How Internal Combustion Engines Work

This article pertains to internal combustion engines which are used for especially automobile propulsion and electricity generation.

Internal combustion engines burn fuels such as natural gas, gasoline, hydrogen, diesel, and alcohols to produce mechanical energy either to directly drive a mechanism or to turn an electricity generator such as an alternator. As complex as combustion engines are mechanically and electronically, the combustion process itself is simple.

To understand the operation of an internal combustion engine, you have to learn some basic physics facts first, such as:


  1. A spark plug is a basic part of a gasoline engine which passes the high voltage electric current that it is supplied with through the air, this is called arcing.
  2. What a spark plug does is create a spark, which is also an electric arc. An arc is when electricity flows through the air from one point, to another usually very close point.
  3. Electric sparks are extremely hot, and heat causes gasoline, air as well as other substances to expand.
  4. The more a substance expands, the less dense it becomes, the more it is compressed, the more dense it becomes.
  5. Expansion is when the particles of a substance move further away from each other, causing the substance to take up more space. When a substance expands, it normally absorbs heat from whatever is in contact with it.
  6. As the mixture of gasoline and air expands inside the cylinder (shaped similarly to a bean can, but is obviously much stronger), it exerts more pressure on the lining of the cylinder, pressing on all parts of it which are above the piston, and of course, down on the piston itself. Imagine that the gasoline is trapped in the cylinder and trying to get out, so it is pressing against the walls of it and everything that is in it.
  7. Naturally, the piston is forced down due to the pressure exerted on it by the gasoline, and this is when the engine starts to generate what is called mechanical energy.
  8. After the above process is completed, the piston reciprocates (moves back up in the opposite direction). That is why this type of combustion engine is called a reciprocating engine. I am just trying help you to understand the very basic thermodynamic operation of a combustion engine, not the details of the mechanical process.
  9. As the piston reciprocates, a valve opens and allows the piston to push the burnt gasoline and other byproducts of the process out.


  1. Before the process above takes place, the air in the cylinder has to be compressed. Compression is the exact opposite of expansion, which I explained above, so that means that compression is the process of forcing the particles of a substance closer together as opposed to further apart.
  2. What I said above, means that the air in the cylinder becomes hot. Remember that the more a substance is compressed, the more dense it becomes, and the greater it’s pressure.
  3. Due to high pressure caused by combustion, the mixture of gasoline and air expands very rapidly, forcing the piston down at a very high speed.
  4. Don’t worry about the details which I have left out, you will see them in the next section, I was just explaining some basic facts which you needed to know, before actually reading about the operation of a combustion engine from the beginning of the cycle, to the end.

Operation of A Standard Gasoline Powered Engine

A typical gasoline-powered engine (the most common type of engine for vehicles, not power plants) is an apparatus which converts chemical energy such as gasoline into mechanical energy, but usually rotary mechanical energy.

They do this by compressing a mixture of gasoline and air in airtight cylinders to increase it’s density, temperature, and pressure and then a high voltage high temperature electric spark which causes the gas mixture to expand rapidly/explode, causing pressure inside the cylinders to increase and that pressure forces the pistons downwards.

As the pistons are forced downwards, they turn a crankshaft with a mechanism that uses the pistons’ linear (up and down in a straight line) motion to rotate a crankshaft. The device used to achieve this linear to rotary motion is called the connecting rod.

In order to be started, a battery-powered electric motor called the starter motor has to be used to crank the engine so that it can pump fuel into it (unless an electric pump is being used) and then compress the gas mixture so that it can be ignited.