2-Stroke and 4-Stroke Engines

Richard L. Parish  |  11/18/2004 8:10:55 PM

Most small, hand-carried lawn and garden tools use 2-stroke engines (Figure 1), and most larger machines such as lawn and garden tractors use 4-stroke engines (Figure 2).
Lawn mowers 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 four 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 (Figure 3). 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 much 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. 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.

The fact that 2-stroke engines are lubricated by oil in the gasoline and do not have a crankcase makes them more amenable to operation at different angles.

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.

Four-stroke engines must usually be kept level to provide proper lubrication, thus they often are not suited to use on some hand-held tools.

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.

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