Diesel Engine Systems Part 1: Low Pressure System

December 21, 2016 - 9:00 AM by Samantha Pudney


Ready to learn more about your diesel engine? Understanding the different parts and purposes is the best way to get familiar with best practice and ensure your engine maintenance is at the highest standard.

Diesel fuel systems consist of two parts, a low-pressure system and a high-pressure system, as well as numerous different elements. Each component of your engine is critical to smooth running.

For now, we’ll delve into the details of the low-pressure side. The function is to deliver a clean supply of uncontaminated fuel to the injection pump. This is important to ensure that the entire fuel system can remain free of any buildup in debris and water contamination.

The various elements of the low-pressure system are, in order:

1. Fuel tank

Your fuel tank should ideally be placed above the engine level, where the head of the fuel will provide a positive pressure force throughout the system. In the case of a leak, the fuel will seep out rather than air ingestion into the fuel system.

If your tank is alongside or lower than your engine, then any leaks that develop air can be drawn into the system when the engine is stopped. A small header tank will be enough to provide positive pressure, but must also be vented to fresh air outside to allow flow into the engine fuel system.

NB: This is not necessary if your engine has a self-air bleed fuel filter fitted to the engine.

A fuel tank can be made of various materials including, but not limited to:

  • Stainless steel: strong and corrosion resistant.
  • Mild steel: coated inside and out with a paint system to protect against corrosion.
  • Plastic: normally translucent so that the fuel level can be seen and offers less condensation and no risk of rust forming.
  • Heavy-duty tanks: flexible and ideal for temporary tanks to endure a long journey.

2. Supply Line

Your engine will vibrate relative to your boat, so it is only logical that a fuel system composed entirely of solid pipe will fracture. A section of the pipework between the tank and the engine must be flexible, such as a purpose-made diesel fuel hose with a protective wire mesh cover.

This will usually be placed next to the engine between your water separator and the lift pump, allowing for the rest of the fuel line to be made of solid pipe and securely attached to your vessel.

Solid piping can be made of anything from thick-walled copper to stainless steel. All sealing joints should be metal-to-metal. A sealing compound or PTFE tape should never be used in this situation as you can run the risk of particles entering the fuel system.


3. Fuel Pre-Filter

Most systems will have a separate water trap maintained between the fuel tank and the mechanical or electric fuel lift pump for the water to settle out of the fuel and be drained off routinely. Fuel passes through the internal passages which allow water and debris to settle out.

If there are more than a few drops of water settled or a small amount of debris, inspect the tank and, if possible, clean it out immediately. The water trap will not need any maintenance but the cylindrical container should also be removed and cleaned if it is badly affected.

Most water separating filters will consist of a disposable element filter, fitted above a clear plastic cup as a water trap. This way you are able to see if there is any water or dirt on the bottom. Give this a routine check over and be prepared to drain out if necessary.

4. Fuel Lift Pump

The mechanical lift pump is operated continuously from the engine camshaft and offers a consistent supply of fuel at a low-pressure to the injector pump, keeping things topped up nicely. Many pumps also have internal mesh screens to filter out debris and prevent non-return valves from staying open.

Two non-return valves flow inward and outward of the chamber above the oscillating diaphragm. Once the pressure in the injector pump body is established, the spring becomes compressed and the displacement reaches an optimal level to maintain pressure.

The Electric type pump is operated via the ignition switch once the key is turned to the on position.

5. Fine Fuel Filter

The last line of defence for the high-pressure pump is the fine filter, which works to trap debris before it is able to clog either the pumps or injectors. Considered by many to be something of a throwaway item, your filter should actually be changed every 250 to 300 hours. Always refer to the maintenance guide in your engine operations manual.

Frequency will depend on the standard of fuel used and the effectiveness of the filter. If in doubt, best practice is to change the fine fuel filter when the oil filter is changed. Take care to ensure that the seals and the holding bolts are in good condition and correctly placed.

This simple system maintenance will help to ensure the longevity of your fuel system and your boat.