Compression Comprehension

A four-stroke (or four-cycle) 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 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. We need to bear that in mind as we move on, because 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. In essence what we are going to look at here is a measure of how effectively our theoretical compression ratio is translated into real world pre-combustion cylinder pressure. This is highly influenced by such things as ring and valve seal and valve opening/closing events.

You may well have heard the term Compression Ratio (CR) many times, but may not know exactly what defines the CR or how it's calculated. If so, you need to refer to the nearby sidebar.

Also it may all look like we are treading a well-worn path here, but it's worth taking a quick look at the four strokes, as each of the other three is intimately tied to the compression stroke. Check out the four-stroke sequence of events in the sidebar. Every one of these strokes must accomplish its goal effectively for an engine to be able to produce a high output. Let's start with the intake stroke. The more efficiently the cylinder is filled on the induction stroke, the more rpm the engine can turn before it "runs out of breath." The better the intake filling is, the higher the pressure achieved on the compression stroke. This, along with as high a compression ratio as the fuel will stand, means significantly higher pressures on the power stroke.

Moving on to the compression stroke itself, we find that 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 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. There are other less obvious but more influential implications that we will deal with later when we look at the CR and compression factors in detail.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 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.

Thermodynamics Made Easy
It takes the barest of mental agility to appreciate that increasing the CR will raise cylinder pressure, thus causing torque output throughout the rpm range to simply follow suit. What is less obvious is that the increase in output from the higher CR comes about largely due to an increase in thermal efficiency. The thermal efficiency is a measure of how effectively the engine converts the heat-generating potential of the fuel, when burned with an appropriate amount of air, into mechanical power. To explain all this (starting from the raw fuel and air to the output at the flywheel) is rather more complex than we have the space (or inclination) to deal with, but no matter, as the most pertinent and relatively simple part applying here is not.

To more clearly appreciate how the thermal efficiency is improved, we need to consider what is essentially the opposite side of the coin to the CR. This is the Expansion Ratio (ER) and describes what occurs as the piston moves down the bore on the power stroke rather than what happens as it moves up on the compression stroke.

Take a look at the Cylinder Pressure Decay chart and then let's go through the characteristic difference (computed taking into account typical heat losses) between a high-compression cylinder versus a low-compression one. For a moment, let us imagine that both the 15:1 and the 2:1 cylinders start off at TDC with 1,000 psi. As the piston of each cylinder moves down the bore, the drop in pressure follows a distinctly different line. The 15:1 cylinder drops pressure much faster than its 2:1 counterpart because of its more rapid change in volume. It only has to go down the bore a short way for the original volume to have doubled, whereas the 2:1 cylinder must travel to the bottom of the bore to double its original volume. At the bottom of the stroke, the 15:1 cylinder has dropped down to about 25 psi above atmospheric whereas the 2:1 cylinder is still at some 260 psi. In simple terms the high-compression cylinder, when the exhaust valve opens at BDC, is only dumping 2.5 percent of its original pressure whereas the 2:1 cylinder is dumping 26 percent!

Up to this point we have assumed that both cylinders start with 1,000 psi. In reality, the best that the 2:1 ratio cylinder would generate is about 200 psi. That produces the lower curve (light blue line) on our graph. Both the 2:1 and 15:1 cylinders will draw in about the same amount of fuel and air. But we can see that the 15:1 cylinder has more area under the curve by an amount equal to the green shaded area. The addition of the green shaded area under the curve amounts to about a doubling of the power output from the same amount of fuel and air. That means from the same heating value of fuel we have doubled the thermal efficiency and in so doing extracted twice the power.

From what we have covered so far, you can see why a high-compression cylinder produces better power and fuel economy. It is not solely because the charge is squeezed harder and the resulting combustion pressure is increased, but also because the higher expansion ratio allows more energy to be extracted from the original high-pressure charge.