There’s only so much we can upsize with a single piston cylinder arrangement before hitting the mechanical upscaling effect that are, beyond a point, almost impossible to overcome. To get more power imagine doubling the cylinder stroke as well as piston diameter.
The resultant will be a 8 times increase in the engine volume, for same MEP the power output will be 4 times and torque 8 times but the weight of the piston, connecting rod and crank will also increase 8 times making previous high rpm’s from the simple single-cylinder engine impossible to achieve without breaking the crank or the connecting rods! All such complications apart, there’s what is called the ‘flywheel effect’ that also needs attention. In a 4 stroke cycle, it is only the power stroke that is producing power while the rest of the 3 strokes ride piggy back on it.
Going by this fact the piston comes down the fastest during the power stroke and will progressively slow down during the rest of the 3 strokes, the greatest slowing down occurring during the compression stroke. This fluctuation in crank speed will make a large single cylinder engine extremely jerky in operation and the jerkiness at high rpm’s will mean harsh vibrations which in turn might even damage the engine itself. Of course the jerkiness can be reduced by using a heavier crankshaft, or heavy flywheel, that stores more energy and so is not so affected as the rest of the cycle consumes it.
Older motorcycles had often a heavy crank which made them feel oh-so-smooth at lower rpms and also allowed a really slow idling rpm that was music to the motorcycle’s enthusiasts of the day. But heavy cranks mean slower responding engines and also heavy engines – not really very good at least as far as motorcycling goes.
So in, comes the second or the third, the fourth or even the sixth cylinder. There’s simple reason going for multi-cylinders when talking of higher engine volumes – the more the cylinders the lesser are the intervals between power strokes and so the more even and smooth is the engine while running. Again with multi-cylinders you can have shorter strokes and so higher peak rpms. The smaller pistons and other reciprocating components mean lighter parts which can go up and down faster, allowing higher rpms.
The smaller a cylinder the greater is its surface to volume ratio, which allows higher compression ratios without the resulting overheating of the engine, because there’s more surface area to dissipate the extra generated heat at higher compression. Multi-cylinders can have lighter cranks and/or flywheels and so will respond far quicker to throttle inputs than would an equivalent volume single.Tag: EngineSingle-CylinderMulti-CylinderPerformancePower-StrokeTechnicalMechanicalTechnology