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Old 10-03-2017, 04:25   #1
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Fuel tank gets warm when motoring

Since we just had our tank replace we had the covers off to monitor for any leaks. I noticed the tanks does heat up a bit after our 5 hour motor journey yesterday. Is this normal? I assume the fuel that gets returned from the fuel pumps on the engines builds up some heat.

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Old 10-03-2017, 04:57   #2
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Re: Fuel tank gets warm when motoring

That's what I would have thought, the return fuel being warmed.
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Old 10-03-2017, 05:40   #3
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Re: Fuel tank gets warm when motoring

Sounds as if you didn't have the opportunity to check before the install but I go with Mark in agreeing with your thoughts. The prior tanks probably heated as well due to returned fuel. Is there an Orana owner's group that maybe could answer your question?
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Old 10-03-2017, 05:50   #4
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Re: Fuel tank gets warm when motoring

Return fuel is of course warm, a large part of why there is return fuel is the excess fuel flow is used to cool the injection pump, you think an air compressor gets hot pumping air up to 100 PSI? Pressurize a liquid to 800 or more, you generate some heat.

My Duramax pickup truck that I used to have even had a fuel cooler for return fuel, it looked like a little radiator, reason is there was concern that fuel when the
tank got low could get too hot, a common rail pressurizes it's fuel to 30.000 PSI and really generates some heat doing so. It's also used to cool the electronics too though.
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Old 10-03-2017, 06:35   #5
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Re: Fuel tank gets warm when motoring

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Originally Posted by a64pilot View Post
Return fuel is of course warm, a large part of why there is return fuel is the excess fuel flow is used to cool the injection pump, you think an air compressor gets hot pumping air up to 100 PSI? Pressurize a liquid to 800 or more, you generate some heat.

My Duramax pickup truck that I used to have even had a fuel cooler for return fuel, it looked like a little radiator, reason is there was concern that fuel when the
tank got low could get too hot, a common rail pressurizes it's fuel to 30.000 PSI and really generates some heat doing so. It's also used to cool the electronics too though.
Diesel injection systems rely upon close tolerance components rather than elastomer seals to minimise leaking nonetheless there is leakage of a significant volume of fuel past the plungers of the fuel pump and the pop valves in the injectors. In going from pressurised to ambient there is an energy conversion which results in heat production in the fuel.
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Old 10-03-2017, 07:43   #6
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Re: Fuel tank gets warm when motoring

Typical rotary pumps return up to 30% of the fuel back to the tank, and considering that returned fuel has gone through the engine/block it will have picked up a lot of energy. So, depending upon the size of your engine, size of the fuel tank and fill level you will get more or less warmth.

p.s. While the fuel will heat up while being pressurized, it will cool down when being depressurized and since the fuel returns to the tank at the same pressure it was taken from the tank the equation works out to zero heat gain from that.
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Old 10-03-2017, 08:32   #7
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Re: Fuel tank gets warm when motoring

And we shall not forget that this fuel return have an important function: remove heat from the injector (including the tip) to keep it from overheating in the combustion chamber. I like the idea that the return line passes trough some cooler to remove heat from the fuel before it reach the tank. The capacity of the fuel tank, or more precisely the amount of fuel left in the tank, along with the time the engine is runing, affects its temperature.
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Old 10-03-2017, 08:37   #8
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Re: Fuel tank gets warm when motoring

Perfectly normal, for a diesel. More fuel than can possibly be burned, by the engine, is circulated through the pump and injectors to lubricate and cool those components. There is an "Excess Fuel Return line" you'll see, which carries this warm fuel back to the tank.
Good job of observation; keep it up.
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Old 10-03-2017, 10:29   #9
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Re: Fuel tank gets warm when motoring

Quote:
Originally Posted by zerompg View Post
Since we just had our tank replace we had the covers off to monitor for any leaks. I noticed the tanks does heat up a bit after our 5 hour motor journey yesterday. Is this normal? I assume the fuel that gets returned from the fuel pumps on the engines builds up some heat.

Thanks
You have it! I doubt I would worry about it.
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Old 10-03-2017, 11:02   #10
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Re: Fuel tank gets warm when motoring

I have a friend with twin cats in his trawler. He did have large (I think around 1000 gals) tanks so it would take a couple of days at anchor for the fuel temp to get to ambient. In the summer the added heat in the tanks made the cabin warmer than he liked so he put heat exchangers on the return lines. He said that it made a noticable difference in the cabin temp in the tropics.
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Old 10-03-2017, 11:07   #11
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Re: Fuel tank gets warm when motoring

Liquids are not compressible and hence do not get warm when compressed.
This only works for compressible media like gases.
Physics 101.

The return fuel picks up heat from the engine either directly through contact in the pump and injector or indirectly from the engine room air and heat radiation.
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Old 10-03-2017, 15:23   #12
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Re: Fuel tank gets warm when motoring

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Liquids are not compressible and hence do not get warm when compressed.
This only works for compressible media like gases.
Physics 101.

The return fuel picks up heat from the engine either directly through contact in the pump and injector or indirectly from the engine room air and heat radiation.
Yes Boyles Law and Charles Law deal only with gasses so (Physics 101) you're quite correct to say that this does not apply to fluids. However (Physics 201) fluids are compressible at the extreme pressures found in modern diesel injection systems. We observe a volume reduction of about 8% at around 45,000psi which is pretty standard for piezoelectric injectors.

Your statement regarding where the return fuel picks up heat is correct.
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Old 10-03-2017, 15:30   #13
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Re: Fuel tank gets warm when motoring

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Originally Posted by zerompg View Post
Is this normal? I assume the fuel that gets returned from the fuel pumps on the engines builds up some heat.

Thanks
YES! You assumed right?
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Old 10-03-2017, 15:50   #14
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Re: Fuel tank gets warm when motoring

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Liquids are not compressible....
If this were true we could put water in the brake system of our vehicles.
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Old 10-03-2017, 15:57   #15
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Re: Fuel tank gets warm when motoring

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Originally Posted by a64pilot View Post
My Duramax pickup truck that I used to have even had a fuel cooler for return fuel, it looked like a little radiator, reason is there was concern that fuel when the
tank got low could get too hot, a common rail pressurizes it's fuel to 30.000 PSI and really generates some heat doing so. It's also used to cool the electronics too though.
Yep, in common rail trucks without a fuel cooler, the fuel can get excessively hot.
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