To understand the mechanical differences between a two stroke and four stroke engine, lets first consider how the four stroke engine works. The four strokes are:
Intake: The piston travels down the cylinder while the intake valve is opened to allow a mixture of fuel and air to enter the combustion chamber.
Compression: The intake valve is closed and the piston travels back up the cylinder thereby compressing the gasses.
Combustion: The spark plug ignites the compressed gas causing it to explode, which forces the piston down.
Exhaust: The piston rises up the cylinder as the exhaust valve is opened, allowing the piston to clear the chamber to start the process over.
Each time the piston rises and falls it turns the crankshaft that is responsible for turning the wheels. This is how fuel is converted into forward motion.
Of note here is that the spark plug only fires once every other revolution. Also, there is a sophisticated set of mechanisms working in synchronization to create the four strokes. A camshaft must alternately tip a rocker arm attached either to the intake or exhaust valve. The rocker arm returns to its closed position via a spring. The valves must be seated properly in the cylinder head to avoid compression leaks. In other words, a symphony of mechanical events occurs.
In the two stroke engine, all four events are integrated into one simple downward stroke, and one upward stroke. Two strokes. Intake and exhaust are both integrated into the compression and combustion movement of the piston, eliminating the need for valves. This is accomplished by an inlet and exhaust port in the wall of the combustion chamber itself. As the piston travels downward from combustion, the exhaust port is exposed allowing the spent gasses to rush out of the chamber. The downward stroke also creates suction that draws in new air/fuel through an inlet located lower in the chamber. As the piston rises again, it blocks off the inlet and port, compressing the gasses at the top of the chamber. The spark plug fires and the process starts over. Significantly, the engine fires on every revolution, giving the two stroke its power advantage.
However, at the lowest point of travel of the piston when the chamber is filling with fuel/air, the exhaust port exposed above allows some fuel/gasses to escape the chamber. This is easily seen with an outboard motorboat, evident by the multicolored oil slick surrounding the engine, but it happens with all two stroke engines. This — along with burning oil -- creates pollution and fuel-efficiency issues.
For these reasons, two stroke engines are reserved for intermittent use, where weight-to-power ratio or orientation issues are important and where mileage isn't primary. Meanwhile manufacturers are looking for ways to add advantages to four stroke motors, making them smaller, lighter and more robust.
To further understand the difference between a two stroke and a four stroke engine let us consider the advantages and disadvantages.
Advantages of the 2 - Stroke
Has more get-up-and-go because it fires once every revolution, giving it twice the power of a four stroke, which only fires once every other revolution.
- Packs a higher weight-to-power ratio because it is much lighter.
- Is less expensive because of its simpler design.
- Can be operated in any orientation because it lacks the oil sump of a four stroke engine, which has limited orientation if oil is to be retained in the sump
These attributes make two stroke engines very popular for a variety of uses from dirt bikes, mopeds, jet skis, and small outboard motors, to lawn and garden equipment such as mowers, edgers, leaf blowers, chain saws and hedge trimmers.
But there are other differences between the two stroke and four stroke engines that aren't so favorable, which is why you won't see two stroke engines in cars.
Disadvantages of the 2 - Stroke
Faster wear and shorter engine life than a four stroke due to the lack of a dedicated lubricating system.
- Requires special two stroke oil ("premix") with every tank of gas, adding expense and at least a minimal amount of hassle.
- Heavily pollutes because of the simpler design and the gas/oil mixture that is released prior to, and in the exhaust (also creates an unpleasant smell).
- Is fuel-inefficient because of the simpler design, resulting in poorer mileage than a four stroke engine.
- Has a high-decibel whine that may exceed legal noise limits in some areas, depending on the product and local applicable laws.
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