Design and Simulation of Two Stroke Engines

Design and Simulation of Two-Stroke Engines
engine the cyclic period is 360° and the modeler would perform the summation appropriately.
If the engine were a multi-cylinder unit of n cylinders then the accumulation process would be
repeated at all of the intake ports of the unit. To transfer that data to a delivery ratio, DR, value
the conventional theory is used where pref is the reference density for the particular industry
standard employed and Vsv is the swept volume of all of the cylinders of the engine:
cylinder = n
Z m a
Delivery Ratio pR _ cylinder=i (2.18.72)
Pref v SV
The term for cyclic air flow rate for a two-stroke engine is called scavenge ratio, SR,
where the reference volume term is the entire cylinder volume. This is given by:
cylinder = n
Scavenge Ratio SR = cylinder^ (2.18.73)
Pref ( V SV + V CV)
The above example is for one of the many such terms required for design assessment
during modeling. The procedure is identical for the others. Some more will be given here as
further examples.
Work done during an engine cycle
The work output for an engine is that caused by the cylinder pressure, pc, acting on the
piston(s) of the power unit. The in-cylinder work is known as the indicated work and is often
reduced to a pseudo-dimensionless value called mean effective pressure; in this case it is the
indicated mean effective pressure. Let us assume that there are n cylinders each with an identical
bore area, A, but a total swept volume, Vsv- The piston movement in any one cylinder at
each time step is a variable, dx, as is the time step, dt, and the volume change, dV
The work done, 8W, at each time step in each cylinder is,
8W = PcAdx = PcdV (2.18.74)
The total work done, Wc, for that cylinder over a cycle is given by the summation of all
such terms for that period. Remember that the thermodynamic cycle period is 0c, and for a
four-stroke engine is 720° and for a two-stroke engine is 360°. Thus this accumulated work
done is,
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