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-   -   Return diesel fuel to the tank? (http://www.sailnet.com/forums/diesel/97726-return-diesel-fuel-tank.html)

CarbonSink62 03-20-2013 02:23 PM

Return diesel fuel to the tank?
 
I read the posts regarding the loss of the HMS Bounty last year and one item confused me. This is a quote about the chief engineer from the linked article :

"That he wasn’t licensed doesn’t mean he was professionally deficient – that he couldn’t correctly answer the simplest questions about diesel engines did. Believing that Bounty’s engines burned all the supplied fuel and that her engines “didn’t return any fuel to the tanks – they burned it all,” was all anyone needed to hear."

I'll admit to being ignorant of an awful lot of engine info; especially diesel engines, but I don't get this. The quote makes it sound like anyone knowledgeable of large (I assume?) diesel engines knows that they return fuel to the tank.

Can someone tell me why?

I can only guess that it is some sort of fuel polishing. i.e. 'Draw and filter twice as much fuel as you need, and return the filtered fuel to the tank.'

I that it or is there more going on?

Five decades on the planet and still a newbie,

Ken

chucklesR 03-20-2013 03:05 PM

Re: Return diesel fuel to the tank?
 
Because the low pressure side of the fuel pump system ALWAYS sends more to the high pressure side than it needs.

For a direct example my westerbeke 30b pumped 25 gallons per hour, and ran full time.
The engine, at full RPM, consumed 1.1 gallon per hour.
The rest (23.9 gallons) returned to the tank via the fuel return hoses.

The same is true whether it's a mechanical fuel pump (cam operated) or an electrical pump on the low pressure side.
The high pressure side of the system is cam driven and boosts the PSI to 800 or so (engine dependent), that amount of pressure is needed to overcome the springs in the injectors and get good spray into the cylinder.
You would not want the entire fuel system at 800 PSI for obvious reasons.

Because of that you can literally, depending on filter placement, clean your fuel just by running the pump (electrical) or running the engine.

Any professional should in fact know that.

jrd22 03-20-2013 03:09 PM

Re: Return diesel fuel to the tank?
 
Yep, like Chuck said. I don't know of any diesel that doesn't, not sure about some of the small air cooled diesels though.

CarbonSink62 03-20-2013 03:12 PM

Re: Return diesel fuel to the tank?
 
Thanks, guys.

aa3jy 03-20-2013 03:29 PM

Re: Return diesel fuel to the tank?
 
Yep..my Yanmar has a low pressure(about 3 psi +/-) return line to the diesel tank. Easy to see from pictures from following the high and low rail lines(metal piping) to and from the top of the injectors.

Letrappes 03-20-2013 05:05 PM

When that line plugs up you have a problem. Guess how I know this...

TQA 03-21-2013 11:07 AM

Re: Return diesel fuel to the tank?
 
All injector pumps HAVE to have a 'leak off' pipe.

It may do but does not HAVE to return to the tank.

It may return to the fuel feed pipe anywhere between the tank and the inlet side of the low pressure 'lift' pump.

So what the Bounty guy said might be correct.

Now as the qty of fuel that leaves the high pressure pump through the leak off line, is small, much less than the qty actually consumed by the engine using it as a way of polishing the fuel is a non starter. [ There is an exception, the old CAV rotary pumps did return significant amouts. I am not sure if it would be enough to 'polish' fuel though.

Ex diesel mechanic and instructor.

casey1999 03-21-2013 03:12 PM

Re: Return diesel fuel to the tank?
 
Many of the small Hp Yanmars have the fuel return line connected at the top of each fuel injector. The lines are run together then connect to a hose that takes fuel back to fuel tank. I guess you could connect the line to the fuel intak, but because this fuel return line may have air in it, that could cause a problem by air locking your fuel system. I do not seem to get any fuel returned to the tank through this line at idle (have not checked at full throttle), but the engine runs fine.

I do not understand how the injector works. Is it the fuel pressure that causes the injector nozzel to spray fuel? Why do you need a fuel return. Does not the fuel injector pump meter correct fuel to the injector? I can see where the fuel mechanical pump (or electric if that is what you have) could push too much fuel, but with a diesel, I would think the fuel would be restricted by the positive displacement fuel injection pump.

TQA 03-22-2013 12:30 PM

Re: Return diesel fuel to the tank?
 
See FIG 1 for a good pic on how an old fashioned injector pump / injector works.

Fuel from the pump is delivered to the injector at 1500psi + .

The injector piston [or spool if you prefer the part beneath the spring anyway] is a good fit in its cylinder but still leaks a small part of the fuel into the the very top area connected to the return .

This leakage MUST occur to keep things lubricated. How much they leak is not that important as long as they all leak approx. the same amount. Hence the manufacturers rebuildes supplying matched sets of injectors.


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