Most consumer and professional gas trimmers on the market have two-cycle engines, which use gasoline that has lubricating oil added to it. But some manufacturers have developed four-cycle engines, which use the same type of gasoline as your lawnmower or car. Two-cycle engines and four-cycle engines each have benefits and drawbacks. When you're choosing a new gas trimmer, decide between a two-cycle-engine trimmer and a four-cycle-engine trimmer based on your trimming needs.
Power vs. Weight
Four-cycle engines will use two pistons, as opposed to one, to generate the power needed to turn the crankcase. Since two pistons are moving, more power is being generated. This usually translates into more power at the cutting head. For the thickest, toughest weeds you may need the power of a four-cycle engine, but for most homeowner trimming needs, a two-cycle engine will deliver more than enough power. With more power comes more weight, and four-cycle-engine gas trimmers can weigh up to 4 lbs. more than the average two-cycle-engine gas trimmer. If you have long trimming hours ahead, you may want to reduce the strain on your lower back by purchasing a two-cycle gas trimmer.
Most two-cycle engines burn a mixed oil-and-gas fuel. This fuel can create harmful exhaust fumes. A lot of four-cycle trimmers use regular gasoline for fuel and regular motor oil, separately, for lubrication. This is less harmful to the environment because such an engine will burn cleaner. It burns cleaner in part because it uses a catalytic converter. With a two-cycle engine, you will need to mix the fuel in a gas can. Such a mix will separate after about 30 days, becoming useless; you will need to dispose of it according to local environmental regulations.
Maintenance and Repairs
Generally speaking, the more parts an engine has, the more likely it is to suffer maintenance issues. Two-cycle engines have been designed with simplicity in mind and use fewer breakable parts than a four-cycle engine. Two-cycles are much easier to repair. But because two-cycles use mixed fuel, which is dirtier, you may have parts that break down quicker than a four-cycle engine's parts would.
Most four-cycle trimmers will be more expensive up front, but they may last longer. Four-cycle maintenance and repair, when it is needed, may be more expensive than that of a two-cycle. But since two-cycles require mixed fuel, you’ll be purchasing more oil.