What % of diesel returns to the tank?

[dutyhog]

On a wind-less day, our return pipe failed, unknown to us, somewhere near Duart on the way south, and the fuel tank became empty as we tried to pass through the Dorus Mor. From the amount of diesel sent to the bilge I calculated that less than 50% of the fuel pumped from the tank was used by the Beta engine at 2000 rpm. And it looked like that sort of proportion when I rigged up the spare can to gravity feed the engine and sent the return to the main tank.

 

[Mccuedan]

Dutyhog: I experienced the exact same problem - here in California when sailing to Catalina Island with sufficient new fuel in main tank. Fuel was pumped into the secondary tank - resulting in an ‘out of fuel’ main tank emergency - a problem that has since caused loads of headaches. My question to all is how much fuel should return? Attaching a clear hose to the return line shows very little diesel returning. All lines appear clear. New filters. New mechanical lift pump. Engine runs fine if manually primed and bled. But, regardless of throttle speed - no return fuel is seen through the transparent hose attached to the return outlet on filter and leading to a bucket. The engine is ISUZU C-240…reliable as a hammer till now! All ideas will be appreciated.

 

[Bru]

Our Merc OM636 can, due to our dual tank arrangement, transfer all the fuel from one 88 litre tank to the other in well under an hour (if I forget to switch the return over) which means well over 95% of the fuel pumped is being recirculated back to the tank!

Apparently this is normal as the OM636 has a constant pressure fuel manifold or so I've been told

 

[Mark26]

All of the above is very good for fuel polishing, and makes me think we need nothing more (fuel polishing equipment) than spare filters. Ideally a switchable double filter set up, so we can just switch over to the other whilst we replace one if needed.

 

[savageseadog]

I had to rig a temporary Tank once on a Yanmar 3GM30. The return wasn't rigged initially until I realised the 25l can of fuel wasn't going to get us back. The amount of fuel going back was very high, at least 50%. The return was put into the can and all was well.

 

[rotrax]

Ditto.

After a serious fuel blockage I rigged a full 25 litre can of diesel, only to find it emptying very rapidly. Disconnecting, joining and lengthening the return so it went into the can fixed it immediatly.

My own experience on most modern diesels is that at least 75% is constantly recirculated via the return.

I have installed a Racor copy on my return with a 10 micron filter as a constant fuel polisher.

 

[PaulRainbow]

Ditto.

After a serious fuel blockage I rigged a full 25 litre can of diesel, only to find it emptying very rapidly. Disconnecting, joining and lengthening the return so it went into the can fixed it immediatly.

My own experience on most modern diesels is that at least 75% is constantly recirculated via the return.

I have installed a Racor copy on my return with a 10 micron filter as a constant fuel polisher.

The fuel being returned has already been through your primary and secondary filters, so if the Racor in the return is capable of doing any polishing, the filter setup isn't doing its job. I'd suggest the Racor would do better as a primary filter, with a 30 micron filter.

 

[scottie]

Lazarus ?

 

[geem]

According to my Perkins manual, my fuel consumption at full power is 17.62l/hr. Pretty epic. However, we only use 3.3l/hr at 1200rpm and 6 kts boat speed. Our fuel lift pump is electric. Assuming the fuel lift pump can pump as much as the engine needs at full revs then we are circulating about 500% of the fuel actually consumed by the engine. Unlike a mechanical lift pump, our circulating fuel volume flow rate is fixed regardless of revs. Who needs fuel polishing?

 

[Rappey]

I was under the impression a common rail returns far more fuel than a mechanical injector?
I have a rotary cav pump and a very old diesel. My return is drips.

 

[geem]

I was under the impression a common rail returns far more fuel than a mechanical injector?
I have a rotary cav pump and a very old diesel. My return is drips.

Ours is mechanical injection

 

[vas]

Lazarus ?

nah, 9 and a half years only, we've seen worse

 

[Graham376]

Ours is mechanical injection

So is ours. Electric fuel pump runs at constant speed as soon as ignition turned on, regardless of engine running or not.

 

[geem]

All of the above is very good for fuel polishing, and make me think we need nothing more (fuel polishing equipment) than spare filters. Ideally a switchable double filter set up, so we can just switch over to the other whilst we replace one if needed.

Our filters are very accessible and 1/4 turn type. You can swap them in a few seconds. The fuel system is self priming so easy peasy. No need for dual filters. The only drawback is the filters are expensive but I only change them every three years as our engine hours annually are quite low. I keep the tank clean by sucking up any sludge from the sump annually so the filters get an easy life

 

[rotrax]

The fuel being returned has already been through your primary and secondary filters, so if the Racor in the return is capable of doing any polishing, the filter setup isn't doing its job. I'd suggest the Racor would do better as a primary filter, with a 30 micron filter.

