Marine Diesel Engines – How They Work
Marine diesel engines are feisty beasts. They are much more robustly built than their close cousins, the petrol (gasoline) fuelled type. Both fall into the general category of internal combustion engines. This means there’s an explosion of vaporised fuel that drives a piston down a cylinder, where linear motion is converted into rotary motion by means of a crankshaft.
Unlike petrol engines, which employ an electrical spark to initiate combustion, diesel engines rely on ‘pressure-ignition’, a phrase which describes the principle well. When air is compressed rapidly it becomes extremely hot. Once it reaches about 400°C (752ºF) it’s hot enough to ignite a suitable fuel injected into it. This temperature is reached when the air is compressed to about a fourteenth of its original volume – and an engine capable of achieving this would be said to have a ‘compression ratio’ of 14:1. In practice, higher compression ratios are used to provide a comfortable margin – the range 18:1 to 20:1 being typical. This compares to about 10:1 for a petrol engine and explains why diesels must be more solidly constructed to cope with the higher loads.
And those two commodities, air and fuel, are all a diesel engine needs to run. Yes, battery power is almost invariably employed to crank and start them, but (with certain exceptions, including electrically governed and common rail type engines) once started they will operate without electricity.
For both petrol and diesel engines there are two-stroke and four-stroke (also known as two-cycle and four-cycle) forms. The marine diesel engines we are familiar are almost always four-strokes
So, in order to function, a diesel engine must gather together a quantity of air, compress it very quickly so that it heats, then introduce some fuel which will combust and expand. The four cycles are known as ‘induction’, ‘compression’, ‘expansion’ and ‘exhaust’ – often less elegantly expressed as:
Suck: The piston withdraws down the cylinder and draws in air from outside through the open inlet valve. The exhaust valve is closed.
Squeeze: Now the piston is moving back up into the cylinder. Both the inlet and exhaust valves are closed. Having no escape, the air is being compressed and gets hotter and hotter. Just a little before it reaches the top of the stroke, atomised diesel fuel is sprayed into the cylinder at high pressure and …
Bang! The fuel and air mixture in the cylinder ignites and burns – essentially a contained explosion. The expanding gases drive the piston down the cylinder with great force which, once converted to rotary energy by the crankshaft, turns the propeller.
Blow: The exhaust valve opens and the piston pushes back up the cylinder to expel all the spent gases. Once it reaches the top, the exhaust valve closes and the inlet valve opens. The cycle is ready to start again.
More on engines: Marine Diesel Engine Fuel System - the roundabout route