The Basics

The 4 Stroke Diesel Cycle

 
 

 

Nickolaus Otto invented the 4 stroke cycle in 1862. More details of how the four stroke spark ignition cycle works, together with pictures of Otto's first engines can be found here

 

In 1892 Rudolph Diesel invented the compression ignition engine, now named after him. The first working engine was built  at the Augsburg Maschinenfabrik (now part of the MAN B&W group) in 1897. The single cylinder engine was used to power stationary machinery. It weighed five tonnes and produced 20 hp at 172 rpm! The engine operated at 26.2% efficiency, a very significant improvement on the 20% achieved by the best petrol engines of the time.

 

In 1912 the first ocean going vessel to have diesel engines installed was the Selandia. The engines were 8 cylinder 4 strokes. An idea of their size can be got from the man standing by the engine controls half way down the engine.

The four stroke cycle is so called because it takes four strokes of the piston to complete the processes needed to convert the energy in the fuel into work. Because the engine is reciprocating, this means that the piston must move up and down the cylinder twice, and therefore the crankshaft must revolve twice.

The four strokes of the piston are known as the induction stroke, the compression stroke, the power stroke, and the exhaust stroke. Students sometimes remember this as "suck, squeeze, bang, blow."

1. INDUCTION: The crankshaft is rotating clockwise and the piston is moving down the cylinder. The inlet valve is open and a fresh charge of air is being drawn or pushed into the cylinder by the turbocharger

2. COMPRESSION: The inlet valve has closed and the charge of air is being compressed by the piston as it moves up the cylinder. Because energy is being transferred into the air, its pressure and temperature increase. By the time the piston is approaching the top of the cylinder (known as Top Dead Centre or TDC) the pressure is over 100 bar and the temperature over 500C

3. POWER: Just before TDC fuel is injected into the cylinder by the fuel injector. The  fuel is "atomised" into tiny droplets. Because they are very small these droplets heat up very quickly and start to burn as the piston passes over TDC. The expanding gas from the fuel burning in the oxygen forces the piston down the cylinder, turning the crankshaft. It is during this stroke that work energy is being put into the engine; during the other 3 strokes of the piston, the engine is having to do the work.

4. EXHAUST: As the piston approaches the bottom of the cylinder (known as Bottom Dead Centre or BDC) the exhaust valve starts to open. As the piston now moves up the cylinder, the hot gases (consisting mostly of nitrogen, carbon dioxide, water vapour and unused oxygen)  are expelled  from the cylinder.

As the Piston approaches TDC again the inlet valve starts to open and the cycle repeats itself.

 

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