Marine Diesel: Low-Pressure Fuel Return Lines

Diesel fuel not used for combustion is circulated in the low-pressure side of the system for lubrication and cooling.

16th March 2013.
By Ed Sherman

Question: Recently I was poking around on my new boat checking things out on the Volvo diesel engine. The picture I sent in shows my engine. The red arrow is pointing to a small rubber hose that runs from each of my engine’s fuel injectors to the other and then back down to my injection pump. What’s weird here is that the injectors have heavy duty metal pipes going to them and these hoses feel like rubber and don’t even have any hose clamps on them. Is this a problem waiting to happen? I’ve heard that these systems run at pretty high pressures.

Low-pressure return lines

The softer fuel-return lines shown by the red arrow are on the low-pressure side of the fuel circulation system.

Answer: You have a good eye! Not to worry though. You are right that on your engine the fuel injector pressures are quite high. As I recall, on the Volvo engines with the system your engine uses, injector opening pressures approach 4,000 psi. But, that’s all going on at the high-pressure side of the system, contained by those metal lines with the hex fittings holding them to the injectors. The rubber hoses you refer to are on the low-pressure, return side of the system.

Diesel injection systems are closed, full loop systems. In other words, fuel gets sent to each cylinder, but the system always sends more fuel than will actually be used as the engine runs. Diesel fuel serves multiple functions when the engine is running, more than just providing a flammable fuel for combustion. It also acts as a lubricant and coolant for the extremely close-tolerance parts inside your injectors and injection pump. The fuel in a typical system of this type will recirculate either by sending the excess fuel all the way back to the engine’s fuel tank, recirculating it through the fuel injection pump, or on some engines via the engine-mounted fuel filter assembly. The hoses you point out are friction-fitted to metal nipples on the side of each injector. The really low pressure at this point in the system is not a problem for this arrangement; the push-on fit is adequate for the pressure here.


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About the author:

Ed Sherman

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Ed Sherman is a regular contributor to boats.com, as well as to Professional Boatbuilder and Cruising World, where he previously was electronics editor. He also is the curriculum director for the American Boat and Yacht Council. Previously, Ed was chairman of the Marine Technology Department at the New England Institute of Technology. Ed’s blog posts appear courtesy of his website, EdsBoatTips.
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