Gasoline powered weed eaters are available with 2 types of engines: 2-stroke and 4-stroke. The biggest difference between them will be the fuel-type required for operation.
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A 2-stroke engine requires a lubricating oil to be added to the fuel mixture. A 4-stroke engine will operate on gas alone, requiring no additives.
2-stroke engines do not have valves, and less moving parts will translate into a longer operational life. 4-stroke engines are much more fuel efficient, allowing for longer use for the same amount of fuel.
If you are environmentally conscious, the 4-stroke is the preferred option as it produces significantly less air pollution. 2-stroke engines are generally louder when operating as well.
The 4-stroke engine was developed by Nikolaus August Otto in 1876. Two years later, the 2-stroke engine was invented by Sir Dougald Clerk in 1878.
Gasoline powered automobile engines utilize the 4-stroke design. However, diesel engines can operate on either the 2 or 4-stroke cycle.