Design and Simulation of Two Stroke Engines

Design and Simulation of Two-Stroke Engines
significance is such as to justify an experimental program to provide a more accurate value of
the constant, a, in the Annand model than that which is assumed here as applicable to a twostroke
engine crankcase. The values determined for the convection heat transfer coefficient,
Ch, lying beteen 200 and 320 W/m 2 K, are much as one would determine from an even simpler
analysis, or could be culled from the literature describing somewhat similar physical situations
in other fields of engineering.
5.4 Mechanical friction losses of two-stroke engines
As the designs of two-stroke engines contain details of physical construction which are
relevant variables as far as the friction losses are concerned, it is not possible to determine a
fundamental theoretical approach to this topic. The best I can offer is a series of empirical
relationships which have been shown to correlate quite well with experimental observations
on various types of two-stroke engines. The situation is more complex for the two-stroke
engine in comparison to the four-stroke unit in that two-stroke engines are manufactured with
both "frictionless" bearings, i.e., rolling element bearings using balls, rollers or needle rollers,
and also with hydrodynamic bearings, i.e., the oil pressure-fed plain bearings as seen in
virtually all four-stroke cycle engines. The friction characteristics of any engine are also
related to the type of lubrication, but virtually all engines which have crankshafts supported
by rolling element bearings employ a total loss oiling system into the crankcase of the engine.
This is normally in the form of (i) pre-mixed lubricating oil and fuel supplied via the carburetor
and hence to the various bearings and friction surfaces, (ii) a pump supplying a metered
quantity of oil directly to the bearings and friction surfaces, or (iii) a pump supplying a metered
quantity of oil into the inlet tract to be broken up by the air flow and distributed to the
bearings and friction surfaces. As a historical note, the British word for pre-mixed oil and
gasoline (petrol) was "petroil."
The friction characteristics of two-stroke engines can be divided into several classifications,
i.e., those with rolling element bearings or those with plain bearings, and for those units
which employ spark ignition or compression ignition. Virtually all compression-ignition, twostroke
engines use plain bearings. All of the input or output data in the empirical equations
quoted below are in strict SI units; the friction mean effective pressure, fmep, is in Pascals;
the engine stroke, Lst, is in meters; the engine speed, N, is in revolutions per minute.
It will be observed that the equations below relating friction mean effective pressure to
the variables listed above are of a straight-line format:
fmep = a + bLstN
where a and b are constants. It is interesting to note, and it would be supported by more
fundamental theory on lubrication and friction, that the value of the constant, a, is zero for
rolling element bearings. The term in these equations, which combines stroke and engine
speed, is piston speed in all but name.
Spark-ignition engines with rolling element bearings
This classification covers virtually all small engines from industrial units such as chainsaws
and weed trimmers to the outboard, the motorcycle and the snowmobile. From the experi-
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