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post #1 of 9 Old 07-09-2004 Thread Starter
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Fuel Return Ratio

Here''s a highly variable question: How much diesel gets returned to the fuel tank in the process of running your engine, verses what gets burned.

For example, if I run my engine for 10 hours at a consumption rate of 1.15 gallons per hour, that''s 11.5 gallons of fuel used. My question is, how much got sent from the tank to the engine in the process and then returned to the tank? I''ve heard people say that as much gets sent back as is used (ie. the pump sent 22 gallons to the engine and 11 got returned to the fuel tank).

Does anyone have a definitive notion of what the return ratio is??? I''ve recently had a hard time calculating "what''s in my tanks" because regardless of which tank I''m using, the engine only returns fuel to the primary tank. HELP!!!
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post #2 of 9 Old 07-10-2004
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Fuel Return Ratio

For obvious reasons, this is a poor practice (not returning fuel to the tank that supplied it, and unacceptable under ABYC "Standards & Recommended Practices..."
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post #3 of 9 Old 07-10-2004
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Fuel Return Ratio

Gord is coreect. This is very bad! On my boat when I switch tanks I also switch the return line with a Y valve. On one side I mark red and the other green. The valve sits next to the tank selector.

It''s not that hard to add a valve to redirect the return line to the proper tank it came from.
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post #4 of 9 Old 07-10-2004 Thread Starter
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Fuel Return Ratio

Gord and Paul,

While I appreciate your replies, neither of you answered my question. Yes I''m aware that it is not a premium system configuration (as you said, "for obvious reasons"). So what I really need is not an "ABYC standards recommendation." I would love a "ball park figure" or even guestimated range of "diesel returned" during normal function. Once again, thanks for reading my inquiry and posting a response, but what I need is a "fuel return ratio." Even a guess would be useful. Thanks,
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post #5 of 9 Old 07-11-2004
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Fuel Return Ratio

The FUEL RETURN vs FUEL CONSUMPTION RATIO is ABOUT 2 to 1.
That is, 2 gallons are returned for each gallon burned (3 gallons total supply flow)
NOTE: I have read where others state the ratio at about 1:1 - so caveat emptor applies!

FUEL FLOW RATIO:
A diesel engine’s TOTAL FUEL FLOW RATE is often cited as APPROXIMATELY: horsepower x 18% = gph
thus a 30-hp diesel would have a theoretical total flow rate of 5.4 gph. (30 x 0.18 = 5.4)

Diesel engines consume about 1 gallon per hour for every 18 hp used. You can estimate the number of gallons consumed per hour by multiplying horsepower used by 0.055.
thus a 30-hp diesel would have a conservative theoretical consumption rate of 1.65 gph (30 - 18 or 30 x 0.055 = 1.65)
NOTE: This is an obvious approximation, as diesel''s have widely varying operating curves.

The Return rate would be Total Flow less Consumption = Return
thus (from above)
5.4 Flow - 1.65 Consumption = 3.27 gph Return
then
3.27 return divided by 1.65 consumption = 1.98:1 Return / Flow Ratio
hence the Rule of Thumb:
Return is Twice Consumption (henceTotal Flow is 3 Times Consumption)
or
5.4 Flow divided by 1.65 Consumption = 3.27:1 Flow/Consumption Ratio

FWIW
Gord

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post #6 of 9 Old 07-11-2004
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Fuel Return Ratio

If your return is rubber tubing, it would be easy to disconnect it and measure the amount returned in a given time. If copper, there would be a flex connection near the engine that could be undone.
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post #7 of 9 Old 07-11-2004
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Fuel Return Ratio

You know, this post is in two places, thus the disfunctional understanding of what the deal is. Check out General Discussions. It PMO when things are in two places, the thread can''t be followed.
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post #8 of 9 Old 07-12-2004 Thread Starter
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Fuel Return Ratio

Gord,

Thanks for your very detailed response. This precisely what I was hoping for.

BoatBum, Sorry for the two posts. I didn''t realize this would confuse anyone (though I''d just recommend reading "one thread" if two are baffling).
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post #9 of 9 Old 07-22-2004
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Fuel Return Ratio

From www.yanmarhelp.com:
The total engine fuel flow, on a 3GM30 ( 27hp ) is not much greater than the engine fuel consumption as the return flow is low.
On a 6CX-ETE ( 420hp ) the total flow is 846 litres per hour at full power. The return flow is 756 litres per hour give or take a bit.

So, you may wish to take my previous advice with a grain of salt, and do what you can to confirm the approximation.

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