Design and Simulation of Two Stroke Engines

Chapter 5 - Computer Modeling of Engines
where Ck is the value of the thermal conductivity of air at the instantaneous crankcase temperature,
Tcc, and consequently may be found from Eq. 2.3.10.
The value of Tccw is defined as the average temperature of the crankcase wall surface, the
exposed crankshaft and connecting rod surfaces, the surface of the underside of the piston,
and the exposed cylinder liner surface. The heat transfer, 5QCC, over a crankshaft angle interval,
d6, and a time interval, dt, can be deduced for the mean value of that transmitted to the
total surface area exposed to the cylinder gases:
as
dt
d9 60
x
360 rpm
(5.3.5)
then "Qcc - ChAccw(TCc Tccw)dt (5.3.6)
The total surface area within the crankcase, Accw, is composed of:
Accw = A.crankcase + Apiston + Acrankshaft (5.3.7)
It is straightforward to expand the heat transfer equation in Eq. 5.3.6 to deal with the
individual components of it, by assigning a wall temperature to each specific area noted in
Eq. 5.3.7. It should also be noted that Eq. 5.3.6 produces a "positive" number for the "loss" of
heat from the cylinder, aligning it with the sign convention assigned in Sec. 4.3.4.
The typical values obtained from the use of the above theory are illustrated in Table 5.1.
The example employed is for a two-stroke engine of 86 mm bore, 86 mm stroke, running at
4000 rpm with a flywheel diameter of 150 mm. Various state conditions for the crankcase gas
throughout the cycle are selected and the potential state conditions of pressure (in atm units)
and temperature (in °C units) are estimated to arrive at the tabulated values for a two-stroke
engine, based on the solution of the above equations. The three conditions selected are, in
table order: at standard atmospheric conditions, at the height of crankcase compression, and
at the peak of the suction process.
Table 5.1 Crankcase heat transfer using the Annand model
Pec
(atm)
1.0
1.2
0.6
Tec
(°C)
20
120
60
Nu
1770
1448
1078
Re
299,037
224,483
146,493
ch
(W/m 2 K)
It can be seen that the convection heat transfer coefficient does not vary greatly over the
range of operational conditions within the crankcase, but the variation is sufficiently significant
as to warrant the inclusion of the analytical technique within an engine simulation. The
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