If we take an engine and scale it up, the power to weight ratio goes down. This is because all engines, at their maximum speed, have a similar piston speed (about 8 m/s mean, 12 m/s peak) so power is proportional to piston area rather than swept volume. This rule applies all the way from tiny 1 cm3 model aircraft engines to huge 1 m bore marine Diesels. Basically it achieves:
- the same peak stress in the connecting rod
- the same Mach number for the air in the inlet ports
- same parameters for the lubricating oil
The same rules apply to gas turbines (though they start with a much higher power:weight for a given power) and animals (there are no 300 ton flying birds, fortunately).
It follows that to increase the power to weight ratio, we should use many small cylinders. Sometimes also there is a limit on the bore (about 1 m for Diesels due to thermal stresses; 100-200 mm for petrol engines to avoid knocking, depending on octane number and compression ratio) and more power can then only be achieved by adding more cylinders.
The simplest step up from a single cylinder is a twin. We have two options:
- Single crank pin. Perfectly balanced if a 90 degree V; other angles a little better than an inline twin, particularly as angle approach 90 degrees.
- Separate crank pins. Can achieve force balance by suitable phasing of the cranks, e.g. 180 degree parallel twin or flat twin; 52 degree Honda Transalp with 76 degree cranks.
Historically, many inline twin cylinder motorcycles (e.g Triumph Bonneville, Norton Commando, hundreds of others) were 4-strokes and had 360 degree cranks i.e. both pistons reached TDC simultaneously and fired on alternate revolutions. In terms of balance, these were no better than a single cylinder.