Fuel to injectors vs fuel to return

Isn't it true that more fuel returns to the tank than get sent trough the engine? Isn't also true that your fuel filter or filters are basically scrubbing fuel all the time?
My question. What percent of fuel gets burned vs returned?

What make and model engine do you have? More information is needed to give an accurate answer.

Yes it is true but as mentioned varies based on engine. Detroit diesels return a lot, ford Lehmans 120 maybe .25 gals/hour.

I would assume that Mainship has one or two Yanmars. My single Yanmar 315 HP is likely a bit smaller that the single six they use in some MS trawlers but larger a with probably a wholly different injector pump. However, for what it is worth, I equipped my boat with a Floscan which can tell me how many gallons per hour go into the engine and how many are returned. Up at fast cruise, I see 21 GPH going in and about 11 GPH being returned. The reason there is so much return is because the fuel is also used to cool part of the engine. If you have a return fuel cooler, located in the seawater cooling hose just after the seawater cooling pump, you probably have more fuel being returned than being used by the engine.

And that is why you must ensure the fuel out tank is also the fuel return tank under normal circumstances. Otherwise you risk a fuel overfill / spill.

Woodland Hills said:

What make and model engine do you have? More information is needed to give an accurate answer.

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MS 400 single Yanmar 6lya stp 370 HP. Didn't know different engines returned varied that much.

READY said:

Yes it is true but as mentioned varies based on engine. Detroit diesels return a lot, ford Lehmans 120 maybe .25 gals/hour.

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Curious what is a lot. 50%?

Pmcsurf1 said:

Curious what is a lot. 50%?

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In my case, Cummins 6BTA5.9-M 370 (each engine)

Fuel Consumption @ Rated Speed - 20 gal/hr
Approx Fuel Flow to Pump - 73 gal/hr
Approx Fuel Flow Return to Tank - 53 gal/hr

These engines return 73.6% of the fuel to the tank at WOT.

My 8V71 Detroits return 40GPH. It's called "built in polishing". Due to the different injection system.

Yes, it does vary greatly depending on the engine. My Lehmans return very little but the Detroits in a previous boat returned more than they burned.

My 12/71’s return so much hot fuel that the cabin sole over the tanks gets warm. This is great in the fall, but not so good in summer.

Cummins QSB 5.9 engine

What percentage of fuel is returned to the tank.

OldDan1943 said:

Cummins QSB 5.9 engine

What percentage of fuel is returned to the tank.

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OldDan, There are quite a few QSB 5.9 models. Seaboard Marine lists the Cummins Marine Diesel Performance Curves for most of their marine diesels. You can find your specific numbers there.

Yes it is built in polishing especially if you run on low tank levels. then the fuel in cycled faster than if tank is full

LarryM said:

OldDan, There are quite a few QSB 5.9 models. Seaboard Marine lists the Cummins Marine Diesel Performance Curves for most of their marine diesels. You can find your specific numbers there.

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Yes, it talks about consumption but does not indicate the % returned to the tank.

The detailed specs on the second page of those performance charts show total fuel pumped, return to tank at WOT and burn at WOT. Looks like the few QSB 5.9 models I looked at all pull 50 gph from the tank at rated RPM, so return flow is less on the higher power models (as they burn more at full power).

LarryM said:

OldDan, There are quite a few QSB 5.9 models. Seaboard Marine lists the Cummins Marine Diesel Performance Curves for most of their marine diesels. You can find your specific numbers there.

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Hmmm something does not add up. This must be without a load.


Avg. Fuel Consumption - ISO 8178 E3 Standard Test Cycle: l/hr [gal/hr] 49.7
Fuel Consumption at Rated Speed: l/hr [gal/hr] 76.2
Approximate Fuel Flow to Pump: l/hr [gal/hr] 189.3 Maximum Allowable Fuel Supply to Pump Temperature: °C [°F] 60.0
Approximate Fuel Flow Return to Tank: l/hr [gal/hr] 113.0 Approximate Fuel Return to Tank Temperature ......................................................................°C [°F] 65.6 Maximum Heat Rejection to Drain Fuel ..........................................................................kW [Btu/min] 1.5 Fuel Transfer Pump Pressure Range..................................................................................kPa [psi] 76 Fuel Pressure - Pump Out/Rail ...Mechanical Gauge ....................................................................kPa [psi] N.A.
[13.1] [20.1] [50.0] [140] [29.9] [150] [84] [11]
[20885]
N.A. = Not Available

OldDan1943 said:

Hmmm something does not add up. This must be without a load.


