2-Stroke and 4-Stroke Engines

Richard L. Parish, PhD, PE

Most small, hand-carried lawn and garden tools such as string trimmers and chainsaws use 2-stroke engines and most larger machines such as lawn and garden tractors use 4-stroke engines. Lawnmowers are available with either type of engine. What are the differences and why are 2-stroke engines more commonly used on small tools?

“2-Stroke” or “2-Cycle”? Most people use the terms “2-cycle” or “4-cycle” to describe the two engine types. This common terminology, while readily understood, is incorrect. 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 4 strokes of the piston (or two revolutions of the crankshaft).

2-Stroke Engines: 2-stroke engines are not lubricated by a reservoir in the crankcase; they rely on oil mixed with the gasoline to lubricate the engine components. Much of this oil is not burned by the engine and is expelled with the exhaust gas, thus increasing emissions and smoke. The power:weight ratio of 2-stroke engines is higher than most 4-stroke engines, thus the preponderance of 2-stroke engines 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 some of the higher power per pound. Also small 2-stroke engines use intake and exhaust ports instead of valves, thus saving the weight and cost of a valve train. Furthermore, a 2-stroke engine has a power stroke with every revolution instead of every other revolution. Spark plug fouling is more common on 2-stroke engines because of the oil in the fuel, thus more frequent spark plug changes may be needed.

4-Stroke Engines: 4-stroke engines are lubricated by oil in the crankcase, thus oil is not 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; on larger engines, such as those used on garden tractors, an oil pump distributes the oil under pressure and through an oil filter.

Oil Mixtures for 2-Stroke Engines: Different engine manufacturers recommend different oil mix ratios for their engines. If you have more than one 2-stroke engine and they are not the same brand, you may need to keep separate fuel:oil mixes for the machines. Typical ratios range from 32:1 (gasoline:oil) to 50:1. You can buy small containers of oil that are premeasured to provide the right amount of oil for a gallon of gasoline for a given recommended mix ratio, but buying oil in larger quantities may be more economical. Another concern is that you might have trouble finding a small container premeasured for the ratio you require unless you go back to the dealer. Most hardware stores, garden centers, discount stores, etc. 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; check the ratio on the label. It is helpful to know how much oil you need for your required ratio in case you don=t have the correct premeasured container. The table below gives the amount of oil required per gallon of gasoline to achieve various mix ratios:

Ratio	Ounces of oil	Ratio	Ounces of oil	Ratio	Ounces of oil
30	4.3	38	3.4	46	2.8
32	4.0	40	3.2	48	2.7
34	3.8	42	3.0	50	2.6
36	3.6	44	2.9

You should start with a clean, empty container and pour in the correct amount of oil. Next, add one gallon of gasoline, close the container, and shake to mix. 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. If you have a 2-stroke engine, be sure to mix the oil and gas correctly.