Design and Simulation of Two Stroke Engines

Design and Simulation of Two-Stroke Engines
where the constant, a, has a value of 0.26 for a two-stroke engine and 0.49 for a four-stroke
engine. The Reynolds number is calculated as:
Re = ?°£]&>L (4.3.29)
Hey
The value of cylinder bore, dcy, is self-explanatory. The values of density, pcy, mean
piston velocity, cp, and viscosity, |icy, deserve more discussion.
The prevailing cylinder pressure, pcy, temperature, Tcy, and gas properties combine to
produce the instantaneous cylinder density, pcy.
Pcy
Pcy = ^cy x cy
During compression, the cylinder gas will be a mixture of air, rapidly vaporizing fuel and
exhaust gas residual. During combustion it will be rapidly changing from the compression
gas to exhaust gas, and during expansion it will be exhaust gas. Tracking the gas constant,
Rcy, and the other gas properties listed in Eq. 4.3.29 at any instant during a computer simulation
is straightforward.
The viscosity is that of the cylinder gas, u.cy, at the instantaneous cylinder temperature,
Tcy, but I have found that little loss of accuracy occurs if the expression for the viscosity of air,
|Xcy, in Eq. 2.3.11 is employed.
The mean piston velocity is found from the dimension of the cylinder stroke, Lst, and the
engine speed, rps:
cp = 2Lst rps (4.3.30)
Having obtained the Reynolds number, the convection heat transfer coefficient, Ch, can
be extracted from the Nusselt number, as in Eq. 2.4.3:
CkNu
C h " d (4.3.31)
where Ck is the value of the thermal conductivity of the cylinder gas and can be assumed to be
identical with that of air at the instantaneous cylinder temperature, Tcy, and consequently may
be found from Eq. 2.3.10.
Annand also considers the radiation heat transfer coefficient, Cr, to be given by:
u cy
Cr = 4.25 x 10" 9 (Tc 4 y - TC 4 W) (4.3.32)
However, the value of Cr is many orders of magnitude less than Q,, to the point where it may
be neglected for most two-stroke cycle engine calculations. The value of Tcw in the above
306