How a Diesel Engine Works

How a diesel engine works

This page is a very brief, everyday description of how a diesel engine works.
The different types of diesel equipment, old and new which are powering today’s diesel cars.

The main components of a diesel engine are


diesel engine glow plug

Diesel engine glow plug

Basically the diesel engine is like the petrol engine. They are both internal combustion engines.
One of the  main differences is the way in which the fuel is ignited.

  • in a petrol engine a spark ignites the fuel
  • in a diesel engine fuel ignites because of high compression.

Diesel engines of any type work because diesel fuel is injected into a cylinder of highly compressed air.

Because the air is compressed it is very hot; the injected fuel ignites and  forces the piston down into the cylinder and turns the crankshaft.

Diesel fuel has to be atomised (turned to mist or microscopic droplets) before it will combust (burn) properly –  the injectors are designed to make this happen.

Diesel is broadly divided into

  • Rotary diesel and

    diesel injectors

    Rotary diesel injectors

  • Common Rail diesel.

 

This is diesel equipment which we now call early diesel. It generally has little or no electronic control and has been replaced in passenger cars by the common rail diesel system.

A rotary diesel engine typically has a distribution pump and injectors. The pump is timed to the engine so it sends fuel under pressure to each injector at the correct time in the engine’s combustion sequence.

The pressure at each injector is somewhere in the region of 150 bar.

Typically a rotary diesel pump has a solenoid which allows the driver to cut off the supply of fuel and stop the engine; without this the engine will continue to run as long as it has a supply of fuel!

Rotary diesel can be divided into ”direct’ and ‘indirect’ injection.
Direct injection engines are designed for the fuel to be injected directly into the cylinder, typically into a bowl recess in the crown of the piston.
With indirect injection the fuel ignites in a separate combustion chamber recessed into the cylinder head.

The rate of combustion is slowed down and the engine is generally quieter and less stressed than direct injection. When starting, indirect engines generally require glowplugs to warm the fuel until the engine is running and getting warm.

Later Ford Transit 8 valve diesel engines (up to c.1999) are of the direct injection type and have a characteristically noisy diesel sound.

THE DIESEL ENGINE – an OVERVIEW
DIESEL ENGINES COMPRESS AIR – NOT DIESEL FUEL

Strange as it may seem the diesel engine doesn’t compress the diesel fuel it uses – combustion occurs when diesel is injected into compressed (hot) air.

Glow plugs are used to warm diesel fuel when the engine first starts…and not to provide a spark.

Diesel that are very worn will most likely suffer low compression and become more difficult to start.

DIESEL IS ‘GREENER’ THAN PETROL
Not the colour, of course, but it’s environmental impact!

It’s possibly a little surprising that cars with a diesel engine emit less CO2 than equivalent petrol cars.

The downside with diesel is the amount of soot (particulates) which they emit.

DIESELS NEED TO COMPRESS
Compress to impress!
Did you know that the CR compression ratio (difference in volume when piston goes from top to bottom of cylinder) in a diesel engine is roughly double the petrol CR?

DIESEL HAS EVOLVED
Early diesel engines have mechanical pumps and mechanical injectors.

In theory a mechanical diesel engine will continue to run for as long as it has fuel because it has no electrical/electronic control.

Mechanical injectors operate around 200bar pressure (your car tyre is typically 2 bar!)

Electronically controlled pumps (EDC and VP) were fitted to lower emmisions and give greater economy.

Common Rail Diesel has superceded the EDC and VP systems. As a high precision setup the pressure at the injectors is around 2000bar

COMMON RAIL DIESEL PRESSURES ARE 10X HIGHER

Did you know that in the common rail diesel engine fuel is under very high pressure e.g. 2000 bar? this compares with 200bar or less in an earlier rotary diesel engine.