What is a compression ignition (CI) engine?

Short Answer:

A Compression Ignition (CI) engine is a type of internal combustion engine where the fuel is ignited by the high temperature produced by compressing air inside the cylinder. In this engine, only air is compressed during the compression stroke, and fuel is injected at high pressure near the end of the stroke. The heat of compressed air causes the fuel to ignite automatically without a spark plug.

CI engines generally use diesel as fuel, so they are also known as diesel engines. These engines are widely used in trucks, buses, trains, and heavy machines because of their high efficiency, better fuel economy, and strong torque production.

Detailed Explanation:

Compression Ignition (CI) Engine

A Compression Ignition (CI) engine is an internal combustion engine in which ignition of the fuel occurs due to the high temperature of compressed air inside the cylinder. Unlike Spark Ignition (SI) engines, CI engines do not use spark plugs. Instead, air is compressed to a very high pressure and temperature, and then fuel is directly injected into the combustion chamber at the end of the compression stroke. The heat of the compressed air is sufficient to ignite the fuel spontaneously, causing combustion and producing power.

These engines are commonly known as diesel engines, as they mainly operate using diesel fuel. Rudolf Diesel invented this engine in 1892, and since then, it has become one of the most reliable and efficient power sources for automobiles and industrial equipment.

Working of CI Engine

The working of a CI engine takes place in four strokes—suction, compression, power, and exhaust. The complete cycle converts the chemical energy of fuel into mechanical energy through combustion.

  1. Suction Stroke:
    During this stroke, the piston moves downward, and the inlet valve opens. Only air enters the cylinder (no fuel is mixed at this stage). The exhaust valve remains closed.
  2. Compression Stroke:
    The inlet valve closes, and the piston moves upward, compressing the air inside the cylinder. The compression ratio in a CI engine is very high (between 14:1 and 22:1). This increases the air temperature to around 500–700°C, which is high enough to ignite diesel fuel.
  3. Power Stroke:
    Near the end of the compression stroke, diesel fuel is injected into the cylinder at very high pressure through the fuel injector. The high-temperature air causes the fuel to ignite automatically. The rapid combustion produces high-pressure gases that push the piston downward, creating the power stroke.
  4. Exhaust Stroke:
    After the power stroke, the exhaust valve opens, and the piston moves upward again, expelling the burnt gases from the cylinder. The cycle then repeats for continuous power production.

Main Components

The key components of a CI engine include:

  • Fuel Injector: Delivers diesel into the combustion chamber in fine droplets.
  • Cylinder and Piston: Form the main combustion chamber where power is produced.
  • Crankshaft: Converts the reciprocating motion of the piston into rotary motion.
  • Air Filter: Cleans the air before it enters the cylinder.
  • Fuel Pump: Supplies fuel at high pressure to the injector.
  • Valves: Control the flow of air and exhaust gases.

Features of CI Engine

  • High Compression Ratio: Leads to higher efficiency and better fuel combustion.
  • No Spark Plug: Combustion is achieved through heat generated by compression.
  • Use of Diesel Fuel: Diesel has higher energy content and ignition temperature.
  • Strong Torque Output: Ideal for heavy-duty applications.
  • Lower Speed Range: Compared to petrol engines, CI engines run at lower speeds but with greater power.

Advantages of CI Engines

  • High fuel efficiency and better mileage.
  • More torque and pulling power.
  • Greater durability and longer life span.
  • Lower fuel cost compared to petrol.
  • Suitable for heavy-duty vehicles and machines.

Disadvantages of CI Engines

  • Heavier and more expensive construction.
  • Produces more vibration and noise.
  • Higher maintenance cost.
  • Slower speed and poor acceleration compared to SI engines.
  • Emits more smoke and pollutants if not maintained properly.

Applications

Compression Ignition engines are widely used in:

  • Trucks, buses, and tractors.
  • Ships and submarines.
  • Power generators.
  • Industrial machinery and construction equipment.
  • Railway locomotives and heavy transport vehicles.
Conclusion:

A Compression Ignition (CI) engine works by compressing air to a very high temperature and pressure, which causes the injected fuel to ignite automatically. It is also known as a diesel engine and is preferred for heavy-duty applications due to its high efficiency, durability, and torque output. Although it is heavier and more expensive than petrol engines, its fuel economy and reliability make it an essential power source in many industrial and transportation sectors.