Fuel system options?

[justin.schoeman]

It looks like I will need to redo my fuel system. I am using a gravity fed Aero Injector (metered drip style carb) on a VW engine, fed from a fuel tank mounted behind the instrument panel.

Unfortunately, even though the fuel system is almost identical to the Sonex that this set up is designed for, I am getting too much mixture variation with fuel level. Perhaps the fuel tank is a bit taller (more variation in head than in the Sonex)? Whatever the cause, I will need to switch to a regulated fuel feed in the future.

This means that I will need to add a fuel pump. The AeroVee does not have provision for a mechanical fuel pump, so I will need to use an electric pump. This is where things get complex - how can I add some redundancy? If everything is dependant on an electric pump, what happens if something goes wrong with the electrical system?

Probably my best idea so far is to have a small battery pack of 8 penlight alkaline cells to power an aux pump. Test once before each flight, and replace at annual. A pack like this should provide ~1 hour of flight time.

I can also easily add a cam to the end of my secondary ignition shaft (extension of the center of the crank shaft). If I can find a simple mechanical pump that can run dry (without oil), then that would also be an option.

Does anybody else have any ideas?

Thanks,
Justin

[t-bird]

Hi Justin

Keep the current setup with a T piece. Put a non-return valve in current line before the T piece. Have a second line with fuel pump and regulator into T-piece.
The non-return valve will stop the fuel pumping back into the tank through old line. The fuel should flow as usual if you have electrical failure.
The only issue would be that non return valves reduce pressure and you would need one which does not.
I have seen a non-mechanical return valve in a fuel line that looks like a piece of floppy rubber inside that should not reduce the pressure. I can’t remember the name of the product.

If that doesn’t work put in two electrical fuel pumps similar to the rotax 914 setup.

Cheers

Callie

[justin.schoeman]

Hi Justin

Keep the current setup with a T piece. Put a non-return valve in current line before the T piece. Have a second line with fuel pump and regulator into T-piece.
The non-return valve will stop the fuel pumping back into the tank through old line. The fuel should flow as usual if you have electrical failure.
The only issue would be that non return valves reduce pressure and you would need one which does not.
I have seen a non-mechanical return valve in a fuel line that looks like a piece of floppy rubber inside that should not reduce the pressure. I can’t remember the name of the product.

If that doesn’t work put in two electrical fuel pumps similar to the rotax 914 setup.

Cheers

Callie

Thanks Callie. That was my original plan, but the pressure difference between the lowest available pressure regulator and gravity feed is just too high - if the engine is tuned for regulated pressure, it won't run on gravity feed.

Also, the non-return valve is an issue. I have searched long and hard for a commercially available check valve with very low opening pressure and high flow rate. They don't seem to exist. I do have a partially built one at home that I started making (basically a flapper valve that is held open by gravity, but is closed by fuel attempting to flow the wrong way). Gave up on it, when I couldn't stop it from sticking...

Do you know how the 914 pumps are wired. Do they use a backup battery, or just run both from the main bus?

[t-bird]

Hi Justin

The 914 have 2 electric pumps running from the main battery. A light on my dash will come on when the alternator stop charging the batteries. This has happened 45 minutes from nearest landing spot. I completed the 45 minutes by switching to one pump and pulling circuit breakers for all the other stuff. A loose wire on the rectifier was the problem.

There is one pipe from the fuel tank that splits in two for the two pumps. This is another weakness to the system. This pipe came loose on my gyro and both pumps sucked air which lead to a forced landing.

After the two pipes is two filters and the pipes combined again into one.

This system is definitely not redundant and adds failure points.

[Low Level]

Probably my best idea so far is to have a small battery pack of 8 penlight alkaline cells to power an aux pump. Test once before each flight, and replace at annual. A pack like this should provide ~1 hour of flight time.

I agree with Callie on the fuel system, and rather use a small motorcycle battery, wired directly via a three way switch, to the pumps as a backup if you are really concerned about redundancy. The dual electrical pump system, without electrical backup, works well on many gyros - 914's.

It is VERY seldom that a battery will die on you in an instant. The problem is, the only time it is going to happen, is when your regulator fails, in such a way that it cooks the battery, then your backup battery will also go up in smoke. This now leads to a new complicated bypass charging system. Question now, when is redundancy enough ? Two fuel pumps, one main battery.

It is also my opinion that It is much easier to build an electrical backup system, than to do the mods on the engine for a mechanical pump.

Strongs Justin. I trust you will sort this thing and start enjoying your hard work.

[justin.schoeman]

I am leaning more and more towards a simple alkaline battery pack. R60 per year for a fresh pack of Evereadys, and no charging hassles, and not much extra weight (certainly less than a gel cell). I will do some testing when I get a chance...