What % of diesel returns to the tank?

[MoodySabre]

In a separate thread I mentioned that we brought a boat back to Bradwell using a temporary tank. The return was left going into the old tank. I suggested we would need some spare fuel and indeed we ran out just entering the marina. Fortunately there was just enough way on to get near enough for me to do a giant leap on to the first hammerhead.

So the question arose - what % of the fuel gets returned to the tank? On a Beta 35 in question we reckon it must be 50% therefore effectively doubling our consumption frm the new tank.

If I have a VP 2003 and a VP2001 has the same lift pump does that mean that the % for a small engine is even higher.

 

[Bilgediver]

In a separate thread I mentioned that we brought a boat back to Bradwell using a temporary tank. The return was left going into the old tank. I suggested we would need some spare fuel and indeed we ran out just entering the marina. Fortunately there was just enough way on to get near enough for me to do a giant leap on to the first hammerhead.

So the question arose - what % of the fuel gets returned to the tank? On a Beta 35 in question we reckon it must be 50% therefore effectively doubling our consumption frm the new tank.

If I have a VP 2003 and a VP2001 has the same lift pump does that mean that the % for a small engine is even higher.

It should be no where near 50 % unless there are some very crappy injectors with high leakage rates.

Check out the way the volvo s are installed as Volvo had a habit with some smaller engines such as these of returning the spill leakage to the low pressure line in the region of the injector pump. This will not show up as consumed from tank . If the spill is to the tank then should ideally be to tank in use.

 

[MoodySabre]

It should be no where near 50 % unless there are some very crappy injectors with high leakage rates.

Check out the way the volvo s are installed as Volvo had a habit with some smaller engines such as these of returning the spill leakage to the low pressure line in the region of the injector pump. This will not show up as consumed from tank . If the spill is to the tank then should ideally be to tank in use.

It was just a guess. The Beta is a brand new engine. We know the spill must be back to the tank. It was a quick 'get home' measure. Just a question out of interest.

 

[Latestarter1]

Very difficult to answer........

Some in-line pumps models (Bosch P7100) return at least 40/50% of fuel to the tank, and Cummins PT returns over 80%.

Rotary pumps return around 20/30%.

This is the very reason why people screw up when sizing fuel filters.

 

[nigelmercier]

It should be no where near 50 % ...

By this I assume you mean it should be a lot higher? When doing some fault finding I connected a one litre bottle of fuel direct to the lift pump, it lasted about a minute, so over 95% would be my estimate.

 

[Salty John]

Very difficult to answer........

Some in-line pumps models (Bosch P7100) return at least 40/50% of fuel to the tank, and Cummins PT returns over 80%.

Rotary pumps return around 20/30%.

This is the very reason why people screw up when sizing fuel filters.

And why you should always try to have the return fuel line going back to the tank from whence it originated.

 

[srp]

Both the Yanmar 1gm10's that I have installed didn't appear to return anything whenever I pulled the pipe off. I asked my (mooring) neighbour, a retired marine diesel engineer, about this and he said that some engines don't produce very much excess at all.

 

[A1Sailor]

I don't have a clue - but having done the same as MoodySabre (see my post in the other thread) the amount removed from the fuel can by my new (in 2003) Beta35 was "alarmingly high". The return was going to the original tank...
A secondary question, if I may, is:
does it depend on throttle setting?
Is the amount removed by the fuel lift pump fixed, with more returned to the tank at low throttle settings and less at higher settings?

 

[AntarcticPilot]

The quick answer is "Don't really know". But the longer answer might be more useful! On Capricious (VP2003), you can hear the returned fuel running into the tank - it is a continuous trickle; certainly sounds like quite a few litres per hour, for an overall consumption of 2 litres an hour (ish) at economical revs. So, I'd suspect that it is at least 50%; possibly even more than that. But I am quite sure that it will be different for different engines, fuel pumps, injector pumps and injector design. Capricious has recently had new injectors and the fuel pump checked out and so I am sure it is not a problem there.

This is one reason why I'm a bit suspicious of "fuel washing" systems - after all, after a few hours motoring, most of the fuel in the tank will have been through the engine's fuel filtration system.

