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Basically the same thing arcosine stated.
While there are many factors that would influence efficiency, the best argument for fewer cylinders is the combustion chamber volume in relation to the surface area of the cylinder walls exposed to combustion gasses and conducting heat that would otherwise be available for expansion and useful work.
3.1416 X diameter gives the circumference of the cylinder wall, while 3.1416 X radius (half the diameter) squared gives the area of a circle.
Most efficient large diesels have a bore of 3 feet and a stroke of 9 feet and run at less than 100 RPM. Low RPM, long stroke, and less cylinder wall area to absorb the heat of combustion.
4 cylinders give you a power stroke every 180 degrees of crank revolution so its easier to make them run soothly compared to less than 4 cylinders.
The area being a function of the square of the diameter means the area increases exponentially in relation to the circumference.
regards
Mech
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