In technical parlance, 2-stroke and 4-stroke denote types of engines. Thus a 2-stroke lawn mower constitutes one with a 2-stroke engine, and a 4-stroke lawn mower constitutes one with a 4-stroke engine. Enough differences exist between these two types of engines to deserve careful examination before deciding which type of mower to purchase. Each offers its advantages and disadvantages, which consumers should carefully weigh before deciding to go with one over the other.
Engine Stroke and Power
The names of these two types of engines refer to how many times the cylinder of the engine moves up and down the shaft per revolution. A 2-stroke engine cylinder moves up once and down once -- thus, two strokes -- per cam revolution, meaning it fires once every revolution. A 4-stroke cylinder takes twice as long to complete the same process, firing once for every two revolutions of the engine shaft. Therefore, a 2-stroke engine fires twice as many times, generating more energy than a 4-stroke, which means a 2-stroke engine possesses the capability of producing significantly more power in a lawn mower than a comparably sized 4-stroke engine.
Engine Strokes, Gasoline and Heat
The 4-stroke lawn mower uses a system of valves to control gasoline combustion within the engine block. These valves help lower the heat within the engine block by allowing hot air to escape and keeping engine shafts well lubricated. The 2-stroke engine possesses no such system. Because of this, the gasoline in a 2-stroke engine must be combined with oil to assist with engine lubrication and heat dissipation. Lawn mowers with 2-stroke engines produce more smoke than mowers with 4-stroke engines and therefore more pollution. Operating a 2-stroke engine also proves more expensive because the gasoline must be combined with oil.
Generally, 4-stroke engines last longer than 2-stroke engines. Two factors contribute to this: stroke wear and heat and lubrication wear. The increased stroke ratio of 2-stroke engines creates more wear on the engine, which causes it to age more quickly. Additionally, the extreme heat produced within a 2-stroke engine, and lack of sufficient lubrication when compared to 4-stroke engines, creates stress that causes the engine to age quickly. Common 2-stroke engines require more frequent repair than 4-stroke engines, especially when mower owners properly maintain 4-stroke engines through regular maintenance. Of course, because lawn mower engines don't undergo the constant wear of motorcycle or boat engines, the actual benefits of longevity are negligible.
As with all generalizations, the differences between 2-stroke and 4-stroke engines are prone to exceptions. Writing on web resource Deep Science, author Michael Harrison points out that these generalizations only hold true for the most basic of 2-stroke and 4-stroke engines. A number of advanced or complex 2-stroke engines, such as those found on boats or large industrial equipment, contain engineering modifications designed to reduce engine heat and noxious emissions while increasing lubrication and overall engine longevity. Such engines create more power than 4-strokes while eliminating many of the drawbacks of 2-stroke engines. However, lawn mower engines generally don't have such complex engines. When shopping for a lawn mower, ask your salesperson about the specs of 2-stroke and 4-stroke engines, specifically heat dissipation, lubrication, general wear and longevity.