Explain the working principle of a diesel engine.

Short Answer:

A diesel engine works on the principle of compression ignition, where air is compressed to a high temperature, and then fuel is injected into the hot air, causing it to ignite automatically without a spark. This combustion pushes the piston down and produces mechanical work.

The diesel engine operates through four strokes: intake, compression, power, and exhaust. It uses diesel fuel, which has a higher energy content and needs high pressure for ignition. Diesel engines are known for their fuel efficiency, high torque, and durability, making them ideal for heavy vehicles and industrial applications.

Detailed Explanation:

Working principle of a diesel engine

A diesel engine is a compression ignition engine that converts the chemical energy of diesel fuel into mechanical energy. Unlike a petrol engine (spark ignition), a diesel engine does not use a spark plug. Instead, it relies on high pressure and temperature to ignite the fuel.

It operates on the Diesel cycle, which includes four processes: isentropic compression, constant pressure heat addition, isentropic expansion, and constant volume heat rejection. Diesel engines are widely used in trucks, buses, generators, trains, and ships due to their robust design and better fuel economy.

Main Components of Diesel Engine

  • Cylinder – Where the fuel burns and power is generated.
  • Piston – Moves up and down inside the cylinder.
  • Fuel Injector – Sprays diesel fuel into the cylinder.
  • Crankshaft – Converts the piston’s linear motion into rotary motion.
  • Valves – Control the flow of air and exhaust gases.
  • Flywheel – Maintains engine momentum and smooth operation.

Four-Stroke Operation of a Diesel Engine

  1. Intake Stroke
    • The intake valve opens, and only air is drawn into the cylinder as the piston moves downward.
  2. Compression Stroke
    • The intake valve closes.
    • The piston moves upward, compressing the air to a very high pressure and temperature (up to 40 bar and 600°C or more).
  3. Power (Combustion) Stroke
    • At the end of compression, the fuel injector sprays diesel into the hot compressed air.
    • Fuel ignites automatically due to high temperature.
    • The explosion forces the piston downward, producing mechanical work.
  4. Exhaust Stroke
    • The exhaust valve opens, and the piston moves up again to push out the burnt gases.
    • The cycle then repeats.

Key Features of Diesel Engine Working

  • No spark plug is used.
  • High compression ratio (typically 14:1 to 22:1).
  • More torque and better fuel efficiency than petrol engines.
  • Diesel fuel is injected directly into the combustion chamber.
  • Heat required for ignition comes from compression only.

Advantages of Diesel Engine

  • Fuel efficient – More kilometers per litre
  • Longer engine life due to strong build
  • High torque – Suitable for heavy loads
  • Reliable performance in rough and long operations
  • Lower CO emissions compared to petrol

Disadvantages

  • More noise and vibration
  • Higher initial cost
  • Heavy engine structure
  • Produces nitrogen oxides (NOx) and particulate matter
  • Cold starting is difficult without heaters

Applications

  • Heavy vehicles like trucks, buses, and tractors
  • Generators and backup power units
  • Railway locomotives and marine engines
  • Construction and mining equipment
  • Industrial machinery
Conclusion

The diesel engine works by compressing air to a high temperature and injecting diesel fuel, which ignites due to heat from compression. This creates a powerful expansion that moves the piston and produces mechanical energy. Diesel engines are preferred where power, efficiency, and durability are needed. Their unique compression ignition system makes them suitable for heavy-duty applications across transportation and industry.