You’ll see both “cycle” and “stroke” used to describe engines that run weed trimmers. They mean the same thing. In general, noisier two-cycle engines are cheaper to buy and are lighter and more powerful, but you have to mix oil with gas, and they wear out more quickly. More expensive four-cycle engines are heavier but they are quieter, cheaper to run, last longer and they are more apt to meet EPA pollution standards. To understand these differences you need to know how they work.
Basic Engine Difference
Internal combustion engines operate on a cycle of five functions: intake of air; compression of air and fuel; ignition of air and fuel; combustion of air and fuel; and exhaust. A two-cycle engine completes these functions with two strokes of the piston and one revolution of the crankshaft. A four-cycle engine requires four strokes of the engine and two revolutions of the crankshaft.
You have to mix oil with gas for two-cycle engines in a ratio ranging from 1 part oil to 50 parts of gasoline up to 1 part of oil to 20 parts of gasoline. You need to check the label of the two-cycle oil to make sure it is appropriate for the ratio of your engine. A four-cycle engine has a lubrication system using oil in a crankcase.
Weight and Power
Two-stroke engines are lighter than four-cycle engines because four-cycle engines have valves that require a valve train. They deliver from 20 to 60 percent more power for their weight, but four-cycle engines deliver more torque. Torque is the twisting or turning force, in this case, like the spinning strings that trim plants.
Two-cycle engines are generally cheaper to buy than four-cycle engines because they are less costly to make. Since they have fewer moving parts, they are cheaper to maintain. They also wear out more quickly because they run hotter and are lubricated by oil in their fuel instead of having an independent lubricating system. The oil in the fuel of two-cycle engines tends to foul the spark plugs, so you’ll have to change the spark plugs more often. Two-cycle engines also use more gasoline than four-cycle engines.
Two-cycle engines are easier to start. Four-cycle engines on trimmers usually have tabs on the bottom of the pistons that splashes oil up from the crankcase into the cylinder. That means you need to keep your trimmer close to level for proper lubrication. A 2-cycle engine does not have a crankcase so you can operate it any angle you want.
Noise and Pollution
Four-cycle engines run quieter than twp-stroke engines, making them less objectionable to your neighbors. Two-cycle engines also release up to 30 percent of the oil and gas mixture as exhaust smoke. Increasingly strict EPA pollution requirements may mean that two-cycle engines may one day be phased out.