[dcb]

This is a little hard to explain, and I have to make some generalizations...

If you hold displacement and stroke equal, an engine with less cylinders will be more fuel efficient. The ideal (and the limit) being one cylinder, where the shape of the total piston area is a single circle.

Lets say that you have a 2 cylinder engine with pistons of radius 1, the total circumference around both pistons is 12.57.

Whereas if you have a single piston engine with the same stroke/displacement it would have a piston radius of 1.414 and a total circumference of 8.88.

This means that the single piston engine has about 1/3 less contact with the cylinder wall, and thus significantly less friction, even though the stroke and displacement are identical.


Also consider that less pistons means less connecting rods and bearings, and less valvetrain and camshaft lobes.


And if we are comparing a larger 4 cylinder to a smaller 3 cylinder, the larger 4 cylinder is going to have more pumping losses for a given power output