What Are The Three Purposes Of a Piston?

Engines are complicated beasts with a lot of parts that move around. But a piston is one of the most recognizable parts by name. You may have heard how important it is, but you probably can’t tell me what it does. Understanding how the piston works give you to comprehend far more about the engine, including how they and the rings can fail.

What Is a Piston?

A piston is a moving disk inside a cylinder. Piston rings seal the cylinder so that gas can’t leak out. As a liquid or gas inside the cylinder expands and contracts, the disk moves inside the cylinder. A piston helps turn heat energy into mechanical work and back the other way around. Pistons are a very important part of heat engines because of this.

Pistons work by sending the force that comes from gas in the cylinder expanding to a crankshaft. This gives a flywheel the momentum to turn. This kind of machine is called a reciprocating engine.

For a piston to keep turning heat energy into work, it has to go through a cycle, and there are many ways to go through this cycle.

What Does a Piston Do?

The piston moves up and down in the cylinder when the engine is running. When the piston gets to the point where it turns, it suddenly slows down and then speeds up again. This makes forces of inertia that push on the piston.

When you add this to the force caused by the pressure of the gas, you get the force of the piston, which is then sent to the connecting rod and crankshaft. Connecting rods are only perfectly straight where they turn at the top and bottom. The piston is pushed against the side of the cylinder by the angle of the connecting rod.

During the combustion cycle, the amount and direction of this force are always changing. They depend on the force of the piston and the angle between the piston crown and the connecting rod axis. Pistons have rings that go around them. They make a seal between the combustion chamber and the crankcase.

They also clean the oil off the cylinder walls, which keeps the amount of oil used in the check. The heat that the piston takes in during combustion is also sent to the cool running surface of the cylinder liner by the piston rings.

In general, here are the 3 purposes of a piston:

Compression Stoke

As the piston goes down within the cylinder, the air and fuel are pulled (or injected) into the chamber. As the piston rises up the cylinder again (called the power stroke), the air and fuel are compressed by the piston. The closer the piston is to the head of the cylinder, the more compressed the fuel and air end up being. As it gets to the top, the spark plug ignites the fuel/air mixture causing the piston to be pushed back down the cylinder.


As the piston is pushed down by the explosion of gases back down, it develops an area of low pressure within the cylinder, producing a vacuum. That’s exactly what draws the mix of air and fuel into the cylinder for the next power stroke when it will certainly be compressed before being fired up again.


The connecting rods linking the bottom of the piston are linked to the crankshaft, which transforms the up and down movement of the piston into a circular movement which is then connected through other drive shafts, gearboxes etc, to the wheels.

Do pistons wear out?

Over time, however, the pistons come under some stress and get worn, so they no longer make a good seal. At that point, the piston rings need to be changed. It’s important that the pistons fit tightly within the cylinder in order to produce sufficient compression and vacuums. Without that tight seal and resulting compressions, it will not be possible to generate the right power, which will lead to poor performance in terms of speed and fuel economy.