Design and Simulation of Two Stroke Engines

Design and Simulation of Two-Stroke Engines
PORT PLAN LAYOUT
Fig. 1.4 QUB type of deflector piston of cross-scavenged engine.
diesels of the uniflow-scavenged two-stroke variety with thermal efficiencies in excess of
50%. However, these low-speed engines are ideally suited to uniflow scavenging, with cylinder
bores about 1000 mm, a cylinder stroke about 2500 mm, and a bore-stroke ratio of 0.4. For
most engines used in today's motorcycles and outboards, or tomorrow's automobiles, borestroke
ratios are typically between 0.9 and 1.3. For such engines, there is some evidence
(presented in Sec. 3.2.4) that uniflow scavenging, while still very good, is not significantly
better than the best of loop-scavenged designs [1.11]. For spark-ignition engines, as uniflow
scavenging usually entails some considerable mechanical complexity over simpler methods
and there is not in reality the imagined performance enhancement from uniflow scavenging,
this virtually rules out this method of scavenging on the grounds of increased engine bulk and
cost for an insignificant power or efficiency advantage.
1.2.4 Scavenging not employing the crankcase as an air pump
The essential element of the original Clerk invention, or perhaps more properly the variation
of the Clerk principle by Day, was the use of the crankcase as the air-pumping device of
the engine; all simple designs use this concept. The lubrication of such engines has traditionally
been conducted on a total-loss basis by whatever means employed. The conventional
method has been to mix the lubricant with the petrol (gasoline) and supply it through the
carburetor in ratios of lubricant to petrol varying from 25:1 to 100:1, depending on the application,
the skill of the designers and/or the choice of bearing type employed as big-ends or as
main crankshaft bearings. The British term for this type of lubrication is called "petroil"
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