What are the four strokes in an internal combustion engine?

Short Answer:

The four strokes in an internal combustion engine are: intake stroke, compression stroke, power stroke, and exhaust stroke. These four strokes complete one full operating cycle in the engine, and they occur in sequence within each cylinder to produce mechanical work from the combustion of fuel.

During the cycle, air-fuel mixture is drawn in (intake), compressed (compression), ignited to produce power (power), and the burnt gases are pushed out (exhaust). This process is repeated rapidly to rotate the crankshaft and keep the engine running. These strokes form the base of most petrol and diesel engines used in vehicles and machines.

Detailed Explanation:

Four strokes in an internal combustion engine

An internal combustion engine (IC engine) converts the chemical energy of fuel into mechanical energy using a process that happens inside the engine cylinder. This process is called the four-stroke cycle, and it is the most common operating cycle used in cars, bikes, trucks, and generators.

Each stroke refers to the movement of the piston from top to bottom or bottom to top within the cylinder. The piston is connected to the crankshaft through a connecting rod. Two revolutions of the crankshaft (720°) are needed to complete the full four-stroke cycle.

The Four Strokes Explained

  1. Intake Stroke
  • Also called suction stroke.
  • The piston moves downward from top dead center (TDC) to bottom dead center (BDC).
  • The intake valve opens, and a mixture of air and fuel (in petrol engines) or only air (in diesel engines) is drawn into the cylinder.
  • The downward motion creates a vacuum, which pulls the charge into the combustion chamber.
  1. Compression Stroke
  • The piston now moves upward from BDC to TDC.
  • Both the intake and exhaust valves are closed.
  • The trapped mixture is compressed, which increases its pressure and temperature.
  • In petrol engines, the mixture is ready for spark ignition, while in diesel engines, only air is compressed.
  1. Power Stroke
  • Also called the expansion stroke or working stroke.
  • At the end of compression, spark plug fires (in petrol engines) or fuel is injected (in diesel engines).
  • Combustion of the air-fuel mixture produces high-pressure gases, which force the piston downward.
  • This stroke produces the actual mechanical power that turns the crankshaft.
  1. Exhaust Stroke
  • The piston moves upward again from BDC to TDC.
  • The exhaust valve opens, and the burnt gases are pushed out of the cylinder.
  • After this, the intake valve opens again and the cycle starts over.

Importance of Each Stroke

  • The intake stroke ensures proper supply of fresh charge.
  • The compression stroke prepares the mixture for efficient combustion.
  • The power stroke is where energy is generated.
  • The exhaust stroke clears the cylinder for the next cycle.

Each of these strokes is carefully timed with the opening and closing of valves, controlled by the camshaft and timing system. Smooth and correct operation of all four strokes is necessary for engine performance.

Applications of Four-Stroke Engines

  • Petrol engines in cars, motorcycles, scooters, and small machines.
  • Diesel engines in buses, trucks, trains, ships, and generators.
  • Lawn mowers, pumps, and industrial machines.

Four-stroke engines are preferred because they are fuel-efficient, reliable, and produce less pollution compared to two-stroke engines.

Conclusion

The four strokes in an internal combustion engine are intake, compression, power, and exhaust. These steps work in a continuous loop to ensure that the engine takes in fuel, compresses it, burns it for energy, and removes waste gases. This cycle is the heart of all four-stroke petrol and diesel engines, making it essential to understand how modern engines work efficiently and power vehicles and machines across the world.