What Is The Four-Stroke Piston-Engine Cycle?

Let's begin with just one cylinder of the four-stroke piston engines that power nearly all of today's cars, trucks, and motorcycles. A crankshaft rotates in main bearings and carries an off-center crankpin. On the crankpin is a connecting rod whose other end is fastened to a piston. The piston is a close fit inside a cylinder, such that rotation of the crankshaft causes the connecting rod to drive the piston up and down in the cylinder. Thus, the connecting rod converts the rotary motion of the crankshaft into the back-and-forth motion of the piston in its cylinder. The piston is effectively sealed to the cylinder by two or more piston rings.

The end of the cylinder above the piston is capped by a cylinder head, into which are fitted valves that can open and close. In the simplest case, there are two valves—one intake valve to let fresh fuel-air mixture into the cylinder and one exhaust valve to let spent combustion gas out of the cylinder. The other end of the cylinder is open to allow the connecting rod to do its work.

Heat engines operate on a simple concept: When we heat a gas (air, for example) in a sealed volume, its pressure rises. If we give that rising gas pressure something movable to push against, we can get mechanical work from it. In the case of the piston engine, the sealed volume is that trapped in a cylinder between a movable piston and a closed cylinder head. If we toss that assembly into a tub of boiling water, when the heat reaches the air trapped in the cylinder, its temperature and pressure will rise, driving the piston away from the cylinder head. Nice idea as far as it goes but how can we use it to get home for Thanksgiving?

We need a way of rapidly repeating this process of heating a trapped volume of gas to generate mechanical power. We need a power-producing cycle whose steps can be repeated over and over. One possible cycle would be to alternately submerge our piston and cylinder, first in boiling water and then in ice water, alternating back and forth, causing the piston to move down the cylinder in the hot boiling water, and move up the cylinder in the cold ice water. This would be slow and inefficient because it takes a long time to heat and cool all those metal parts, when all we need to heat is the gas inside the cylinder.

The obvious way to very rapidly heat a volume of air is to mix some fuel with that air and ignite it. When we look into this, we find that burning a chemically correct mixture of gasoline and air inside a sealed volume causes its pressure to jump up to about seven times its original pressure. Now we’re talkin’—we might make it to Thanksgiving yet. But after the fuel and air are burned, how can we repeat the process? How do we get rid of the burned gases and replace them with fresh mixture? We think this through and come up with the necessary steps.

First, we have to remove the hot but expanded combustion gas from the cylinder. To do this, we open the exhaust valve and push the piston toward the cylinder head, pumping out the spent combustion gas. This is the exhaust stroke of the piston.

With the piston near to but not touching the cylinder head, we close the exhaust valve and open the intake valve. Pulling the piston away from the cylinder head, we draw in a fresh charge of air with fuel mixed into it (fuel is added to the in-rushing stream of air by a carburetor or fuel injector). This is the intake stroke of the piston, which ends with the piston at the bottom of its stroke (closest to the open end of the cylinder) with both valves closed.

Next comes a need for some thought. If we ignite the fuel-air mixture now, with just atmospheric pressure in the cylinder (14.7 pounds per square inch at sea level), combustion will heat and raise the pressure in the cylinder to 7 x 14.7 = 103 pounds per square inch (psi). While that's a useful pressure, we can easily get a lot more. Here's how: Instead of igniting the fuel-air mixture with the piston at the bottom of its stroke, we now drive the piston toward the cylinder head, compressing the fuel-air mixture in the cylinder to a much higher pressure, say, 150 psi. It takes power to do this, but it's an investment because when we ignite our trapped fuel-air mixture compressed to 150 psi instead of at 14.7 psi, we now get 7 x 150 = 1,050 psi. That gives us some really serious force against the piston. It is to achieve this much higher combustion pressure that we compress the fuel-air mixture before we ignite it. This is the compression stroke, which, with both valves closed, begins with the piston at its bottom position and the cylinder full of fresh fuel-air mixture, and ends with the piston rapidly nearing the cylinder head.

The most convenient and rapid way to ignite the compressed fuel-air mixture is by electric spark, which is made to jump between the metal electrodes of a spark plug, screwed into the cylinder head. The spark ignites the fuel-air mixture, transforming the chemical energy of the fuel into heat and pressure in the resulting combustion gas, which drive the piston away from the cylinder head. As the piston moves down its cylinder, the temperature and pressure of the expanding combustion gas fall rapidly. As the piston nears its bottom position, the exhaust valve begins to open, starting the release of the expanded combustion gas. Combustion is rapid burning, not an explosion. Grenades explode, which is not something an engine’s mechanical parts can tolerate.

As the exhaust valve opens and the piston starts moving toward the cylinder head again, exhaust gas is cleared from the cylinder so that another intake stroke can follow.

With our piston connected to a rotating crankshaft by a connecting rod, the piston makes two strokes for every crank rotation, completing the four-stroke cycle in two rotations of the crank. To save excess verbiage, we measure the speed of the crankshaft as revolutions per minute, rpm for short. As we cruise to Thanksgiving dinner in my four-cylinder econobox, 2,400 rpm translates to 2400/60 = 40 revolutions of the crank per second. That, with two power strokes per revolution, gives 80 power strokes per second.

To get more power, we can do what Glenn Curtiss originally did: Make the piston and cylinder bigger so we can draw in, compress, and burn a larger volume of fuel-air mixture. Or we can add more cylinders, making such arrangements as V-twin, parallel twin, inline-four, V-6, etc. This makes engines look more complicated, but it’s just a cluster of one-cylinder engines sharing a common structure; the operating cycle is the same for all the cylinders. Some engines have more than two valves in each cylinder’s head. Four valves per cylinder is the most usual arrangement today—two intakes and two exhausts.

This cycle, consisting of the four piston strokes (usually given as intake, compression, power, and exhaust) can be repeated with truly amazing speed. Four-stroke Formula 1 engines of the V-8/V-10 era or MotoGP engines of the 800cc era could reach 20,000 rpm. Research engines have operated at least to 27,000 rpm. Although such rpm sound fantastic, we know that thousands of riders of 600cc production sportbikes have seen their tach needles come swinging back past 16,000. It's all a matter of good engineering.