Got that already, 30 micron primary in OE Racor 500 series, Yanmar secondary but fitted with Baldwin eqivilent canister - the Yanmar canisters are 57 quid from a Yanmar dealer - and the Racor copy with a 10 micron element on the return.

It must be doing something as the lower third of the element was a darker colour than the other two thirds when I changed it after two seasons - about 250 engine hours - use.

I was wary of putting a 2 micron element in in case it added too much resistance to the return.

 

[Ian_Edwards]

Mechanically governed Diesel engines are designed keep a constant RPM, which is set by the throttle.
If the engine is only lightly loaded (eg motor sailing), then only a small amount of diesel is needed to maintain the rpm, conversely if the engine is heavily loaded (eg punching to a head wind and waves) at lot of diesel is required, to the point where the engine can't efficiently combust all the diesel and you get black smoke from the exhaust.
The amount of diesel which gets into the cylinders depends on the pulse of diesel produced by the injector pump, pressure, flow rate, and duration, and the point at which the injector opens up and then closes, which, in turn, depends on how the spring is set-up and the nozzle . The bit that goes back down the return line, is the difference between the amount of diesel pulsed by the injector pump and the amount allowed into the cylinder by the injector.
Initially this down to the design of the system, the designer may want to allow a significant quantity of diesel to return to cool the fuel system, and I've seen day tanks get quite hot with returned diesel, but they may not.
It is also a function of wear and maintenance, of both the injector pump, and more likely the injectors, which may need to be cleaned and reset.
So the question is really impossible to answer in a general sense, it depends on the design, how well the fuel system is maintained , and perhaps to a small extent the throttle setting.
I think I've got that right, it's been awhile (about 30years!) since I had a hand in designing and specifying diesel engines, but no doubt someone will correct me if I'm wrong.

 

[jwfrary]

From experience its massively different engine to engine.

Common rail engines pump far more fuel (mainly for cooling) than is used. I filled a 1 gallon bucket in a minute at idle on our qsc cummins when we tested it before install. It's only using the quanity of the bucket an hour at idle!

The v1505 kubota on the Onan Generator produces a good steady stream into the tank, which you can hear in the tank room! Quite comforting in a way.

Conversely my yanmar 3gm 30 produces practically nothing.

 

[David1912]

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

I had a similar problem. On m’y Beneteau 42.3 with Yanmar 4j engine there is a metal gauze on the tank feed. When I had diesel big this gauze blocked preventing any fuel reaching the filter

 

[Beneteau381]

Mechanically governed Diesel engines are designed keep a constant RPM, which is set by the throttle.
If the engine is only lightly loaded (eg motor sailing), then only a small amount of diesel is needed to maintain the rpm, conversely if the engine is heavily loaded (eg punching to a head wind and waves) at lot of diesel is required, to the point where the engine can't efficiently combust all the diesel and you get black smoke from the exhaust.
The amount of diesel which gets into the cylinders depends on the pulse of diesel produced by the injector pump, pressure, flow rate, and duration, and the point at which the injector opens up and then closes, which, in turn, depends on how the spring is set-up and the nozzle . The bit that goes back down the return line, is the difference between the amount of diesel pulsed by the injector pump and the amount allowed into the cylinder by the injector.
Initially this down to the design of the system, the designer may want to allow a significant quantity of diesel to return to cool the fuel system, and I've seen day tanks get quite hot with returned diesel, but they may not.
It is also a function of wear and maintenance, of both the injector pump, and more likely the injectors, which may need to be cleaned and reset.
So the question is really impossible to answer in a general sense, it depends on the design, how well the fuel system is maintained , and perhaps to a small extent the throttle setting.
I think I've got that right, it's been awhile (about 30years!) since I had a hand in designing and specifying diesel engines, but no doubt someone will correct me if I'm wrong.

Don’t forget the governor decides the amount of fuel to be injected and the throttle decides how many revs by what you set them to. So set it at 1500 rpm for example and the governor decides how much to inject depending on the load. So flat calm sea and not much injected, start climbing swells and the governor gives more fuel to keep the revs constant.

 

[RunAgroundHard]

Old thread but I have no fuel return to tank.

The return line goes into the top of the last filter on the engine. Engine never misses a beat and when I have had to bleed the engine post filter change, it bleeds air very fast with minimum cranking. This system was an upgrade when all the fuel lines from the tank were replaced and new filter unit installed. The fitter decommissioned the old fuel return lines to the tank. Upgrade was 12 years ago.