Most small, hand-carried lawn and garden tools such as string trimmers and chainsaws use 2-stroke engines, while larger machines such as lawn and garden tractors use 4-stroke engines. Lawn mowers are available with either type of engine.
LSU AgCenter engineer Dr. Dick Parish explains the differences and why 2-stroke engines are more commonly used on small tools.
"Most people use the terms ‘2-cycle’ or ‘4-cycle’ to describe the two engine types," Parish says. "This common terminology, while readily understood, is incorrect."
The engineer says that every reciprocating internal combustion engine operates through a "cycle" of five functions – intake, compression, ignition, combustion and exhaust. In a 2-stroke engine, all five functions of the cycle are completed in only two strokes of the piston (or one revolution of the crankshaft). In a 4-stroke engine, the five functions require four strokes of the piston (or two revolutions of the crankshaft).
Parish says 2-stroke engines aren’t lubricated by a reservoir in the crankcase but by oil mixed with the gasoline. Much of this oil is not burned by the engine and is expelled with the exhaust gas, thus increasing emissions and smoke.
He also says the power-to-weight ratio of 2-stroke engines is higher than most 4-stroke engines, so the lighter 2-stroke engines work well in hand-carried applications such as blowers and string trimmers.
"Typically, 2-stroke engines rev up to higher speeds than 4-stroke engines, and this accounts for much of the higher power per pound," Parish says. "Also, small 2-stroke engines use intake and exhaust ports instead of valves, thus saving the weight and cost of a valve train.
Parish adds that spark plug fouling is more common on 2-stroke engines because of the oil in the fuel, meaning more frequent spark plug changes may be needed.
The engineer says 4-stroke engines are lubricated by oil in the crankcase, so oil isn’t mixed with the gasoline.
"On small engines, such as those used on lawnmowers, a tab on the bottom of the piston rod splashes oil up into the cylinder," he says. "On larger engines, such as those used on garden tractors, an oil pump distributes the oil under pressure and through an oil filter."
Parish cautions that different engine manufacturers recommend different oil mix ratios for their engines.
"If you have more than one 2-stroke engine and they aren’t the same brand, you may need to keep separate fuel-to-oil mixes for the machines," he says. Typical ratios range from 32 to 1 (gasoline to oil) to 50 to 1.
"You can buy small containers of oil that are measured to provide the right amount of oil for a gallon of gasoline for a given recommended mix ratio," Parish says. "But buying oil in larger quantities may be more economical."
Another concern Parish cautions about is having trouble finding a small container measured for the ratio you require unless you go back to the dealer. Most hardware stores, garden centers, discount stores and similar stores carry only one or two ratios.
"Don’t just assume that the small container of 2-cycle oil you pick up at the discount store will give the right ratio for your engine," Parish warns. "Check the ratio on the label. It’s helpful to know how much oil you need for your required ratio in case you don’t have the correct measured container."
Parish recommends starting with a clean, empty container and pouring in the correct amount of oil. Next, add 1 gallon of gasoline, close the container and shake it to mix the fuel. Agitate the container before pouring each time you use the gasoline mix.
"Either type of engine can do a good job if used in an appropriate application," Parish says. "If you have a 2-stroke engine, be sure to mix the oil and gas correctly."