Definition
A four-stroke engine is an internal combustion engine that makes one burst of power while the piston moves four times inside the cylinder (down twice and up twice). It repeats four steps, intake, compression, combustion, and exhaust, like gears turning, and converts the chemical energy in fuel into the spinning force that moves a car.
The Piston's Four-Beat Dance
Pull back the plunger of a syringe and water rushes in. The piston in an engine does the same thing. It moves down and pulls air (in some engines, air mixed with fuel) into the cylinder. This first move is called the intake stroke.
Next comes the compression stroke, where the air gets squeezed hard. With all the valves closed, the piston rises again and packs the gas it just inhaled down to about one-tenth of its original volume in a gasoline engine. The tightly squeezed mixture gets very hot and its pressure shoots up. Now even a tiny spark can set off a big explosion.
Right at this moment, a gasoline engine's spark plug flashes. A diesel engine has no spark plug. It squeezes the air even harder until it is hot, then sprays in fuel, and the fuel lights by itself. The fire spreads in an instant, the gas expands violently, and that huge pressure slams the piston back down. This combustion stroke is where the real force that moves the car is born.
Finally, the piston pushes back up and shoves the burned leftover gas out through the exhaust port. This fourth move is the exhaust stroke. These four steps repeat dozens of times every second without a break.
Four Moves, Just One Push of Power
Strangely, out of the four moves, the engine makes power by itself in only one: the combustion stroke. The other three don't make energy. They actually use it, getting ready for the next combustion.
So how do those three powerless moves keep going without stopping? The secret is a heavy iron wheel at the end of the engine. Combustion gives this wheel a strong spin, and just like a spinning top keeps turning, the heavy wheel's inertia keeps the pistons moving up and down on their own and carries them through the next steps. In an engine with several cylinders, the combustions in the other cylinders take turns adding their push.
A piston that only moves straight up and down can't turn a wheel. That's why a bent, rod-shaped part called a crankshaft is attached beneath the pistons. Think of pedaling a bicycle: your feet push up and down, and the wheel goes around in a circle. The crankshaft is the key part that turns the piston's up-and-down motion into round spinning motion.
To Be More Precise: Why Use Four Strokes?
To be more precise, not every engine in the world goes through four steps. Lawn trimmers, some small motorcycles, and huge ship diesel engines sometimes use a two-stroke engine, which fires once for every two piston moves.
But a small two-stroke engine takes in fresh air and pushes out exhaust at the same time, so some unburned fuel leaks out. That makes more smoke and tends to waste fuel. A four-stroke engine, on the other hand, mostly opens its intake and exhaust valves separately, so it tends to have better fuel efficiency and cleaner exhaust.
Today's passenger cars choose four-stroke engines almost without exception. Lately, computers control the timing of the valves opening and closing down to a thousandth of a second, so a single drop of fuel can carry a car farther and more smoothly.
Common misconceptions
An engine ignites and makes power every single time the piston moves up or down.
During the piston's four moves, only the combustion stroke ignites and makes power, once. The other three moves run on the flywheel's spinning inertia and the push from other cylinders, getting ready for the next combustion.
Where you meet it
The engines in ordinary gasoline and diesel passenger cars
During one cycle of four piston moves (two up, two down), the crankshaft inside the engine turns exactly two times.
In one sentence
It repeats four steps, intake, compression, combustion, and exhaust, turning the piston's up-and-down motion into the spinning force that drives the wheels.