How important is Diesel Fuel Polishing?

Wxx3 · Jun 17, 2014

Larry M said:
I think it depends on the engine and we don't run at WOT. Our FL SP135 returns about 1.8 GPH at 1800 rpms. Not what I would call enough to really polish fuel (we carry 700 gallons) but enough to get you in trouble if you don't return the fuel to same tank you're drawing from.

I estimate that my sp135 returns about the same amount of fuel that it uses. So, I use it to transfer fuel to trim the boat a lot. Also bc the port fuel feed valve is hard to reach!

By the way, I'm no engineer, but it seems to me that the return's main function is to cool the fuel charge and keep it cooler than it would be otherwise. So if your just returning it to the fuel manifold, you're defeating its purpose.

Capt.Bill11 · Jun 17, 2014

Delfin said:
The point of polishing is to prevent the tanks from getting filled with poop in the first place. It is what keeps the tanks clean. Filtration just removes the crud before it hits the engine but does nothing to clean the tank. If you go to any yard that has serviced a fuel tank you will hear them tell you that when polishing systems are used as designed, the tanks have no sediment build up even after years of use.

Since "polishing" (what a stupid term IMO) does nothing to stir up the fuel in the tank, how exactly does it keep the fuel "polished"?

Unless the crud that may be in the fuel is in suspension the extra filtration is just filtering clean fuel over and over. Even if the system draws from the lowest point on a tank it won't do much unless the tank is low on fuel and is being stirred up in a rough seaway since fuel tanks are baffled.

I think your money is far better spent on dual change on the fly filters with large surface area elements then on a "polishing" system.

psneeld · Jun 17, 2014

Wxx3 said:
I estimate that my sp135 returns about the same amount of fuel that it uses. So, I use it to transfer fuel to trim the boat a lot. Also bc the port fuel feed valve is hard to reach!

By the way, I'm no engineer, but it seems to me that the return's main function is to cool the fuel charge and keep it cooler than it would be otherwise. So if your just returning it to the fuel manifold, you're defeating its purpose.

That may be truein a 1:1 mix...hot fuel mixing with fresh...

But before I did the return to the manifold trick...I did a lot of research.

Still not trusting everything I see and read..I decided to test my engine (and genset to be sure) setup.

I disconnected and placed the return in a jar. After 15 minutes I has less than a oz in the jar so I increased the RPMs...at 1500 RPM and 10 minutes later I had maybe a quarter ounce more....

So I figured the total heat gain after travelling though all the piping, brass return manifold and the big brass fuel manifold, plus a racor and fuel pump plus on engine fuel filter...and being mixed with well over 10X the amount of fresh fuel...just maybe it doesn't make a difference....and it didn't over the last 2500 mile trip.

Others here have posted that their return line was plumbed back into the supply line from the factory is what got me thinking in the first place.

Delfin · Jun 17, 2014

Capt.Bill11 said:
Since "polishing" (what a stupid term IMO) does nothing to stir up the fuel in the tank, how exactly does it keep the fuel "polished"?

Unless the crud that may be in the fuel is in suspension the extra filtration is just filtering clean fuel over and over. Even if the system draws from the lowest point on a tank it won't do much unless the tank is low on fuel and is being stirred up in a rough seaway since fuel tanks are baffled.

I think your money is far better spent on dual change on the fly filters with large surface area elements then on a "polishing" system.

I think you may have missed the oft repeated injunction that fuel polishing is done while underway. The lower the level of fuel, the more agitated the fuel is, even with baffles. Plenty of sloshing around and plenty of contaminant removal, which is why cruisers using polishing systems never seem to need to have their tanks cleaned out. They stay clean.

koliver · Jun 17, 2014

When I had a sailboat, small diesel tank, but less usage too, I had frequent plug ups of my Racor. Especially in the predictable conditions of lots of sloshing around, and of course in conditions where use of the engine was absolutely necessary.
But... that stage of my life ended when I bought Retreat, in 1994. Since then, I have noticed a marked improvement in the clean-ness of the fuel that I purchase, I now have two rather large tanks, 300US Gal each, and I try to leave them full over the winters, so the avg age of the fuel in the tanks is now over two years. Yet, I now go through Racor filter elements about every 4 years, (I keep track of this in my maint. log) and have never had a plug up. The return of my Volvos is likely more than the consumption, but nowhere close to the volumes suggested above as being necessary to a polishing system. I think there is a lot to be attributed to the modern refining and filtering that goes on before I get my fuel. It seems like it gets polished, so all I have to do is keep it from degrading and no furhter polishing is necessary.

Delfin · Jun 17, 2014

koliver said:
When I had a sailboat, small diesel tank, but less usage too, I had frequent plug ups of my Racor. Especially in the predictable conditions of lots of sloshing around, and of course in conditions where use of the engine was absolutely necessary.
But... that stage of my life ended when I bought Retreat, in 1994. Since then, I have noticed a marked improvement in the clean-ness of the fuel that I purchase, I now have two rather large tanks, 300US Gal each, and I try to leave them full over the winters, so the avg age of the fuel in the tanks is now over two years. Yet, I now go through Racor filter elements about every 4 years, (I keep track of this in my maint. log) and have never had a plug up. The return of my Volvos is likely more than the consumption, but nowhere close to the volumes suggested above as being necessary to a polishing system. I think there is a lot to be attributed to the modern refining and filtering that goes on before I get my fuel. It seems like it gets polished, so all I have to do is keep it from degrading and no furhter polishing is necessary.

Refinery standards filter the fuel to 30 microns. 2 to 5 microns can damage common rail systems, so on that basis the freshest fuel is already contaminated. The asphaltene precipitates out after that final filtration and collects in the bottom of the tank absent polishing and those particles are big. All polishing is designed to do is keep the fuel at refinery specs over the period time you use it. We polish at 30 microns and transfer fuel at 10 microns to reduce the load on the secondarys. You can run just fine for years on the filters placed on the engine, but the reason you read about people having their engines stopped when they get frisky in a seaway is because that precipitate gets stirred up and overloads the system.

How important is Diesel Fuel Polishing?

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