Marine diesel engine fuel systems

A typical marine diesel engine fuel system forms a circuit, around which the fuel passes in an endless parade. Let’s trace the fuel’s roundabout journey.

• The story starts at the tank – basically the food store for the marine diesel engine. This can be made from a variety of materials: these days stainless steel, aluminium and polyethylene being the most popular.
• Assisted by gravity and pulled by the pump, the fuel exits the pump, passes through a stopcock and goes immediately to the pre-filter. The fuel enters through the inlet and is sent downwards into a bowl. There, an arrangement of fixed vanes causes the fuel to rotate, creating a centrifugal effect that separates water particles and heavy sediment and deposits them in the bottom of the bowl.
• The bowl may either be of glass or metal or plastic. Transparent bowls allow you to see if any contaminants have been collected, in which case they can be drained off through a valve at the lowest part of the bowl.
• The fuel then rises inside the casing and passes through a filter element before passing on to the ...
• ... lift pump. This is basically the heart of the fuel supply system. The most common type of lift pump is driven directly by a cam inside the engine, and often has a small external handle so it can be operated manually for bleeding air from the system. Lift pumps don’t supply the fuel to match the demand. They deliver an excess of it, leaving the injector pump to take what it wants. But, before it gets there it has to pass through the…
• ... fine filter. Filters are categorised by the size of the particles they allow through. The customary unit used to define particle size is the micron– one millionth of a metre. And, whereas a typical pre-filter might fall into the 10–50 microns range, the fine filter is more stringent at 2–10 microns.
• At this point we should sing a short hymn of praise in favour of two-stage filter systems. Diesel fuels contain tarry particles called ‘asphaltenes’. Being small, soft and pliant, they present no serious threat to the fuel injection process, but they tend to stick to the fibres of the first filter element they encounter. It’s the pre-filter’s job to ensnare most of the asphaltenes to prevent them choking up the fine filter, whose primary responsibility is to remove any tiny particles that could damage the exceedingly delicate and expensive injector pump.
• The role of injection pumps is crucial to the proper working of diesel engines. And why this should be so is easily understood, since they perform an extremely demanding job. After receiving the fuel from the fine filter they must deliver precisely metered quantities of fuel to each injector, exactly on time and at very high pressure– and they must do so over and over again.
• Neither does all the fuel go on to the injectors. The surplus is used to lubricate the injection pump before being sent back to the tank along the fuel return line.

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