How Do Marine Fuel Systems Work?

Marine diesel engines are internal combustion engines that use heavy fuel oil and compressed air to generate power. Within the larger system, the main engine is used for propulsion while the auxiliary engine generates electricity for the ship. Additionally, marine diesel engines work on the principle of the dual combustion cycle; similar to engines used in many modern cars, this may include a four-stroke cycle involving intake, compression, power, and exhaust. However, due to the combustive nature of heavy fuel oil used in diesel engines, the combustion cycle may also be confined to two strokes wherein the compression of hot air and diesel fuel is enough to cause the mixture to self-ignite. 

As the first step of operations, air is taken in by the engine and compressed inside the cylinder. Following this, the fuel is injected into the burner along with the compressed air to generate power output. Finally, the unburnt gasses are blown out of the engine in the exhaust stroke.

Major Components of Marine Fuel Systems

Aside from the central engine system, there are various support networks that ensure the safe and effective storing and transfer of fuel oil. Here, we will explain the various networks and their usage:

Fuel oil transfer system: This system receives fuel and then delivers it to settling tanks where water and solids are naturally separated from the oil. In the process, fuel oils are loaded through deck-fill connections and then sent into storage tanks fitted with heating coils. As the heavy fuel oil is transferred to the fuel oil settling tanks, it is also strained so that the resulting liquid is cleaner and more efficient.

Fuel tank overflow system: All tanks overflow to an overflow tank via a pipeline with an observation glass. This line is also connected to a flow alarm so that a level alarm will be activated when the tank is a quarter full. The purpose of this system is to prevent oil from overflowing onto the deck or surrounding machinery where it could potentially cause a fire.

Fuel oil supply for a two-stroke diesel engine: With many marine diesel engines operating in a two-stroke cycle, there must be a fuel oil supply system that processes the fuel so that it is fit for combustion. In the fuel oil supply system, the oil is stored in tanks and pumped to a settling tank to be heated and filtered. After passing through centrifuges, the cleaned oil is pumped to a daily service tank and then to engine-driven fuel pumps when needed. Finally, the fuel pumps will discharge high-pressure fuel to their respective injectors.

Within this system, there is also a viscosity regulator which controls the fuel oil temperature to provide the correct viscosity for combustion. Therein, a pressure regulating valve ensures a constant-pressure supply to the engine-driven pumps, and a pre-warming bypass is used to heat the fuel once more before starting the engine.

Various Safety Devices in a Diesel Engine Fuel System:

To lessen the risk of potential fires, leaks, and more, there are several safety devices commonly used in diesel engines powering marine vessels. These devices often include:

  • Quick-closing valves on settling/service tanks
  • Relief valves on two pumps/heaters
  • A quick-closing valve on the mixing/vent tank
  • Pipes lagging/save-alls under pumps and heaters
  • A low fuel oil pressure alarm
  • A high fuel oil pressure alarm
  • A low fuel oil temperature alarm
  • A high fuel oil temperature alarm
  • Emergency remote stops for pumps
  • Double-skinned high-pressure pipes between the fuel injection pump and injector 


Marine diesel engines rely on a number of important components to keep them running safely and effectively. If you are in need of parts for your marine diesel engine or other industrial machinery, Just Parts Unlimited has you covered with a vast inventory of over 2 billion high-grade parts, including top-notch fuel systems. As a premium procurement partner, we have curated a robust catalog of high-quality products that are all sourced from leading global manufacturers on our Approved Vendor List (AVL). Begin the purchasing process with us today to start procuring the items you need with time and cost savings in mind.


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