.,
100% "weak" of stoichiometric. Above this equivalence ratio the value is constant at unity.
As a diesel engine will produce peak power at a A value of approximately 1.25, but at impossibly
high levels of black smoke emission, the equation above is sensibly applicable for equivalence
ratios between unity and 2.0.
4.3.4 Heat transfer during the closed cycle
The experience at QUB is that, particularly for spark-ignition engines, the most effective
and accurate method for the calculation of heat transfer from the cylinder during the closed
cycle is that based on Annand's work [4.15,4.16,4.17]. For diesel engines, it should be added
that the heat transfer equation by Woschni [4.18] is usually regarded as being equally effective
for theoretical computation. Typical of the approach to the heat transfer theory proposed
by Annand is his expression for the Nusselt number, Nu, leading to a conventional derivation
for the convection heat transfer coefficient, Q,. The methodology is almost exactly that adopted
for the pipe theory in Sec. 2.4. Annand recommends the following expression to connect the
Reynolds and the Nusselt numbers:
Nu = aRe 0 - 7
305
(4.3.28)