The Advantages of 4 Stroke Enginesby Richard Rowe
The two-stroke vs. four-stroke argument has been going on ever since Dugals Clarks patented the design in 1881, exactly 20 years after Alphonse Beau de Rochas patented the four stroke in 1861. Though 2-strokes are undoubtedly lighted and produce more power per revolution than four strokes, they do have a number of disadvantages. While modern technology has narrowed the gap between these two engine designs, the fact is that 4-stroke engines are preferred for almost every road-going vehicle on the planet.
The primary reason that 2-strokes tend to get worse fuel economy than four strokes is that they pull air in through the intake port while simultaneously pushing used gases out through the exhaust port. Along with other factors, this crossover often results in fuel being expelled from the exhaust before it has the opportunity to burn. 4-Stroke engines have a dedicated intake, power and exhaust stroke, which keeps fuel-to-exhaust crossover to a minimum. All else being equal, a 4-stroke engine with the same type of direct injection system used by modern 2-strokes will still get better fuel economy.
In general, 4-stroke engines almost always make more torque at low RPM than 2-strokes. This extra torque has a lot to do with the efficiency of the fuel burn; a 4-stroke uses almost all of its fuel to impart power to the crankshaft, whereas fuel crossover in a 2-stroke means that it will produce less power per RPM. 2-strokes do enjoy an advantage in high-RPM power output, but simply don't produce the torque of a 4-stroke.
Because 2-strokes must rev to very high RPM to make any power, most applications using them are geared toward maintaining that RPM. Any engine designer will tell you that the more times an engine goes around, the quicker it will wear out. It's pretty simple math; if an engine can go through ten million RPMs before it wears out, then one that revolves at 5,000 revolutions per minute will go 2000 minutes between rebuilds. The same engine running at 10,000 RPM will only last 1,000 minutes.
Above all else, the primary reason that 2-strokes aren't more popular in mass-vehicle applications is that they tend to run very dirty. 2-stroke engines require that oil be injected with the fuel in order to lubricate the crankcase; that oil gets burned along with the gasoline, which drastically increases emissions and soot. 4-Stroke engines have a dedicated oiling system that's kept largely separate from the combustion chamber, which help to ensure that the only thing burning in the engine is gasoline. If you've ever seen an old car blowing huge plumes of blue smoke from its tailpipe, then you've witnessed the effect that oil burning can have on emissions.
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