Four-Stroke Cycle Engine (Diesel/Petrol Engine):

A four-stroke cycle engine completes the cycle in four strokes (two revolutions of the crankshaft), with the following stages:

1. Suction Stroke: The piston moves downward, and air or a mixture of air and fuel is drawn into the cylinder. The intake valve opens, and the exhaust valve remains closed.
2. Compression Stroke: The piston moves upward, compressing the air or air-fuel mixture. Both valves are closed. In diesel engines, fuel is injected at the end of compression; in petrol engines, a spark plug ignites the mixture.
3. Power Stroke: The ignition of the fuel causes a rapid increase in pressure, pushing the piston downward. This movement is converted into rotary motion by the crankshaft.
4. Exhaust Stroke: The piston moves upward again, expelling the burnt gases through the open exhaust valve.

Characteristics:

• Only one power stroke occurs in every two revolutions of the crankshaft.
• Three strokes (suction, compression, and exhaust) are “idle” strokes, with no power output.

Two-Stroke Cycle Engine (Petrol Engine):

In a two-stroke cycle engine, the entire sequence of events (suction, compression, power, and exhaust) is completed in just two strokes of the piston (one revolution of the crankshaft). There are no valves; instead, ports in the cylinder control the gas flow.

1. Upward Stroke (Suction + Compression):
   • The piston moves upward, covering the exhaust and transfer ports, and compresses the air-fuel mixture.
   • As the piston reaches the top, the spark plug ignites the fuel.
   • Fresh fuel mixture is drawn into the crankcase through the suction port.
2. Downward Stroke (Power + Exhaust):
   • Combustion creates high pressure, forcing the piston downward.
   • As the piston moves down, the exhaust port opens, and the burnt gases exit.
   • The transfer port opens, allowing the fresh fuel mixture from the crankcase to enter the cylinder.

Characteristics:

• Power and exhaust strokes occur during the downward movement of the piston.
• There is a simpler design with no valves but higher emissions and less efficiency than four-stroke engines.

Applications and Efficiency:

• Four-Stroke Engines:
  • Applications: Common in vehicles (cars, trucks), large machinery, and power generation.
  • Power Output: Higher power output due to the four strokes per cycle.
  • Efficiency: Higher thermal efficiency, as there is a better control over combustion and emissions.
• Two-Stroke Engines:
  • Applications: Often used in smaller, portable machinery (chainsaws, motorcycles, lawnmowers).
  • Power Output: Generates more power for the size due to the two strokes per cycle.
  • Efficiency: Less efficient and more polluting, as the exhaust and intake processes overlap, leading to incomplete combustion.

Comparison between Two Stroke and Four Stroke Engines: