Be they two-stroke, four-stroke, diesel or rotary, internal combustion engines all work on the same principle: Fuel and air can be made to combust. Doing so causes rapid expansion of the resulting gasses. This expansion of gasses is “mechanically contained” in a combustion chamber in such a way that the energy from it is used to turn a crankshaft.
Rotary engines are dissimilar enough to require their own discussion, so if these are of interest to you, then I recommend you look elsewhere. The other types of engines mentioned, all feature a piston attached to the crankshaft via a conrod travelling up and down inside a cylinder.
When the fuel-air mixture is ignited the piston is at the top of its stroke and the gasses expand, forcing the piston down. This is the “power stroke”. The distance the piston travels from the top of its stroke to the bottom, is measured and referred to (rather unimaginatively) as the engine’s “stroke”. The diameter of the cylinder in which the piston travels is measured and called the “bore” size.
Together, the bore and stroke of an engine are used to define the engine capacity. The volume of the cylinder , which has the diameter of the bore, and the height of the stroke, gives you the capacity of a single cylinder. Multiply this figure by the number of cylinders in the engine and you have the total engine capacity.
If a four cylinder engine has a bore of 74mm and a stroke of 58mm…
For those of you who do not remember:
V = π * r ² * height.
The radius is 3.7cm (half the bore) and the height is 5.8cm.
Using the formula, a single cylinder, has a capacity of 249.45 cubic centimetres (cc).
Multiply this by the number of cylinders and you end up with a 997.8cc engine.
As a piston travels up and down inside a cylinder, it must come to a complete stop and change direction. Arresting and reversing the piston’s momentum takes energy. The faster the piston is travelling, and the heavier the piston is, the more energy is required. This energy is supplied by the momentum of the crankshaft. This means that some of the engine’s output is spent in changing the velocity of the piston. For this reason, there tends to be an optimum speed at which the piston travels. The longer the stroke of an engine, the further the piston travels. So, in an engine with a long stroke, the piston travels faster at a given crankshaft speed than in an engine with a short stroke. As a rule of thumb: the longer the stroke of an engine, the slower the maximum revolutions per minute (RPM).
There is no ideal size for an engine bore or stroke. A lot depends on what the engine is powering. Engines that need to move a lot of weight, require more torque.
Torque is a twisting force applied to an object, like a wheel or a crankshaft. For our purposes, we will consider that torque is measured in pounds-force feet (lbf-ft) meaning the equivalent of a given force, in pounds, acting on the end of a lever of length in feet. … For example, standing with 180 pounds body weight on a lug wrench one foot long yields 180 lbf-ft of torque.
Work is the application of force over a distance. Unfortunately, the units used are the same (pounds times feet) but we write this as ft-lb just to distinguish it. The real difference is that in this case, the “feet” part means feet of movement.
Power is the application of work within a finite time. 550 ft-lb of work in one second is one horsepower.
From this, we can see that torque and power are related. In fact:
hp = (torque * RPM) / 5250
(Where torque is measured in lbf-ft)
Another rule of thumb is that: engines with a longer stroke produce more torque. There are other factors that influence power and torque outputs of an engine. Cubic capacity never goes astray when you need more power and torque…
A motorcycle does not need to shift a vast amount of mass – but tends to be size constrained. (unless of course, you are prepared to just jam in a big car engine)
More weight conscious sports bikes tend to concentrate on obtaining a high power figure, over obtaining a high torque figure. Given the formula above, we can see that to produce more power, we either need more torque or more engine revs. So, sports bikes tend to concentrate on producing power by spinning quickly, which means they end up having a short stroke.
Getting the fuel-air mixture into the combustion chamber also affects power and torque figures of an engine, but that is a story for another time.