Rotax 912 pump bypass

by in 912 / 914 Technical Questions 13 years ago

The Rotax installation manual shows a bypass circuit for the series electrical pump. I have found the Facet electrical pump flows through if switched off so havent ever used a bypass line for it. However, my concern is for the Rotax engine pump, if it fails I think there would be very little flow through it. I have just tested this: Engine not running but electric pump running, and found only enough flow pushed through the engine pump to give about 30% power. The test was set to give the minimum Bing carb pressure of 2.2 psi.

So if the engine pump fails, I think there is a forced landing, despite having an electric pump that could supply on its own all the fuel the engine could ever use. Surely there should be a bypass circuit with checkvalve around the engine pump to avoid this problem. Has anyone done this? Did you use the Rotax check valve 874532 or some other?

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Re: Rotax 912 pump bypass

by Roger Lee » 13 years ago

Roger Lee

125 5.12k 4.52k

Hi Doug,

The Rotax mechanical pump should always fail open and if the electric pump puts out between 2.2 - 5.8 psi then you should have no issues. There is almost no way for the mechanical pump to seal off the fuel flow. Why did you purposefully reduce the pressure from your own electric pump to the system because that's not how it would work if your mechanical pump failed? I have cut them open to look. If I can find my pictures I'll post them.

Rotaxfuelpump1.JPG (You do not have access to download this file.)

Rotaxfuelpumpscreen.JPG (You do not have access to download this file.)

Rotaxfuelpumpdiaphram.JPG (You do not have access to download this file.)

Rotaxfuelpumpinternalscreen.JPG (You do not have access to download this file.)

Rotaxfuelpump.JPG (You do not have access to download this file.)

Roger Lee
LSRM-A & Rotax Instructor & Rotax IRC
Tucson, AZ Ryan Airfield (KRYN)
520-349-7056 Cell

Thank you said by: Jez O'Hare

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Re: Rotax 912 pump bypass

by itemprop='name' » 13 years ago

0 0 0

Roger,

Thanks for quick reply and the photos (so glad to see these). From the photos it would appear (as you said) that it would fail open. The test I rigged tapped into the fuel distribution block just upstream of the carbs and led past a fuel gauge to a needle valve. The electric pump was on and the engine off, so fuel pumped from the elec pump through the engine pump to the fuel block then through the needle valve to a measuring container. The needle valve was set to give 2.2 psi at the fuel block. The carbs took no fuel of course but 3.6 litre per hour (measured separately) went from the fuel block up the vapour fuel return line to the tank. The flow I measured in the container would have been available to the carbs at their min pressure of 2.2 psi, and was only 8.5 litre per hour (not enough to maintain level flight).

To further check on the engine pump resistance to flow, I repeated the test but this time bypassed the engine pump. Now I got near 27 litre per hour which is enough for 100% engine power.

Before and after the test, the needle valve was closed and the elec pump gave its rated 4.5 max psi pressure. The elec pump is a Facet 40105 whose max psi is just a little over the 4.4 max psi recommended in the Rotax 912 Installation Manual for auxiliary elec pumps.

So engine pump does allow flow through but not enough to get home if it is not working. A bypass around the engine pump, with a check valve seems like a solution for this. Do you know of anyone who has done this or of any reason not to do it? I would much appreciate your input on this .....

Regards Douglas N

Thank you said by: Jez O'Hare

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Re: Rotax 912 pump bypass

by itemprop='name' » 13 years ago

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Here is an update. I found the elec pump was a 40106 instead of the 40105 recommended for the Rotax 912 so replaced it with the 40105. Also found that the battery dying internally so temporarily replaced it with an auto battery.

Now found this elec pump puts through 80 liter per hour if the engine pump is bypassed and almost the same if the flow goes through the engine pump. In both cases, there was no restriction at the final line outlet so the elec pump was pumping against no restriction i.e. at an outlet pressure close to zero. This result shows that the engine pump does in fact allow flow through without much resistance ( a separate test showed that a fuel head of about 2 1/2 ft high (less than a psi) was enough to crack the valves and allow fuel to begin to "free flow" through).

However, if I repeated the above tests with the final fuel outlet restricted to a 2.2 psi pressure the fuel flow is only 6 litre per hour. This may not be a valid result because the pressure gauge used was clearly not accurate at these low pressure and the flow rates from the elec pump are very sensitive to even small changes in these low pressures. Will repeat these with an accurate low pressure test gauge.

In the meantime, here is a key question: Rotax says 2.2 psi is needed for "reliable operation" of the Bing carbs --- is this to force through the float needle valve the 27 litre per hour for 100 % power --- what happens if the pressure is say half a psi? With the elec pump able to put through its high flows only at very low pressures, this is critical to how useful it is if the engine pump dies.

Douglas N

Thank you said by: Jez O'Hare

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Re: Rotax 912 pump bypass

by itemprop='name' » 13 years ago

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This is final update. In summary, if the Facet 40105 elec pump is good and the a/c battery is good (see below) and your fuel lines are of adequate size, the elec pump should push sufficient flow through a Rotax 912 failed engine pump to give full power to the engine. There is no need for a bypass on the engine pump as I originally posited.

In my previous posts to this thread, the flow rate results given are reliable --- but the pressures quoted should be disregarded. I found that the pressure gauge used was innacurate and after replacing it with a proper test gauge obtained quite different results.

With the new gauge, and the pumps in series tested as per Rotax specs, with elec pump on and the engine pump inactive (engine not running) the flow to the carbs at the 2.2 psi spec was 35 litre per hour. This is one-third more than the Rotax 912 ULS uses at 100% power. If the pressure is raised to 2.5 psi, the flow rate drops to 27 liter per hour which equates to 100% power. Deadhead pressure was just over 4 psi. In both tests, the fuel return line to the tanks was active and that would have been taking 3.8 litre per hour at these pressures (from previous test on just the return line).

From a previous test, if the the carb pressure was set to zero, the flow rate was near 80 litre per hour, i.e. three times what the engine could use at full power.

The following points should be noted. The flow rates are very sensitive to changes in the low pressures used in the test so it is essential to have an accurate fuel pressure gauge. The Facet pump is also sensitive to the battery capability; my original battery could easily start the engine and show 13.6 volts while powering the elec pump but this pump could still not put out its full output. The pump pulls hard out of the battery in each of its pulses and it needs a truly good battery to cope. When I backed up my original battery with an automobile one, the pump improved markedly. Similarly when today I put in a new aircraft battery.

Douglas N

Thank you said by: Jez O'Hare

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Re: Rotax 912 pump bypass

by Jez O'Hare » 13 years ago

Jez O'Hare

1 7 0

Thank you Douglas and Roger. Which pump did you do your experiments with? The Pierburg / old type or newest type Italian mechanical fuel pump?

I will replace my leaking 912 mechanical pump with the newest type pump (with drain line) tomorrow. I got 2 andair check valves, one to bypass the electric pump as the installation manual shows, and another to bypass the mechanical pump as I saw other aircraft fuel diagrams with redundant parallel systems doing this..

I was surprised to find your posts this morning, I had just been discussing it with a friend that told me the mechanical pump bypass & valve was unnecessary. I am now considering to not install the mechanical pump bypass.. but is it not safer to have one anyway? Or is it too complicated and could cause problem?

912FuelSystemTrike.jpg (You do not have access to download this file.)

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