Engine Overview — The Four Strokes

An engine is a machine that converts fuel into motion. That is all it does. It mixes air and fuel together, compresses the mixture, ignites it, and uses the force of the expanding gases to push a piston down inside a cylinder. That piston is connected to a crankshaft that converts the up-and-down piston motion into rotational motion — spinning — that eventually turns the wheels. Every gasoline engine you will ever work on repeats four steps over and over, hundreds of times per minute, in each cylinder.

The piston moves down from the top of the cylinder to the bottom. As it moves down, it creates a vacuum — like pulling back a syringe. The intake valve opens and air mixed with fuel gets pulled into the cylinder by that vacuum. Think of it like breathing in. The cylinder fills with the air-fuel mixture.

Both valves close. The piston moves back up from bottom to top and squeezes the air-fuel mixture into a very small space at the top of the cylinder. This compression is critical — squeezing the mixture makes it much more powerful when it ignites. A typical gasoline engine compresses the mixture to about one-tenth of its original volume. That is called a 10-to-1 compression ratio.

At the exact right moment when the piston reaches the top and the mixture is fully compressed, the spark plug fires. A spark jumps the gap at the tip of the plug and ignites the compressed mixture. The fuel burns rapidly, the gases expand with tremendous force, and that force pushes the piston back down with thousands of pounds of pressure. This is the only stroke that produces power. The other three strokes just prepare for this one.

The exhaust valve opens and the piston moves back up, pushing the burned gases out of the cylinder and into the exhaust system. The cylinder is now empty and ready to start the intake stroke again. The whole cycle — intake, compression, power, exhaust — takes two full rotations of the crankshaft. In a four-cylinder engine running at 3,000 RPM, each cylinder fires 1,500 times per minute. All four cylinders firing in sequence produce 6,000 power strokes every minute. That is what makes the engine run.

Every engine needs three things to run. Fuel — the right amount, at the right time. Air — mixed with the fuel in the correct ratio. Spark — at the precise moment. If any one of these three is missing or wrong, the engine will not run correctly. When you are diagnosing a no-start or driveability concern, your first question is always — which one of these three is the problem? That one question focuses every diagnosis.