Fuel rig news

Having stripped and reassembled the electric pump that I’ve used in my fuel rig, I now understand what the problems are. I’ll mention what I shall be doing to try and resolve them but I thought that having put it back together again, I’d give the system another go this morning.

Having connected the system up to my car battery and carefully primed it, I activated the pump to see what would happen. For reasons that I do not understand, it appeared to be returning fuel to the jerrican so I took the step of reversing the connections on my car battery and trying again. Success! This time fuel began to flow out of the delivery tube into the small container that I was using to collect it, and at not that bad a rate either.

Well, partial success anyway, because although this time there seemed to be little if any fuel leaking from the pump’s bottom plate, after a short while a leak did start from where the casing enclosing the pump’s electric motor joins with the pump’s rotor body. And this wasn’t really surprising, because when I disassembled it, I found that both the design and manufacture of the pump are very poor.

Here’s a sketch that I’ve done to show how the pump is put together. It’s just hand-drawn so apologies for the quality.

The pump consists of three sections – a top cap, a central section and a rotor chamber at the bottom.

The top cap contains the electrical connections and the brushes for the electric motor, a solid bearing in which the top of the motor shaft runs and holes to accept two long screws that pass down through the central section into threaded holes in the rotor chamber, which when tightened hold the whole thing together.

The central section comprises just a tube in which the motor magnets are mounted and in which are the motor windings. Otherwise the central section is just an empty void.

The rotor chamber on the bottom is a bit more complex than I’ve shown it to be but that isn’t important. What matters is that it contains a rotor to pump the fuel from its inlet connection out through its outlet, which is driven by the lower end of the motor shaft.

When the pump is disassembled, the central section separates from the rotor chamber taking the motor shaft with it as the windings are strongly attracted to the magnets which are fixed in the central section. You then find that the motor shaft passes through another simple bearing in the top of the rotor chamber and connects to the rotor by way of a slot.

Now for the problem areas, which are fairly obvious. When running, the rotor is said to generate a pressure in the chamber of 14 psi and clearly as the bearing in the top of the chamber through which the shaft passes has no kind of gland packing on either side, fuel in the chamber is forced up through it and into the void of the central section. Then no matter how tightly the retaining screws are screwed down, because there isn’t any kind of sealing gasket at either the top or the bottom of the central section, as soon as the level of the fuel inside it reaches the lower joint it begins to leak out.

And more seriously, the fuel is then in direct contact with the windings of the motor and can potentially rise higher and higher in the central void until it comes into contact with the motor’s commutator and brushes. In fact, when I first tried the pump out, I primed the system very energetically using the primer bulb that I’d incorporated and fuel not only began to leak out of the lower joint between the motor body central section and the rotor chamber but also out of the top joint where it joins the top cap. And afterwards when I stripped the pump I did indeed find that the commutator and brushes were wet.

Quite honestly, the design and manufacture of the pump are shoddy and if I’d known then what I know now I’d never have bought it in the first place. The lower shaft bearing in the top of the rotor chamber and its potential for allowing fuel to pass through it into the main body of the pump’s electric motor is the principal issue.

Now, I’m not a design engineer and therefore have no idea how you design your way around that problem and evidently the designer of the pump didn’t either but for what they’re worth, here are my thoughts.

The lower shaft bearing really needs some kind of gland packing that allows the motor shaft to rotate within it but prevents fuel passing up through it and given how the design is, I can’t see how that can be done properly as a design afterthought. I know for sure, because I tried it today, that if you just pack the bearing with standard grease, the fuel just washes it away with impunity and continues its journey north.

The idea I’ve had is to see what can be done if fuel-resistant grease is used instead – and yes, before the uninitiated and great unwashed say that there’s no such thing, there is. It’s called EZ Turn, it’s for lubricating and packing seals in aircraft fuel systems principally, and is available in cans and, luckily for me, in 5 oz tubes. Take a look at the following link.

EZ Turn

I’ve only found it on the internet being sold by US suppliers like Skygeek, who I think are the most competitive for both the product itself and delivery, and from the German Aircraft Spruce franchise at much higher cost, presumably because they just ship it in from Aircraft Spruce in the US and then just send it out.

I’m going to experiment with packing the lower bearing with it and seeing if I can work out some kind of containment mechanism. The reason for that is that as well as there being pressure generated inside the pump itself, I want to pump the fuel upwards to the level of the Savannah’s wing tanks. This will give plenty of potential because of the ‘U’ tube effect for fuel to fill up the whole of the void of the pump’s central section so it comes into contact with the motor’s commutator and brushes while the motor is running, which would be undesirable to say the least.

I don’t hold out too much hope of succeeding but as I’ve already received a partial refund for the amount that I paid out for the pump and am asking for more, it’s a shot to nothing almost and it’ll be interesting trying. In the meantime I still won’t have a working fuel rig though, which is a bit of a bummer 🙁