What is a Four Stroke Engine?

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  • Written By: R. Anacan
  • Edited By: Bronwyn Harris
  • Last Modified Date: 16 October 2019
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A four stroke engine is a type of internal combustion engine. The term four stroke refers to the number of steps in the cycle that the engine utilizes to create power. This is in contrast to the two-stroke engine, which employs a two-step cycle to convert fuel into energy. The two-stroke engine is commonly used for smaller scale applications such as for small motorcycles, motor scooters, snowmobiles, lawn mowers, and chainsaws. The four stroke version is generally used for larger applications and is the most common type of engine used in automobiles today.

This type of engine is an ingenious and practical design that has powered millions of vehicles. It produces a large amount of power in an efficient and effective manner. It also generally produces less pollution and lasts longer than a two stroke engine. The engine was created by Nikolas Otto in the mid 1800s, and in his honor, it's sometimes known as the Otto Engine. The four main strokes or steps that make up the cycle are intake, compression, combustion, and exhaust, which is sometimes called the Otto cycle.


The first stroke of a four stroke engine’s cycle consists of the intake phase. During this stroke, the piston that is inside of the cylinder moves in a downward motion. The movement triggers an intake valve to open, which allows a fuel and air mixture to enter the cylinder. During this process, a vacuum is created that draws the fuel mixture into the cylinder. The downward motion of the piston also creates room in the cylinder for the fuel and air that is drawn into it.

The compression phase occurs when the piston moves back up to the top of the cylinder. The upward movement of the piston compresses the air and fuel mixture in the cylinder. This is important because it increases the pressure of the air and fuel, which serves the purpose of heating the mixture, which in turn allows for rapid combustion.

Stroke three consists of the combustion of the fuel. The combustion phase, also known as the power stroke, is the stage where an engine’s power is generated. In most four stroke engines, a spark plug ignites the mixture of fuel and air. The force of the combustion of the fuel and air mixture forces the piston back down in the cylinder.

The last stroke of the cycle is the exhaust phase. As the piston returns back to the top of the cylinder, an exhaust valve opens in the cylinder. The upward moving piston expels the combusted air and fuel out. In most vehicles, the expelled exhaust is then directed towards the tailpipe, where it leaves the vehicle.


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Post 3

@Vincenzo -- I don't know. I like two stroke engines. They tend to be more efficient and can be made smaller. Yes, they are dirty and you have to add oil, but they are quite efficient and are great for smaller devices.

Of course, that is just a personal preference but I have always had more luck with four stroke engines on my lawnmowers.

Post 2

@Terrificli -- That is not necessarily true. You can often find four stroke engines in lawnmowers and other things and you should grab those if you can. The problem with two stroke motors is that you usually have to add oil to them and that is expensive. Also, they tend to pollute more than four stroke engines.

Post 1

It is important to point out that these are usually larger engines. Smaller, two-stroke engines are usually used to power things like scooters, lawnmowers and chainsaws.

The type of device you are dealing with, then, tells you a lot about what engine you are using. If you are driving a car, think of a four stroke engine while a two stroke engine is probably powering that weed whacker.

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