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Diesel engines

The key difference between petrol and diesel engines is that there is no spark plug used to ignite the air-fuel mixture. Instead, a diesel engine, which also comes in either two or four stroke guises, uses heat and pressure.

The difference between petrol and diesel in a four stroke process:

  1. Stroke one: As the piston draws back, sucking in air, it does not draw in any fuel, like its petrol counterpart. This means that diesel engine have no use for carburettors to mix the air and fuel.
  2. Stroke two: The piston compresses the air, with diesel fuel only being added (via fuel injectors) as the cylinder is nearing peak compression.
  3. Stroke three: as the fuel is added the heat and compression in the cylinder is already great enough to ignite the fuel. This drives the piston back down the cylinder.
  4. Stroke four: The remaining gasses are ejected.

To get the heat process started, the diesel engine will have glow plugs in the cylinder, which need heating up prior to starting the engine. This technology has improved greatly, with the ability to turn the key and start the engine instantly. However any classic with a diesel engine will usually require you to hold a button for a certain time or wait for a light to go out before trying to start the engine. This allows the glow plugs to heat up.

This higher pressure system, although usually heavier than petrol engines, means a greater torque figure, better suited to industrial use and heavy loads – especially at lower speeds. The advantage of this low down power and fewer electric components, make diesel engines ideal for robust uses in commercial, off-road and military vehicles.

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