Weed eaters, also called trimmers, are marketed with 2-stroke and 4-stroke gasoline engines. Each has its supporters, but generally speaking weed eaters with 2-stroke engines are cheaper, lighter and more powerful than eaters with 4-stroke engines; they also make more noise, vibrate more, emit more pollution, wear out more quickly, and you have to mix oil with the gas. Evolving EPA pollution requirements favor 4-stroke engines.
The term "stroke" refers to the movement of the piston in the cylinder of an engine. In a 2-stroke engine, compressed fuel explodes, pushing the piston outward; fuel is inserted into the cylinder as the piston returns. A 4-stroke engine has separate strokes for compression and exhaustion. A distributor on a 4-stroke engine supplies a spark every second turn of the crank shaft when the piston is close to the top dead center of the compression stroke.
Fuel and Power
You have to mix about 4 oz. of oil for each gallon of gas for 2-stroke engines. Two-stroke engines deliver more power for their size than weed eaters with 4-stroke engines; they are also more difficult to start than 4-stroke engines.
The simple design of 2-stroke engines means they are cheaper to manufacture, so the cost is less to the consumer. The oil that you mix with the gasoline in a 2-stroke engine can be expensive, plus 2-stroke engines use more gasoline than 4-stroke engines.
Two-stroke engines do not have a separate lubrication system as more sophisticated 4-stroke engines do; they rely instead on oil mixed with fuel, so their moving parts wear out faster than those in 4-stroke engines. This means they are more expensive to maintain. If you add too much oil to the fuel mixture of a 2-stroke engine, the engine will become sluggish and smoke will belch from the muffler. If you don't add enough oil, your engine will run hot and can burn out.
Part of the mixture of air and fuel in a 2-stroke engine leaks out through the exhaust port as it is loaded into the combustion chamber. This is one source of exhaust emissions in 2-stroke engines that are a source of concern for the Environmental Protection Agency. The EPA discourages 2-stroke engines, which it considers heavy polluters, and encourages 4-stroke engines, which pollute less.