Construction and Working of Two Stroke Petrol Engine

       A two-stroke cycle engine is an internal combustion engine that utilizes two distinct piston strokes to complete one operating cycle of the engine. The crankshaft turns only one revolution for each complete operating cycle, providing twice as many power strokes in the same number of crankshaft rotations as a four-stroke cycle engine. The valving system in a two-stroke cycle engine requires fewer parts, making it lighter in weight than a four-stroke cycle engine. Weight reduction is especially desirable in applications such as chainsaws, leaf blowers, and other hand-held outdoor power equipment.

Two-Stroke Working Cycle

1. Ignition/Power: 

       The ignition/power stroke occurs while the piston progress toward TDC and the compressed charge in the cylinder is ignited. During this time, the crankcase has already loaded with a fresh air-fuel mixture. When the charge is ignited, the expansion of hot combustion gases forces the piston toward BDC. Piston motion is transferred from the piston through the connecting rod, causing the crankshaft to rotate. When the piston moves near BDC, the exhaust port is opened and exhaust gases are discharged through the side of the cylinder.

2. Exhaust/Intake: 

      The exhaust/intake stroke occurs as the piston continues moving toward BDC. The intake port opens and the air-fuel mixture is routed inside the cylinder. The shape of the piston head helps to divert the incoming air-fuel mixture to the top of the cylinder. This prevents the incoming air-fuel mixture from passing across the top of the piston and out the exhaust port without burning. Diversion of the air-fuel mixture also helps when removing or scavenging exhaust gases.

3. Compression:

     The compression stroke occurs when the piston is at BDC, exhaust gases have been discharged, and the cylinder is filled with a new charge. The compression stroke begins as the piston starts moving toward TDC. The piston closes the intake and exhaust ports, trapping the charge in the cylinder. The piston functions as a slide valve, exposing the intake and exhaust ports as it moves in the cylinder. The charge is compressed as the piston continues to move toward TDC. Piston movement toward TDC causes the more air-fuel mixture to be drawn into the crankcase. When the piston reaches TDC, the engine has completed one full operating cycle.

Applications of Two-Stroke Engine

• Two-stroke engines are preferred when mechanical simplicity, lightweight, and high power-to-weight ratio are design priorities.
• They are lubricated by the traditional method of mixing oil into the fuel, they can be worked within any orientation as they do not have a reservoir dependent on gravity. This makes them desirable for their use in handheld tools such as chainsaws.
• Two-stroke engines are found in small scales propulsion applications such as motorcycles, Mopeds, and dirt bikes.

Characteristics of Two-Stroke Engine

• Fewer moving parts
• Less weight for comparable output
• Higher fuel consumption
• More noise
• Higher operating speed and temperature
• Smaller size for comparable output
• Greater exhaust emissions

See Advantages & disadvantages of Two-stroke cycles over Four-stroke cycles engine HERE

References:

1. IC Engine by Brody Walker
2. Wikipedia 

3. byjus.com