To understand the differences between a two stroke and four stroke engines, we need to know how the four stroke engine works. In four stroke engine there are four stages:
1- 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.
2- Compression: The intake valve is closed and the piston travels back up the cylinder compressing the gasses.
3- Combustion: The spark plug ignites the compressed gas causing it to explode, which forces the piston down.
4- 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.
Fig.1: 4-Stroke engine (www.rc-airplane-advisor.com)
The spark plug only fires once every other revolution. Also, there is a set of mechanisms working together 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 the two stroke engine, all four stages are integrated into one downward stroke, and one upward stroke. 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.
Induction: the piston travels down the cylinder head it 'sucks' the fuel/air mixture into the cylinder.
Compression: As the piston travels up to the top of the cylinder head, it 'compresses' the fuel/air mixture from the inlet port in the top of the cylinder head, making the fuel/air mix ready for igniting by the spark plug.
Ignition: When the spark plug ignites the compressed fuel/air mixture, sometimes referred to as the power stroke.
Exhaust: As the piston returns back to the top of the cylinder head after the fuel/air mix has been ignited, the piston pushes the burnt 'exhaust' gases out of the cylinder & through the exhaust system.
Transfer Port: The port (or passageway) in a 2 stroke engine that transfers the fuel/air mixture from the bottom of the engine to the top of the cylinder
Fig2: 2-Stroke engine
However, after the fuel air explosion, the piston is driven down. As the piston reached the bottom of its stroke, the exhaust port is uncovered. Most of the gases are driven out. This is easily seen with an outboard motorboat, evident by the multicoloured oil slick surrounding the engine, but it happens with all two stroke engines. This â€" along with burning oil -- creates pollution and fuel-efficiency issues.
Development
The idea to build a two-stroke engine goes back to the year 1879. But this engine became a qualitatively good product only after many years, when the German DKW company accelerated its development. Because of its disadvantages compared with the four-stroke engine, the two-stroke engine is used practically just in a small range of capacity, e.g. in small motorcycles. Formerly the engine was even used to power tiny cars.
The main disadvantage of the two stroke engine, would be the loss of fuel (and oil, as oil is mixed with fuel, or induced with fresh air to lubricate the piston....and this oil is burnt and sent through the exhaust), as some of the fresh mixture would leave the combustion chamber unburnt. This would basically cause more emissions, dirtier exhaust and would damage the catalytic converter.
In recent years, the re-appearance of direct fuel injection (as it already existed in diesel, and 1960's petrol Mercedes), is the main cause of this interest...As direct fuel injection (meaning the fuel injector would be in the combustion chamber such as the spark plug, and would induce the fuel directly in there, as opposed to standard injection where fuel is induced in the intake manifold on top of the valve). This direct injection would limit the fresh mixture loss in exhaust.
The thermal post combustion: there is an air pump, called secondary air pump, pumping fresh air into the exhaust when engine is cold. As on cold starts you have a very rich mixture, meaning raw fuel leaving through the exhaust. The fresh air induced, along with the high temperatures of the exhaust, would cause the fuel in exhaust to burn, this would render the emissions cleaner, and would heat up the exhaust faster for the catalytic converter and oxygen sensors to work (as catalytic converters and oxygen sensors need temperatures around 600 degrees C to operate and clean the emissions).
This thermal post combustion would clean the remaining raw fuel or hydrocarbons, and heat the catalytic converters and oxygen sensors, so they always work at optimal temperatures.
Problems of the two-stroke engine
Actually the two-stroke engine should perform twice the performance of a four-stroke engine with the same cubic capacity .Though it is just possible to gain a performance that is about 50% better. The reasons are obvious: The cylinder can't be filled up with the same amount of fuel as in the four-stroke engine, because the individual strokes are separated not so clearly. If more fuel is induced, it leaves the combustion chamber through the ejection pipe without being burnt. Many concepts were developed to provide a better expulsion of the exhaust in way that the fresh gas doesn't leave the combustion chamber. Though all these inventions, the filling of the two-stroke engine is always worse than in the four-stroke engine, which loses fresh fuel only because of the "overlap" of the valve times (both valves are open for an instant). Beside these performance-technical problems, there are also increasing difficulties with the environment. The fuel mixture of the two-stroke engine often gets shifted with a certain quantity of oil because of the necessary lubrication. Unfortunately the oil gets burnt partly, too, and harmful gases are expulsed by the engine.
Two stroke engines already have a lot of advantages over the standard four stroke engines
- Two-stroke engines do not have valves, which simplifies their construction and lowers their weight.
- Two-stroke engines fire once every revolution, while four-stroke engines fire once every other revolution. This gives two-stroke engines a significant power boost.
- Two-stroke engines are lighter, and cost less to manufacture.
- Two-stroke engines have the potential for about twice the power in the same size because there are twice as many power strokes per revolution.
Disadvantages of 2 Stroke Engines:
-Two-stroke engines don't live as long as four-stroke engines. The lack of a dedicated lubrication system means that the parts of a two-stroke engine wear-out faster. Two-stroke engines require a mix of oil in with the gas to lubricate the crankshaft, connecting rod and cylinder walls.
- Two-stroke oil can be expensive. Mixing ratio is about 4 ounces per gallon of gas: burning about a gallon of oil every 1,000 miles.
- Two-stroke engines do not use fuel efficiently, yielding fewer miles per gallon.
- Two-stroke engines produce more pollution from:
1- The combustion of the oil in the gas. The oil makes all two-stroke engines smoky to some extent, and a badly worn two-stroke engine can emit more oily smoke.
2- Each time a new mix of air/fuel is loaded into the combustion chamber, part of it leaks out through the exhaust port.
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