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Diesel Engine Workings


The easiest way to remember how a diesel engine works, is by remembering the phrase "suck, squeeze, bang, and blow". This refers to a cycle of 4 strokes known as the OTTO cycle.

First of all, air is drawn into the cylinder (suck). The air is then compressed by the movement of the piston, and fuel is injected just before the piston reaches the top of the cylinder (squeeze). The compression raises the temperature of the air; this causes the fuel to combust (bang). Finally, the waste gases are forced out of the cylinder (blow) and into the exhaust system.

Stroke 1 - Induction

  • The inlet valve opens as the piston moves down the cylinder.
  • Air only is drawn into the cylinder.
  • The inlet valve closes just before BDC.
  • Both valves are closed and the induction stroke ends.

Stroke 2 - Compression

  • When the piston moves up the cylinder towards TDC, the air in the cylinder is compressed. This increases its temperature to approximately 600 C.
  • The diesel is injected in an atomised form at approximately 20 before TDC.
  • Both valves are closed and the compression stroke ends.

Stroke 3 - Ignition

  • The temperature in the cylinder causes the diesel to ignite.
  • This causes a huge increase in pressure, forcing the piston down cylinder towards BDC.
  • The exhaust valve opens just before BDC in order to let the gasses out and the ignition stroke ends.

Stroke 4 - Exhaust

  • The exhaust gases are forced out of the exhaust valve as the piston travels past BDC towards TDC.
  • The inlet valve opens just before TDC and the cycle starts again.

OTTO Cycle Facts

  • The cycle was first patented in 1854
  • The first prototype was made in 1860
  • The cycle is said to be named after Nicolaus Otto, a German engineer.

Diesel Engine Facts

  • Diesel engine blocks are normally made from steel.
  • The system will operate at a compression ratio of approximately 14:1 - 23:1.
  • The air/diesel mixture ratios of a diesel engine can vary right through the range, but these are the general figures:

Idle 鈥 up to 60:1    Acceleration 鈥 20:1    Cruise 鈥 100:1

Injection Systems

The injection system has many functions; to inject the fuel into the combustion chamber (cylinder) in the correct mixture form, at the correct time and at the correct position. The system also has to inject the correct quantity of fuel to match throttle position and engine speed. Manufacturers use 3 different types of fuelling systems to deliver fuel to the engine; Mechanical, Electronic, and Common Rail.

Mechanical and Electronic are both employed to make a diesel engine work

Mechanical and Electronic systems both use two different types of high pressure pump; In-line and Rotary. The pump will pressurise the fuel to the correct pressure needed, and send the correct amount of fuel to the injectors at the right time. The fuel will be sent from the pump to the injectors via high pressure pipes (usually made from steel). As the fuel reaches its injection pressure, it will force the injector open, delivering fuel to the cylinder. Both Mechanical and Electronic systems will use a lift/transfer pump; this is a low pressure pump that delivers fuel from the diesel tank to the main pump. This is done using a system of low pressure pipes usually made from rubber or plastic.

Common Rail

The Common Rail system also has a lift/transfer pump and a high pressure pump. The high pressure pump pressurises the fuel which is then sent to a rail connected to all injectors. Unlike the other systems, the injectors are solenoids controlled by the ECU (Electronic Control Unit). The ECU uses information from the vehicle sensors to control when the injectors need to deliver the fuel.

Cold Start Injectors

A Cold Start Injector is located in the inlet manifold. This is used to supply extra diesel fuel to the engine when cold to create a richer mixture to assist in starting.

Glow Plugs

Unlike a petrol engine which uses a spark plug to ignite the fuel, a diesel engine relies on compression in the cylinder creating heat to ignite it.

When starting, if a diesel engine block is cold, the heat created by the compressed air gets dissipated through the cold block. To resolve this, a diesel engine uses glow plugs on starting to warm the air in the cylinder; this then warms the block reducing the amount of heat dissipation when the engine attempts to start. This allows the fuel to be ignited and the engine to start easily.

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