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Tuesday, September 19, 2017

Diesel engines

Posted by Krzysztof Lis on March 15, 2008

Diesel engines, called also compression ignition engines, are internal combustion engines operating on Diesel cycle. The principles of this cycle as well as first diesel engine were discovered in 1890s by Rudolf Diesel.

The combustion in diesel engine is started when the pressure in combustion chamber exceeds ignition point of fuel injected to the chamber. During the compression stroke, the air inside the combustion chamber is compressed and because of that it increases it’s temperature. On a proper timing fuel is injected to the combustion chamber and ignites from the hot air inside.

The peak pressure in diesel engines is greater than in gasoline (spark ignition) engines. The ignition doesn’t require any external energy source (as in gasoline engines – the electric spark from spark plug).

In diesel engines, the fuel is injected to:

  • prechamber or ante-chamber in which combustion begins and then spreads to the rest of combustion chamber (indirect injection),
  • the combustion chamber itself (direct injection).

In some diesel engines, the first ignitions are supported by glow plugs. When cold engine is fed with cold air the compression itself is not enough to obtain high air temperature required for igniting the fuel. In such a case glow plugs are heated and allow the engine to start.

Principles of diesel engine operation

As most of internal combustion engines, diesel engines work in 4-stroke cycle.

1. Intake / induction stroke

As the piston descends, it reduces the pressure in the cylinder. Then, air is forced by atmospheric pressure, into the cylinder through the intake port. In some cases the air is blown to combustion chamber by supercharger / turbocharger or blower of some kind.

2. Compression stroke

Air is compressed by upward moving piston. At the very end of this stroke, when the air obtained its maximum temperature, the fuel is fed (injected) to combustion chamber, it vaporises and ignites from the hot air.

3. Power stroke

Fuel is dispersed into very fine mist of small droplets by the injector. Those droplets quickly vaporize and the fuel vapors ignite. The energy is then released and temperature and pressure of air inside the cylinder continues to rise. The rise of pressure causes the piston to move downward. As the hot air expands, the piston takes the energy from the hot air inside the cylinder and does the work in the diesel cycle. This is the stroke in which the thermal energy is changed to mechanical power!

4. Exhaust stroke

When the piston finishes its work, it is turned again to go upwards, to push the combustion gasses from inside the engine.

Diesel engine fuels

The standard fuel for compression ignition engines is diesel or petrodiesel. It is a mixture of hydrocarbons obtained from crude oil during fractional distillation in temperatures between 200-350°C (392-662°F). The most important parameter that describes a possibility for a fuel to be used in compression ignition engine is it’s cetane number. It is a measurement of combustion quality of a diesel fuel, describes how easily can a fuel be ignited in compression ignition engine.

Other, alternative diesel fuels, are:

  • vegetable oil – straight and waste,
  • biodiesel – chemically altered vegetable oil,
  • heavy fuel oil (bunker oil) – used for large marine diesels.

It is important to remind that the first diesel engine was fueled with peanut oil!

Pros and cons of diesel engine

  • Diesel engine has higher NOx emissions, compared to modern gasoline engine equipped with catalyst.
  • Diesel engine has higher particulate emission, if is not equipped with particle filter.
  • Diesel engines are more expensive in production than spark ignition engines.
  • Compared to gasoline engines of the same power, diesel engines are louder and bigger.
  • The ignition is slow and limits the maximum speed of the engine.
  • The internal efficiency of diesel engines is greater than for any other internal combustion engines. Most large, stationary ICE engines are diesels because of that greater efficiency!
  • The diesel fuel is a better lubricant than gasoline, so diesel engines have longer life span. It is said that a diesel engine will work for 400,000 km (250,000 miles) without a need of rebuild, if taken care of.

Diesel engine parameters

  • Compression ratio – 1:16 – 1:25 or more.
  • Compression pressure – 30 – 50 bar (435 – 725 psi).
  • In-cylinder pressure – 50 – 80 bar (725 – 1,160 psi).
  • Fuel injection pressure – 80 – 220 bar (1,160 – 3,190 psi).

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