A four-stroke engine is so called because in the process of producing power, the piston passes up and down the bore four times. These strokes or events are the induction, compression, power and exhaust stroke. As you may suppose, the effective function of all are important toward producing a high-output engine. But of the four, the compression stroke has far less obvious but more far-reaching implications on an delivery valve engine’s optimal spec and its subsequent success as a power producer.
Obviously the principal idea of the compression stroke is to compress the intake charge as effectively as possible, and to do so with minimal leakage. There are two principal factors associated with the compression ratio. The first is the calculated ratio, which we will refer to as the geometric or static ratio. The next, and equally important, factor is how effectively, and to what degree the physical components of the engine compress the charge into the combustion space.
For the combustion stroke itself, the higher the compression ratio is, the higher the resultant combustion pressure is. Not only that, but the charge also burns faster, thus necessitating less advance for an optimal burn event. In addition to this, the amount of residual exhaust remaining in the chamber at the beginning of the nozzle intake stroke is less. This reduces unwanted intake dilution by the exhaust. These are the most obvious power-enhancing factors, but they are not the biggest influencing factors by any means.
Next is the power stroke. Every bit of power the engine will develop is made on this stroke. We need to make sure everything that happens before, during and after this stroke either enhances it or, at the very least, has minimal negative impact on it. That means not only sealing up the cylinder in the first place, but also making sure it does not leak throughout the power stroke and that its sealing ability is not at the expense of high ring-to-cylinder-wall friction.
Last of the four strokes is the exhaust. Here we need to make sure that cylinder emptying is done without undue pumping losses. Any pressure remaining in the plunger cylinder while the piston is on the way up the bore is negative power. As far as exhaust stroke efficiency is concerned, having a higher CR can, as we will see later, lead to significantly reduced pumping losses.