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What Are the Four Steps of the Combustion Cycle?

The engine in your car works on a four-stroke cycle, a design that offers improved efficiency at the expense of more complex construction. Through combustion, your engine extracts heat energy from the hydrocarbons in gasoline and turns it into mechanical energy by rotating the crankshaft. Understanding how this process works will give you a fresh appreciation for what's happening under the hood as you drive your car down the road.

Induction
- Any time your car is running the crankshaft is turning, and as it turns it pushes and pulls on cranks connecting it to the pistons in the cylinders of the engine. During the first step in the cycle, the induction stroke, the crank pulls the piston down, reducing the pressure in the cylinder. The fuel injector sprays fuel into the air stream and the intake valve leading into the cylinder opens so the low pressure in the cylinder can suck the fuel and air in.
Compression
- Next comes the compression stroke. The intake valve closes so the fuel and air are trapped inside. As the crankshaft continues to turn, the piston surges up the cylinder, squeezing the fuel-air mixture into a very small space. In the process, the engine is doing work on the fuel and air, compressing them and heating them up. During this stage and the next, the exhaust valve remains closed, and the fuel injector ceases to fire.
Power
- The power stroke is where the real action begins. It starts once the volume in the cylinder has reached its minimum point. At that moment, the spark plug in the cylinder fires, and the spark ignites the fuel-air mix, triggering a rapid explosion. The force of this explosion drives the piston back down the cylinder. The piston pushes on the crank as it descends, indirectly exerting torque on the crankshaft so it will turn more rapidly.
Exhaust
- The exhaust stroke is the final step. During this part of the cycle, the exhaust valve opens at the top of the cylinder. Driven by the rotating crankshaft, the piston ascends the cylinder, forcing the waste gas out the valve and into the exhaust system. From here, the waste gas passes through the catalytic converter and the muffler before leaving your car via the tailpipe. In the meantime, the cylinder begins the induction stroke anew and repeats the cycle.

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