Design and Simulation of Two Stroke Engines

Design and Simulation of Two-Stroke Engines
(l + AFR0)rhf
n fi 17^
Exhaust gas mass flow rate = M
ex
— kgmol/s ^i.u.i /;
R, t n fl , (1 + AFR0)rhfV%0 (161g)
Exhaust Oo mass flow rate = - kgmol/s ^I.V.LOJ
100Mex
rvi A T7T?
Engine 02 mass inflow rate = 0.2314 — f - 2- kgmol/s C 1 - 6 - 19 )
M o2
The numerical value of 0.2314 is the mass fraction of oxygen in air and 32 is the molecular
weight of oxygen. The value noted as V%Q is the percentage volumetric concentration of
oxygen in the exhaust gas.
Trapping efficiency, TE, is given by:
Hence:
or:
TE =
air trapped in cylinder
air supplied
_ air lost to exhaust _ Eq. 1.6.18
air supplied Eq. 1.6.19
TE = 1- (1 + AFR0)V%02MQ2 (1620)
23.14 x AFR0Mex
Assuming simplistically that the average molecular weight of exhaust gas is 29 and that
oxygen is 32, and that atmospheric air contains 21% oxygen by volume this becomes:
TE = 1- (1 + AFR0)V%Q2 (1621)
21xAFR0
This equation, produced by Kee [1.20], is programmed together with the other parameters
in Prog. 1.4.
The methodology emanates from history in a paper by Watson [1.21] in 1908. Huber
[1.22] basically uses the Watson approach, but provides an analytical solution for trapping
efficiency, particularly for conditions where the combustion process yields some free oxygen.
The use of this analytical technique, to measurements taken in a two-stroke engine under
firing conditions, is described by Blair and Kenny [1.23]; they provide further data on the incylinder
conditions at the same time using the experimental device shown in Plate 3.3.
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