Avg. Fuel Consumption - ISO 8178 E3 Standard Test Cycle: l/hr [gal/hr] 49.7
Fuel Consumption at Rated Speed: l/hr [gal/hr] 76.2
Approximate Fuel Flow to Pump: l/hr [gal/hr] 189.3 Maximum Allowable Fuel Supply to Pump Temperature: °C [°F] 60.0
Approximate Fuel Flow Return to Tank: l/hr [gal/hr] 113.0 Approximate Fuel Return to Tank Temperature ......................................................................°C [°F] 65.6 Maximum Heat Rejection to Drain Fuel ..........................................................................kW [Btu/min] 1.5 Fuel Transfer Pump Pressure Range..................................................................................kPa [psi] 76 Fuel Pressure - Pump Out/Rail ...Mechanical Gauge ....................................................................kPa [psi] N.A.
[13.1] [20.1] [50.0] [140] [29.9] [150] [84] [11]
[20885]
N.A. = Not Available

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OldDan,

Your numbers above are l/hour not [gal/hour].

At Rated Speed or Full Power WOT 50 gal/hr is being supplied by the Lift Pump to the Injection Pump which is using 20.1 gal/hr to fuel the engine and is returning 29.9 to the tank. In this case ~60% (29.9/50.0=0.598) of the fuel supplied is being returned. That looks in line to me for a QSB5.9-380 HO.

Am I missing something?

LarryM said:

OldDan,

Your numbers above are l/hour not [gal/hour].

At Rated Speed or Full Power WOT 50 gal/hr is being supplied by the Lift Pump to the Injection Pump which is using 20.1 gal/hr to fuel the engine and is returning 29.9 to the tank. In this case ~60% (29.9/50.0=0.598) of the fuel supplied is being returned. That looks in line to me for a QSB5.9-380 HO.

Am I missing something?

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I knew it was L/hr. For some reason, when I copied it, the gah were cropped.

That "return to tank" percentage is a variable depending upon the RPM of the engine?

OldDan1943 said:

and that is a variable depending upon the RPM of the engine?

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In the case of a mechanical lift pump yes, but regardless of RPM, it will pump more than is being used to fuel the engine. Those specs and pump curve may actually be included in the factory service manual if you happen to have one.

LarryM said:

In the case of a mechanical lift pump yes, but regardless of RPM, it will pump more than is being used to fuel the engine. Those specs and pump curve may actually be included in the factory service manual if you happen to have one.

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Yes, in the manual but it is so much fun and many times faster to ask in here.
Plus it might generate additional questions.

One should be concerned about fuel consumption at different speed to be able to properly plan your trips. as far as the return is concerned, one needs to only make sure that the return goes the tank(s) that is being used.
Very simple!

Chrisjs said:

And that is why you must ensure the fuel out tank is also the fuel return tank under normal circumstances. Otherwise you risk a fuel overfill / spill.

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Made that mistake once, having four tanks to choose from. Fortunately, smelled fuel overflow before more than several ounces escaped through the vent and onto the deck.

Things can get a little more complicated than simply taking and returning fuel to the same tank.

With a single engine boat very sensitive to unequal fuel loading, I have to pay a lot more attention than I did with twins and four tanks to keep the boat on an even keel. The goofy engineering I inherited here was main engine return to only the port tank but draw from either or both tanks controlled by the supply valves at the tanks into a common line. That meant that despite an inherent tendency to list to port, there was no way to push any fuel to stbd. All I could do was shut off the stbd tank until enough had been used from the port tank to even things up. BUT using the fuel gauge to help with that was complicated by the fact that the fuel gauge read only the port tank despite there being a level sensor in the stbd tank, which was disconnected.

