The two-stroke engine employs both the crankcase and the cylinder to achieve all the elements of the Otto cycle in only two strokes of the piston
Intake
The fuel/air mixture is first drawn into the crankcase by the vacuum that is created during the upward stroke of the piston. The illustrated engine features a poppet intake valve; however, many engines use a rotary valve incorporated into the crankshaft.
Crankcase compression During the downward stroke, the poppet valve is forced closed by the increased crankcase pressure. The fuel mixture is then compressed in the crankcase during the remainder of the stroke.
Transfer/Exhaust
Toward the end of the stroke, the piston exposes the intake port, allowing the compressed fuel/air mixture in the crankcase to escape around the piston into the main cylinder. This expels the exhaust gasses out the exhaust port, usually located on the opposite side of the cylinder. Unfortunately, some of the fresh fuel mixtures is usually expelled as well.
Compression
The piston then rises, driven by flywheel momentum, and compresses the fuel mixture. (At the same time, another intake stroke is happening beneath the piston).
Power
At the top of the stroke, the spark plug ignites the fuel mixture. The burning fuel expands, driving the piston downward, to complete the cycle. (At the same time, another crankcase compression stroke is happening beneath the piston.)
Advantages of 2 Stroke Engines:
- Two-stroke engines do not have valves, simplifying their construction.
- Two-stroke engines fire once every revolution (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:
-- 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.
-- Each time a new mix of air/fuel is loaded into the combustion chamber, part of it leaks out through the exhaust port.
Component of reciprocating engines.
Piston
A piston is located in a cylinder and is made gas-tight by piston rings. Its purpose is to transfer force from expanding gas in the cylinder to the crankshaft via a piston rod and/or connecting rod. In two-stroke engines, the piston also acts as a valve by covering and uncovering ports in the cylinder wall.
Crankshaft
Most reciprocating internal combustion engines end up turning a shaft. This means that the linear motion of a piston must be converted into rotation. This is typically achieved by a crankshaft.
Flywheels
The flywheel is a disk or wheel attached to the crank, forming an inertial mass that stores rotational energy. In engines with only a single cylinder, the flywheel is essential to carry energy over from the power stroke into a subsequent compression stroke.
Carburetor
Simpler reciprocating engines continue to use a carburetor to supply fuel into the cylinder. Although carburetor technology in automobiles reached a very high degree of sophistication and precision, from the mid-1980s it lost out on cost and flexibility to fuel injection. Simple forms of carburetor remain in widespread use in small engines such as lawnmowers and more sophisticated forms are still used in small motorcycles.
Simpler reciprocating engines continue to use a carburetor to supply fuel into the cylinder. Although carburetor technology in automobiles reached a very high degree of sophistication and precision, from the mid-1980s it lost out on cost and flexibility to fuel injection. Simple forms of carburetor remain in widespread use in small engines such as lawnmowers and more sophisticated forms are still used in small motorcycles.
Spark plug
An internal combustion engine requires three key ingredients to operate: air, fuel, and spark. A spark plug is a critical engine component that provides the spark that ignites the air-fuel mixture that drives an engine.
Cylinder head sits above the cylinders on top of the cylinder block. It closes at the top of the cylinder, forming the combustion chamber. This joint is sealed by a head gasket. In most engines, the head also provides space for the passages that feed air and fuel to the cylinder, and that allows the exhaust to escape. The head can also be a place to mount the valves, spark plugs, and fuel injectors.
Cylinder block which is also called as engine block is the main structure of the engine which gives the space for the cylinders, and it also gives passages for the coolant, exhaust, and in taking gases to pass over the engine and host for the crankcase and camshafts. The engine block is the main housing of hundreds of parts found in modern engines. And it is the largest among the engine parts and it also constitutes 20% to 25% of the total weight of the engine.
Piston pin, also known as a wrist pin, is a hardened steel pin that connects an engine's piston to a connecting rod. This pin is hollow to reduce weight and is held in place with a number of different methods. Most factory-stock piston pin designs rely on a pressed fit with the pin being pressed into the connecting rod
Since the two-stroke engine fires on every revolution of the crankshaft, a two-stroke engine is usually more powerful than a four-stroke engine of equivalent size. This, coupled with their lighter, simpler construction, makes the two-stroke engine popular in chainsaws, line trimmers, outboard motors, snowmobiles, jet-skis, light motorcycles, and model airplanes. Unfortunately, most two-stroke engines are inefficient and are terrible polluters due to the amount of unspent fuel that escapes through the exhaust port.
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