Umum

This system manages to pack one power stroke into every two strokes of the piston (up-down). This is achieved by exhausting and recharging the cylinder simultaneously.
The steps involved here are:

1. Intake and exhaust occur at bottom dead center. Some form of pressure is needed, either crankcase compression or super-charging.
2. Compression stroke: Fuel-air mix is compressed and ignited. In case of diesel: Air is compressed, fuel is injected and self-ignited.
3. Power stroke: Piston is pushed downward by the hot exhaust gases.

Two Stroke Spark Ignition (SI) engine:
In a two stroke SI engine a cycle is completed in two strokes of a piston or one complete revolution (360º) of a crankshaft. In this engine the intake and exhaust strokes are eliminated and ports are used instead of valves. In this cycle, the petrol is mixed with lubricant oil, resulting in a simpler, but more environmentally damaging system, as the excess oils do not burn and are left as a residue. As the piston proceeds downward another port is opened, the fuel/air intake port. Air/fuel/oil mixtures come from the carburetor, where it was mixed, to rest in an adjacent fuel chamber. When the piston moves further down and the cylinder doesn't have anymore gases, fuel mixture starts to flow to the combustion chamber and the second process of fuel compression starts. The design carefully considers the point that the fuel-air mixture should not mix with the exhaust, therefore the processes of fuel injection and exhausting are synchronized to avoid that concern. It should be noted that the piston has three functions in its operation:

• The piston acts as the combustion chamber with the cylinder and compresses the air/fuel mixture, receives back the liberated energy, and transfers it to the crankshaft.
• The piston motion creates a vacuum that sucks the fuel/air mixture from the carburetor and pushes it from the crankcase (adjacent chamber) to the combustion chamber.
• The sides of the piston act like the valves, covering and uncovering the intake and exhaust ports drilled into the side of the cylinder wall.

The major components of a two stroke spark ignition engine are the:

• Cylinder: A cylindrical vessel in which a piston makes an up and down motion.
• Piston: A cylindrical component making an up and down movement in the cylinder.
• Combustion chamber: A portion above the cylinder in which the combustion of the fuel-air mixture takes place.
• Intake and exhaust ports: An intake port allows the fresh fuel-air mixture to enter the combustion chamber and an exhaust port discharges the products of combustion.
• Crankshaft: A shaft which converts the reciprocating motion of the piston into a rotary motion.
• Connecting rod: A rod which connects the piston with the crankshaft.
• Spark plug: An ignition-source located at the cylinder head that is used to initiate the combustion process.

Operation: When the piston moves from bottom dead center (BDC) to top dead center (TDC) the fresh air and fuel mixture enters the crank chamber through the intake port. The mixture enters due to the pressure difference between the crank chamber and the outer atmosphere while simultaneously the fuel-air mixture above the piston is compressed.
Ignition: With the help of a spark plug, ignition takes place at the top of the stroke. Due to the expansion of the gases the piston moves downwards covering the intake port and causes the fuel-air mixture inside the crank chamber to be compressed. When the piston is at bottom dead center the burnt gases escape from the exhaust port.
At the time the transfer port is uncovered the compressed charge from the crank chamber enters into the combustion chamber through the transfer port. The fresh charge is deflected upwards by a hump provided on the top of the piston and removes the exhaust gases from the combustion chamber. Again the piston moves from bottom dead center to top dead center and the fuel-air mixture is compressed when the both the exhaust port and transfer ports are covered. The cycle is repeated.
Advantages: • It has no valves or camshaft mechanism, hence simplifying its mechanism and construction • For one complete revolution of the crankshaft, the engine executes one cycle—the 4-stroke executes one cycle per two crankshafts revolutions. • Less weight and easier to manufacture. • High power to weight ratio
Disadvantages: • The lack of lubrication system that protects the engine parts from wear. Accordingly, the 2-stroke engines have a shorter life. • 2-stroke engines do not consume fuel efficiently. • 2-stroke engines produce lots of pollution. • Sometimes part of the fuel leaks to the exhaust with the exhaust gases. In conclusion, based on the above advantages and disadvantages, the 2-stroke engines are supposed to operate in vehicles where the weight of the engine is required to be small, and the it is not used continuously for long periods of time.