The Four-Stroke Power Cycle

Posted June 7, 2013

What is your car’s RPM measurement when idling in neutral? How about when you are cruising at 35 mph? 50 mph? Did you know that if your RPM measurement is not consistent, you could be blindly wasting money every time you fill up at the gas station! Paying attention to these details is vital to your car’s health as well as your automotive knowledge.

RPM stands for revolutions per minute and measures the revolutions of an engine’s crankshaft. This blog will explain the cycle that the RPM gage on your dashboard is measuring, and you can finally understand exactly what your engine is doing to make your car run—a process overlooked by many car owners.

The Cycle

As your engine turns, a crankshaft spins in order to move attached pistons up and down. Each movement of a piston is called a stroke. In a conventional internal combustion engine, the pistons complete a four-stroke power cycle (down, up, down, up) to move the air/fuel mixture through the engine in order to properly power your vehicle.

Here is a brief explanation of what happens during these four strokes:

1. Intake stroke

With its first downward movement, the piston creates a vacuum chamber in the space it created in the cylinder. While the rings on the piston seal off any possible chance for air to enter the cylinder from the bottom, intake valves, located at the intake manifold towards the top of the cylinder, open and let the air/fuel mixture fill the vacuum.

2. Compression stroke

The piston will then make its first upward movement. In doing so, the piston compresses the air/fuel mixture into the combustion chamber—a tiny space between the top of the piston and the top of the cylinder. Located at the top of the compression chamber is the electrode end of the spark plug. The intake valve will then close, ensuring that the compressed mixture can’t leave the chamber.

3. Power stroke

As a spark is produced across the electrode end of the spark plug, it ignites the compressed air/fuel mixture exploding the mixture and forcing the piston down for its third stroke. The power that pushes the piston down is transmitted through the connecting rod to the crankshaft. It continues through the clutch, transmission, driveshaft and so on until the wheels are spinning.

4. Exhaust stroke

Lastly, the burned gasses in the combustion chamber must be removed. The piston’s fourth and final stroke will move upward as the exhaust valve opens and the burned gasses are pushed out of the exhaust manifold into the vehicle’s exhaust system.

If you are having any ignition trouble or are concerned with your RPM measurement our ASE-certified automotive mechanics are ready to listen to your needs. So come down to our White Bear Lake auto repair shop, or call us at (651)-426-0462 to set up an appointment today!