[Machman]

You want the return to be as far from the intake as possible and at the bottom of the tank to avoid aeration of the diesel fuel. Diesel will hold a significant amount of air if it's bubbled / shaken up..

 

[Machman]

Thanks. My J300's fuel tank doesn't have a proper return line. Any reason why I can't tap into the fuel inlet line near the tank and have the returned fuel dump into the inlet isolated from ambient air?

If you're talking about doing a T into the line coming out of the tank and going to the engine.. Remember that the fuel does 2 jobs in the IP: Lube and cooling. However you do it, you probably want to cycle the fuel through the tank to avoid potential overheating. Plus, the return fuel is somewhat aerated to start with, so it needs to be able to "settle" a bit.

 

[Machman]

Both of those probably are far enough away from the pickup part of things to allow the fuel to de-aerate..

 

[Machman]

I have now concluded that it must be aerated in the pump, but how? and where does the air come from?

Diesel fuel that appears "clear" and "solid" will actually have about 10% air dissolved in it. When you put it through the violence and pressure changes of the pump and injectors (because some of that return fuel is bleed from the injectors), then the dissolved air basically gets "jarred" into forming bubbles in the fuel.

 

[Machman]

Thanks, I hope the diesel I buy at the pump isn't 10% air.
This then must mean that the diesel being injected must be 10% air aswell.

Gaza

From Caterpillar Special Instruction 651-1250:

"Normally No. 2 Diesel Fuel contains about 10% air in solution, although the air is not visible."

That's about the size of it...

It does vary with temperature, ambient pressure, etc, but that's the number I keep seeing as a "baseline average".