Fuel Tank Equalization

Our system is similar to MTs. There is a pair of saddle tanks on each side of the engine room. These gravity feed from their lowest points via manual valves to a 60 gallon day tank in the bilge between the engines. Both engines and the generator normally feed from this day tank and the fuel returns go to the day tank.

However each engine is capable of being fed directly from one of the two saddle tanks on its side of the boat and the fuel returns can be valved to return to a saddle tank. But we have never in 13 years of using the boat ever done anything but feed everything from the day tank and return everything to the day tank.

We designed the same configuration with a couple of minor differences.

We have two 675 gallon saddle tanks that feed a 400 gallon center line bilge tank. The saddles gravity feed the center and all engines draw from the center. However the return goes to the starboard saddle. This accomplishes two things; one is that we want the center tank to stay coolest, (the center tank is not in the engine room) cool fuel is more efficient. By having the return go to the saddle it has a chance to cool befor being used again. The second reason for not sending the return to the center tank is with the center tank being gravity fed, it is always full. Adding return fuel to this is just pushing fuel back up into the saddle tanks. The return fuel should be top fed into one or both saddles.

A minifold should allow you to reconfigure this to feed all engines from any individual tank. Ideally you should be able to gravity balance the saddles independent of the center tank or through it.

Lastly the polishing system should allow you to filter between any of the three.

All IMHO.

-- Edited by Just Bob on Monday 26th of December 2011 08:06:37 PM

Just Bob wrote:
cool fuel is more efficient.

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Your engine burns fuel by weight, its efficiency has no relationship to the fuel temperature.

It can't even begin to contribute anything to the process until its temperature is above about 450*F.

The reason fuel coolers exist is to ensure that the fuel temperature is below its flash point while being pumped around the engine room and stays below that temperature until it reaches the fuel injectors. The other reason cool fuel is nice is because it retains the viscosity needed to prevent undue wear and deposits on the fuel injection components.

To the previous comment I would have to add:

Fuel being returned to the tank is the unused fuel from the injectors, remember that for the fuel to be used in the injector it has to be compressed to around 30,000 PSI. * Compressing a non-compressible substance creates large amounts of heat. This heated fuel is what is being returned to your tank.

The reason you*want to have the coolest possible fuel before going into the fuel pump is for the heaviest densest possible amount of fuel to be compressed into the injector lines. The colder the fuel before the pump the colder it will come out after being compressed.*

Thermal dynamics of the fuel come into play. *Say there is a 50*F temp rise during the fuel compression cycle and your fuel is at 60*F, the fuel now going into the injectors is 110*F. *Now a gallon fuel at 60*F takes up say 0.75cuft and that same gallon of fuel at 110*F will take up 0.83cuft. *This means you have a lighter less dense fuel taking up space where you could possibly get a denser fuel to occupy and have more power per cu.ft.

It doesn't take much research to see this is a debatable subject but I would still stand by my opinion that cool fuel is more efficient.

Thank you for your addition though, much of what you say is correct as well.

*

Just Bob wrote:It doesn't take much research to see this is a debatable subject but I would still stand by my opinion that cool fuel is more efficient.

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It doesn't take any at all to see that there is nothing to debate.

Heat transported by the return fuel comes from the injector body which is heated by combustion.

Any heat created by pumping is so far down in the "noise" as to be insignificant.

The fuel does not gain or lose "efficiency" with temperature. Fuel provides a certain heating value or number of BTUs per unit of weight.

Any reduction in weight per unit of volume admitted to the injector pump cylinder caused by increased temperature is compensated for automatically by the governor admitting more volume to inject the weight of fuel required to maintain the selected rpm.

Until the governor is operating at the fuel limit stop, you, the operator will not even know anything has changed. At that point the engine will be unable to maintain the same rpm it would with cold fuel but since your engine rarely, if ever reaches the fuel limit except under aggessive acceleration that point will probably never be seen unless you routinely shove the throttles forward to make large clouds of black smoke.

The fuel* heating value does not change ... its "efficiency" certainly doesn't - whatever efficiency means in your context.

Considering that fuel cannot contribute anything at all to the process until it reaches autoignition temperature - around 450F, by your understanding there would be little point in even using it. The energy required to raise the fuel charge to that level is subtracted from the heat of compression so if the fuel could be heated to 449F before being injected for free and without other consequences, the engine would actually be more efficient.

You are confusing grapes and grapefruit.

(edited for clarity)


-- Edited by RickB on Tuesday 27th of December 2011 06:31:08 PM

Best you return to the tank you are drawing from, that way you never have to worry about an inadvertent fuel dump costing many eco damage $$$.

Having worked with diesel equipment in Yuma AZ, I can tell you hot fuel is not an issue. Having worked with diesel engines in the far north, I can tell you cold fuel is a heck of a problem.

Fuel polishing discussion again - won't hurt but will it really help on a*well cared for vessel? Just get a top notch filter setup, turn your fuel over*and buy clean fuel*- you'll sleep soundly.