Fuel gravity feed

gunnarsilins

gunnarsilins

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www.eilean.se
I´ve seen some sketches in the PBO some time ago about having a day tank above the engine fed by the fuel lift pump and the day tank feeding the injection pump by gravity.

Does anybody have knowledge of the minimum head above the injection pump needed for proper function? In my case the engine is a Perkins 4.236 with a CAV rotary-type injection pump.

Is there a non-return valve in the mechanical fuel lift pump preventing the daytank to empty itself by gravity back to the main tank when the engine is off?

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dont know what the head should be for a gravity feed direct to injection pump, but suspect it needs to be quite hi, 27 " of water = 1psi so 2ft doesnt give much pressure, in fact thinking on all the gravity feeds i have seen have been to the lift pump which then pressurises the injector pump. the second part about non return valves is that there are two in the lift pump, the in and the out, if either leaks the pump isnt worth a s**t.
i suspect that the day tank system worked by manually pumping diesel into the headr tank
s

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i would suggest it is best not to have to much of a head of fuel on a boat engine.
use your lift pump to pull fuel from the day tank.in my industry we would have a solenoid in line so you only open the fuel when you turn on the ignition. maybe you could easily isolate manually, as i have known engines to fill with diesel.

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most day tank systems use a manual pump to fill the day tank from the main fuel tank, although I have seen an electric pump used on one installation. The day tank then feeds to the engine lift pump. Note that a fuel cock should be attached to the day tank, and if a sight gauge is used then it must be operated via a spring loaded valve.

HTH

Paul M

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G'day Gunnar,

A friend has a day tank feeding a diesel outboard on his 30 foot cat, he has a change over valve on the fuel return line to reduce filling, it's filled with a manual in line pump.

The small (4 Litre) day tank is only 3 feet above the engine injector pump and has no problems.

The system was installed because the fuel tank had no baffles and the pump was sucking air when the tank was less than half full in rough seas.

Hope this helps

Avagoodweekend Old Salt Oz /forums/images/icons/cool.gif

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The lift pump dilivers between 1.5 to 2.5 psi so it does not much hieght. Unless like the fella who has no baffles in his tank why add more stuff to go wrong.
Cheers
Mick

<hr width=100% size=1>smile you'l be on your boat soon ;-)
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Many diesels in the smaller sizes, both industrial and marine, are supplied complete with an integral fuel tank, strapped to the top of the flywheel housing or the rocker box - Listers and Petters are good examples. I guess from this, that a head of a few inches is sufficient for these engines, always taking into account angles of heel and pitch which the boat might adopt.

Cheers! Neil

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One thing to bear in mind is that under certain lift pump failure conditions all of this gravity fed fuel will go straight through the pump and into the sump. A.the engine won't like starting like this and may very well cause damage with the dilution of the oil and B. It's a fire risk. And before people say it's unlikely I've seen quite a few that have done this, common on petrol ones as well.

Jim
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if the diaphragm breaks the fuel on the

pumping side can then get into the crankcase thru the push rod or crank mechanism, however there are usually seals (quite crude) to stop this and if as in the majority of cases the tank is above the lift pump, even if you have a stack pipe, it can still syphon thru to the crank case.
so you pays your money and hope that the diaphragm doesnt break. i will also say that in 40 years of maintenance i could put my hand on my heart and say i have seen this condition only twice.
stu

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Re: if the diaphragm breaks the fuel on the

PBO had a letter not so very long ago from a german Doctor(of what I don't know) that advocated that the header tank be filled by the lift pump and that the HP pump be gravity fed . the header also been supplied with a manual pump so that in the event of lift pump failure then the engine could still function. His reasoning here was that air in the low pressure side and lift pump failure were two of the three major causes of engine/fuel related failure. The third cause was blocked filters and in part this was addressed by the fact that the excess leak off that normally returns to tank was in his method fed back to the header and as it had already been filtered was relativly clean, and the filter had a by-pass secondary filter so as to be servicable in use. He also had some issues with fuel temp but I forget now what they were.

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Re: yeah, but they have still got a lift pump

No they don't. I (sad boy!) was looking at a dumper truck fitted with a Petter air-cooled diesel this afternoon. It had a one gallon tank mounted about 6 inches over the injector pump. There was no intermediate lift pump. The Lister 15HP engine fitted in a previous boat of mine had a gravity feed from a tank mounted under the side deck when I bought her. I subsequently fitted a lift pump and moved the tank to a lower position, but the engine functioned satisfactorily as it was.

Cheers! Neil

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This letter was exactly....

...what I read in PBO and made me think about an arrangement like that.

Pros would be:
Two primary filters could be used in parallell so just by switching over from one to the other they could be cleaned and elements changed without stopping the engine and bleeding it. They would of course be in the line between the main tank and header tank.

To bleed after changing the secondary filter would be easy as well, because the low pressure side will be under constant pressure from the header tank.

No subtle air leaks could occur, they would show up as fuel leaks instead.

The over-capacity of the lift pump would make all fuel to constantly circulate, from main tank - primary filters - header tank - back to main tank via the overflow - and over again...
This would mean better control over dirty fuel, and the fuel would, at least in theory, be filtered several times before reaching the HP pump.

But the horror stories of a ruptured membrane in the lift pump making the crankcase fill with fuel really scares me!
But as it is today, if my fuel tanks are more than half full the level is above the lift pump anyway, so it can still happen!






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Re: This letter was exactly....

If your lift pump fed into the top of the header then it could not drain down through the diaphram......the system described required the lift pump to supply the header and the header gravity fed the hp pump....so if lift pump was drawing fuel up from main tank and then sending it on to the header through a top fitting should the diaphram rupture then the lift pump would be unable to draw fuel up from the main tank nor would fuel in the header be able to drain down through the diaphram......i think?

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I have recently researched the same situation with my engine. I have a CAV DPA rotary pump fitted to a Peugeot XDP88 engine. I found that the pump includes a 'lift' pump within itself. Indeed when the engine was used as a car engine the only primary lift pump was a hand pump to prime/bleed the engine. I will therefore install a day tank only slightly higher than the injection pump, allowing gravity fed bleeding. Initial experiments with the engine suggest that I will have no problems with hight, however I am slightly concerned about high diesel temperature in the day tank after prolonged running periods. I intend to fit a electric lift pump to fill the day tank, togther with a filter system and an overflow to return the diesel to the main tank. This will allow me to 'polish' the fuel as it fills the day tank.

Suggest you talk to a diesel expert who knows your pump in detail.

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