This paper describes an experimental investigation into the effect of bore/stroke ratio on a simple two-stroke engine. This was achieved with a special purpose engine of modular design. The engine allowed four combinations of bore and stroke to be contrived to yield a common swept volume of 400 cm3 with bore/stroke ratios of: 0.8, 1.0, 1.2 and 1.4.
Other factors that might affect engine performance were standardised: the exhaust, intake and ignition systems were common, the combustion chamber designs were similar, scavenge characteristics were similar, port timings and time-areas were kept the same, and cylinder and crankcase compression ratios were also kept the same.
The most important conclusions were:
Engine power was greatest with the compromise bore/stroke ratio of 1.0 or 1.2.
Combustion efficiency tended to decrease with increasing bore/stroke ratio.
Mechanical efficiency tended to increase with increasing bore/stroke ratio.
The specific fuel consumption tended to rise with increasing bore/stroke ratio.
The experimental investigation concluded that a square or slightly over square bore/stroke ratio produces an engine with the best brake performance.