 

[rob2]

I've tried thinking this through and of course there don't seem to be any linear relationships. The lift pump only pumps up to a pressure, so its delivery is not a steady rate of flow. The injector pump (Bosch piston type in my case), though delivers the same volume and pressure per engine revolution, so it delivers enough to suit maximum demand plus a little spare, doesn't it? The setting of the "throttle lever" sets how much air is available, but does the injector squirt the same amount of fuel regardless? I guess not as fuel consumption varies with load. The engine speed must also affect how much air is ingested as efficiency increases up to a certain speed. Then there's possibly the effect of the efficiency of the cam timing...

It's all too much for me!

Rob.

 

[westernman]

I probably varies from engine to engine - or rather by type of injcetion pump. In my case - a Yanmar 4JH2 I think it it over 90%. I.e. almost all the fuel circulates and only a small percentage ends up getting burnt in the engine.
If you have two tanks and and the right valvles, you can use this to effectively polish your fuel as you go.

 

[Latestarter1]

And why you should always try to have the return fuel line going back to the tank from whence it originated.

One could say it is better for the environment than simply piping it over the side.........

#1 Restriction on return line will alter pump performance chacteristics.

#2 Anything other than properly designed return can lead to poor starting when vessel has been left for period of time. Air can work its way back up the return line resulting in extended cranking to purge the pump of entrapped air.

#3 Return fuel is 'ot' cos it has been in 'ot' place around the injector performing a cooling function before coming out the leak off pipe. Returning it to the tank allows fuel to cool, hot fuel leads to fall off in engine performance.

#4 Cos Bosch, Delpi, Diesel Kiki and Nippon Denso say so.

Going back to OP's question return fuel volume is also charcteristic of throttle setting, i.e return fuel volume tends to be higher at a lower throttle setting.

 

[A1Sailor]

Somebody needs to take the fuel lines off their tank, do the measurements then let us know...
I'll sit here and wait for the answer!

 

[wklein]

Haven't a clue. Very interesting question though. Congrats.

 

[doug748]

I would go for over 50%

My fuel tank, with ten gallons, feels hot after an hours running. Note, not warm but hot to the touch. This has been in the forefront of my mind as I need to uprate my shabby return system; it is only a matter of good fortune that it has worked so far.

 

[Otter]

Our return goes to the first fuel filter / water seperator which is guess means that the same fuel could spend a long time going round in circles and not being very cold. The advantage of this arrangement however is that with twin fuel tanks, there is just a simple choice of opening one or other fuel tap without worrying about complicated duel return systems.

 

[nigelmercier]

Somebody needs to take the fuel lines off their tank, do the measurements then let us know...

I did just this, see my reply #5, YMMV.

I suspect that as the revs increase, the lift pump output will increase in a linear fashion, but more fuel will be used. So the percentage return will be lower.

 

[oceanpilgrim]

some engines rely on the return fuel to cool the injectors,stops fuel boiling in the lines at high temps

 

[catmandoo]

The quick answer is "Don't really know". But the longer answer might be more useful! On Capricious (VP2003), you can hear the returned fuel running into the tank - it is a continuous trickle; certainly sounds like quite a few litres per hour, for an overall consumption of 2 litres an hour (ish) at economical revs. So, I'd suspect that it is at least 50%; possibly even more than that. But I am quite sure that it will be different for different engines, fuel pumps, injector pumps and injector design. Capricious has recently had new injectors and the fuel pump checked out and so I am sure it is not a problem there.

This is one reason why I'm a bit suspicious of "fuel washing" systems - after all, after a few hours motoring, most of the fuel in the tank will have been through the engine's fuel filtration system.

Yes I also am suspicious of such system . More concern should be about crud in the sump which in my case starts to make its presence felt when the tank gets warm due to recirculation and / or in rough weather I am at present designing a dip pipe / pump and filter system to remove such crud from the tank bottom - sort of like a swimming pool bottom cleaner

 

[Jcorstorphine]

more than 80%

On my Ford 1.6D with Bosch Injectors and a Rotary pump, I emptied a 5 litre tank in 5 mins with the return coupled to the main tank yet only consume about 2 litres per hour at 1500 RPM.

Too pixxed to do the arithmetic but it looks like 30 times the consumption goes back to the tank. hic