When the tanks are FULL (as they are right now for peak hurricane season), I am very careful to return to the supplying tank, but modifications now allow me to return fuel to either or both tanks and use from either or both while watching the fuel gauge readout from the sensor in either tank to ensure what I want to happen does happen because I normally operate locally with between a half and a quarter tank either side.

Not a boat but back when I was young and dumb I used to fly an old twin engine airplane with the screwiest fuel system I’ve ever dealt with. All return fuel went to the main (wingtip) tank on that side so burning from aux1 had to wait until some fuel had been used from the main. Aux2 fuel had to be pumped to the main for use. Things got complicated because the two engines returned different percentages of fuel, the aux tanks were different sizes, the aux2 transfer pumps ran at different rates, aux2 tanks had no fuel gauges and most of the rest of the gauges didn’t work. First time I flew it I landed with one tip tank full (probably over flowing) and the other almost empty. Took full control to keep the wings level. Finally figured out the tank sizes and flow rates to manage it but that was almost a full time job. I keep expecting to read about that plane in NTSB Reports.

PPandE said:

Not a boat but back when I was young and dumb I used to fly an old twin engine airplane with the screwiest fuel system I’ve ever dealt with. All return fuel went to the main (wingtip) tank on that side so burning from aux1 had to wait until some fuel had been used from the main. Aux2 fuel had to be pumped to the main for use. Things got complicated because the two engines returned different percentages of fuel, the aux tanks were different sizes, the aux2 transfer pumps ran at different rates, aux2 tanks had no fuel gauges and most of the rest of the gauges didn’t work. First time I flew it I landed with one tip tank full (probably over flowing) and the other almost empty. Took full control to keep the wings level. Finally figured out the tank sizes and flow rates to manage it but that was almost a full time job. I keep expecting to read about that plane in NTSB Reports.

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A fellow who worked for me was in a plane where differential full tank levels was the cause of a fatal. In getting ready to land at a remote jungle strip horses ran onto the runway. The pilot pulled a hard climbing turn with the low up hill tank starving for fuel. According to NTSB an all to common experience in all manners of general aviation. John Denver and one famous guy I knew (unnamed intentionally) suffered similar fates.

sunchaser said:

A fellow who worked for me was in a plane where differential full tank levels was the cause of a fatal. In getting ready to land at a remote jungle strip horses ran onto the runway. The pilot pulled a hard climbing turn with the low up hill tank starving for fuel. According to NTSB an all to common experience in all manners of general aviation. John Denver and one famous guy I knew (unnamed intentionally) suffered similar fates.

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Yep, fuel mismanagement, either fuel starvation or fuel exhaustion, is the leading cause of general aviation accidents. (loss of control is the leading cause of fatalities) I got lucky and learned a lesson. The same lesson I try to teach my students in the classroom. I'm sorry that your employee learned the hard way.

fuel return

Pmcsurf1 said:

Isn't it true that more fuel returns to the tank than get sent trough the engine? Isn't also true that your fuel filter or filters are basically scrubbing fuel all the time?
My question. What percent of fuel gets burned vs returned?

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I don't know the quantity or percent but yes, fuel being returned to the tanks from the engine or generator has been "scrubbed". The more you run your systems, the less likely you will have "dirty" fuel or tanks. I have a Ford Lehman 120. If you have that engine, call American Diesel Corp at 804-435-3107. They can tell you everything you ever wanted to know about a FL 120.

You can always look for somebody here who has a device installed like my Floscan which can be set to read the flow in and return flow at all RPMs. There are several manufacturers if I am not mistaken.

I have a Ford Lehman SP 135. i thought that the return was about 10:1, total fuel:burned fuel. This seems about right to me or at least in the ballpark, as I can open the sight tubes valve and watch the amount of fuel being returned.

This is done is large part to cool the Injector pump.

Also, when I first got the boat, it had no way to transfer fuel, so I used the return to a different tank to do so, within a couple of hours, it had drained the feed tank and the engine stopped, so 10 to 20 gallons /hr for consumption of 1.5/gal seems reasonable to me.