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How does a diesel engine work?

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Quick answer

A diesel engine compresses air in its cylinders so tightly that the air becomes hot enough to ignite fuel on contact. When diesel is injected as a fine mist, it burns instantly without a spark plug. The expanding gas forces the piston down, turning the crankshaft and powering the vehicle.

A diesel engine burns fuel by squeezing air so hard that it becomes hot enough to ignite fuel sprayed into the cylinder, with no spark plug involved. The expanding burning gas pushes a piston down, and the crankshaft turns that push into the rotation that drives a vehicle or machine.

Step by step

  1. The piston travels down with the intake valve open, filling the cylinder with fresh air, often boosted by a turbocharger.
  2. Both valves close and the piston rises, compressing the air until it becomes intensely hot and highly pressurised.
  3. Just before the piston reaches the top, the injector sprays a precisely timed burst of finely atomised diesel.
  4. The fuel ignites on contact with the hot air, and the rapidly expanding gas forces the piston back down.
  5. The exhaust valve opens and the rising piston pushes burnt gas out, ready for the next intake stroke.

Key things to understand

  1. On the intake stroke, the piston moves down and draws only air into the cylinder, not a fuel mixture.
  2. On the compression stroke, the piston squeezes the air into a small space, typically around 15 to 20 times smaller.
  3. Compressing air that strongly heats it to several hundred degrees Celsius, well above diesel fuel's self-ignition temperature.
  4. Near the top of the stroke, an injector sprays fine diesel mist at very high pressure, and it ignites spontaneously.
  5. The burning gas drives the piston down on the power stroke, then the exhaust stroke pushes spent gas out.

Labelled diagram

  1. Intake

    The piston moves down and the cylinder fills with air only.

  2. Compression

    The piston squeezes the air until it becomes hot enough to ignite fuel.

  3. Injection

    A high-pressure injector sprays fine diesel mist into the hot air.

  4. Power

    The fuel self-ignites and expanding gas drives the piston down.

  5. Exhaust

    The piston rises and pushes burnt gases out through the exhaust valve.

Four strokes of a diesel engine cylinder

Real-world examples

  • Diesel locomotives on Indian Railways use large diesel engines to drive generators that power electric traction motors.
  • Most farm tractors in India run on diesel because the engines deliver strong pulling force at low speeds.
  • Hospitals and office blocks keep diesel generator sets ready to supply electricity during power cuts.
  • Large container ships use huge slow-speed two-stroke diesel engines that turn the propeller directly.

Common mistake

✗ Myth: Diesel engines use glow plugs to ignite the fuel the way petrol engines use spark plugs.

✓ Fact: Diesel fuel is ignited by the heat of compressed air alone. Glow plugs only help warm the chamber during cold starts and are not needed once the engine is running.

People also ask

How does a diesel engine work without spark plugs?
It relies on compression ignition. The engine squeezes air into a small fraction of its original volume, and compressing a gas this much makes it extremely hot. When fuel is injected into this hot air, it ignites spontaneously, so no spark is needed to start combustion.
What is the difference between diesel and petrol engines?
A petrol engine mixes fuel with air, compresses it moderately and ignites it with a spark plug. A diesel engine compresses only air to a much higher ratio, then injects fuel that ignites from the heat. Diesels are usually more efficient and produce more low-speed torque, while petrol engines rev higher and run quieter.
Why do diesel engines have more torque?
Diesels typically have a long piston stroke, high compression and strong combustion pressure, and they are often turbocharged. Together these produce a large turning force at low engine speeds. That is why trucks, buses, tractors and generators commonly use diesel engines for heavy work.
What are glow plugs for in a diesel engine?
Glow plugs are small electric heaters that warm the combustion chamber before and during a cold start. A cold engine loses too much heat to its metal walls for compression alone to ignite the fuel reliably; once running, the engine no longer needs them.
Why are diesel engines more fuel efficient than petrol engines?
Diesels use much higher compression ratios, which extracts more work from each unit of fuel, and they control power by fuel quantity rather than throttling the intake air. Diesel fuel also contains slightly more energy per litre than petrol.
Who invented the diesel engine?
German engineer Rudolf Diesel patented the compression-ignition engine in the 1890s and built a working prototype later that decade. He aimed to create an engine far more efficient than the steam engines of his time.
Why do diesel engines produce more soot and NOx?
Diesel combustion happens in very hot zones with excess air, which encourages nitrogen oxides, and fuel-rich pockets around the spray can form soot. Modern diesels use particulate filters, exhaust gas recirculation and selective catalytic reduction to cut these emissions.

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