reciprocating engine operating principles

To understand the basic idea behind how a reciprocating internal combustion engine works, it is helpful to have a good mental image of how "internal combustion" works. Say that you took a big piece of plastic sewer pipe, maybe three inches in diameter and three feet long, and you put a cap on one end of it. Then say that you sprayed a little WD-40 into the pipe, or put in a tiny drop of gasoline. Then say that you stuffed a potato down the pipe and ignite the fuel air mixture. Like this:

The potato cannon uses the basic principle behind any reciprocating internal combustion engine: If you put a tiny amount of high-energy fuel (like gasoline) mixed with air in a small, enclosed space and ignite it, an incredible amount of energy is released in the form of expanding gas. For example, if you can create a cycle that allows you to set off explosions like this hundreds of times per minute, and then if you can harness that energy in a useful way, what you have is the core of an internal combustion reciprocating engine!

Almost all reciprocating aircraft engines currently use what is called a four-stroke combustion cycle to convert gasoline into motion. The four-stroke approach is also known as the Otto cycle, in honour of Nikolaus Otto, who invented it in 1867. The four strokes are illustrated below. They are:

The intake stroke.
The compression stroke.
The combustion stroke
The exhaust stroke.

The piston is connected to the crank shaft by a connecting rod. Here's what happens as the engine goes through its cycle:

The piston starts at the top, the intake valve opens, and the piston moves down to let the engine take in a cylinder full of air and gasoline. This is the intake stroke. Only the tiniest drop of gasoline needs to be mixed into the air for this to work.

Then the piston moves back up to compress this fuel/air mixture. Compression makes the explosion more powerful.

When the piston reaches the top of its stroke, the spark plug emits a spark to ignite the gasoline. The gasoline charge in the cylinder explodes, driving the piston down.

Once the piston hits the bottom of its stroke, the exhaust valve opens and the exhaust leaves the cylinder to go out the tail pipe.

Now the engine is ready for the next cycle, so it intakes another charge of air and gas. ...

Notice that the motion that comes out of an internal combustion engine is rotational. In an engine the linear motion is converted into rotational motion by the crank shaft. The propeller serves as a flywheel for the engine, smoothing out the irregularities of the motion.