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Bruce Tuncks

Fuel Pressures

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After many years, I bought a 0 to 5psi fuel pressure gauge ( ebay, about $10, digital display ).

 

Previously, I had not done this because of the argument... " so what can you do about it when flying besides turn on the electric pump. Wouldn't you do this anyway?" And " you are paying a price for this info, there is an extra tee in the fuel line and that has a small but non-zero risk. And the extra tee makes more fluid friction in the line".

 

Here's what I found on putting in the gauge...

 

3.6 psi elec pump on ground

 

3.2 psi idle on mechanical pump only

 

3.4 psi idle both pumps

 

So far so good huh, just what I expected. The max is 4 psi from the book. Then I took off.

 

climb-out at max throttle, one pump .. 1.2 psi

 

This was a lot lower than I expected. But is it OK? Should I now do a flow test?. The last flow test was ok, but now I have an extra impediment to the flow, and that is the tee piece for the pressure gauge.

 

The final reading puzzles me too... 5.4 psi at ten minutes after stopping the engine. Where did this pressure come from? Everything was shut down, electric pump and all.

 

I can see that the fuel-lines in the engine compartment can become hot, but the sender is sited between the mechanical fuel pump and the carby.

 

The ambient temperature was maybe 27 C, but I don't think this has much to do with these results. .

 

 

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So far, I can't find info about fuel pressures. There is the thread here about lean running at full throttle which has good comments about flow-rates but no pressures are mentioned. On a rotax site ( Bing carby but probably a different one ) I found the comment that " max pressure should be 2.5 psi as 2.75 psi can cause flooding and 4 psi can damage the float valve",

 

The Jabiru recommended method of measuring fuel supply rate is to disconnect the supply hose at the carby and run fuel into a container while pumping with the electric pump. This is what I have done in the past, but maybe the carby bowl should be removed so that the float-valve obstruction is included?

 

 

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Electric , SD. The sensor is silicon. It claims to be accurate to 1%. You can find them on ebay, the one I got has a digital display window and 2 push buttons on the front. One button is on/off and the other is to select the pressure units you want. It is powered by two AAA cells in the case itself.

 

 

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If you have turned the engine off then I wouldn't worry about the pressure reading. My electric gauges do some weird things when switched off.

 

 

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Interesting problem, Bruce. I presume the pressure drop on climb out is because the engine is drawing lots of fuel from the carby bowl and the pump(s) are working hard to replace it.

 

Having a fuel pressure gauge is one more thing to distract you and stress about, but it can be useful. The mechanical gauge I bought from Ross Millard gives warning of fuel starvation;

 

the pressure drops and the needle flickers crazily for a minute or so before the engine noise starts to change. Plenty of time to change tanks.

 

 

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Bruce: The rise in fuel pressure after shutdown is due to vaporisation in the fuel lines near the engine from heat soak. It is normal. If your pressure gauge is between the last fuel pump and the carbies you will get this unless you have a fuel return line to the tank. Rotax recommend this return line to let fuel vapor vent back to the tank and stop vapor lock.

 

When you were climbing out, did you have both pumps on? I always use both pumps in critical segments of the flight such as take off, climb to altitude and landing.

 

Having said that, there have been times when I forgot to turn the electric boost pump on during TO or climb and I don't recall fuel pressures that low. Maybe some further investigation is warranted, especially seeing as you're flying a high wing with wing tanks.

 

Edit:

 

When I built the Nynja, I did the fuel flow test straight out of the fuel line (disconnected from the carby). I think that is the normal way to do it. It shows what the pump and lines will support if they are called upon.

 

 

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OK. Remember the rule "Pressure is resistance to flow".

 

So at idle or cruise your fuel pump is pumping more than the engine requires.

 

The carby float lifts and blocks off the extra fuel coming into the carb. This restriction, throttling of the fuel if you like, produces pressure.

 

The pump(s) trying to pump fuel to the carb but the carb doesn't want it.

 

This is what you read on your gauge. The 3.2 to 3.6 figures.

 

When climbing out and using maximum fuel/maximum throttle your float is fully (or close to) open. This mean there is LESS restriction to the incoming fuel.

 

Less restriction means LOWER fuel pressure.

 

Effectively you are trading pressure for flow.... ie 1.2 psi

 

Your pump is still pumping more fuel than the engine requires, which is good and correct, which is where the 1.2 comes from..

 

The high press after stopping is as CSC stated above.

 

 

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I have fuel pressure questions too although different

 

There are std aviation methods for testng fuel supply to engine, it DOES NOT allow use of electric pump to achieve 1.3?? Time WOT fuel rate. Camit went into detail on this as Jabiru fitted custom springg and raised pressure which gave Bing trouble.

 

Jabirus fuel supply setup is too restrictive without much doubt. By replacing hoses and rerouting things I got closer to target but lines too small and too long.

 

Jab system doesnt have check valves so should never get above static head so Id reckon the gauge might have issues. Reading pressure this low is very difficult even with expensive gear

 

My problem is that fuel pressure varies from 3 psi to below 1 psi in cruise flight, pump steadies variation up but its still there. Never even sounds like its short on fuel.

 

 

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Yes however two pumps compensating for this

 

 

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Thanks guys, good comments.

 

Here's what I did today... I did a flow test with the carby bowl off and catching the fuel that would have gone into the carby. So the flow impediments of the new tee and the inlet float-valve were both there. I measured 390 cc/min . The pressure gauge was reading zero at this time, which proves Downunder's point. So a reading of 1.2 psi shows that there was excess capacity in what was being pumped compared with what was required.

 

But not by that much... 390 cc/min is 23.4 l/hr and not far from what a full-power climb would use. Mind you, this was after the electric pump was turned off and I did a full-throttle climb at about 2000 ft to see what the fuel pressure would do.

 

As a result of this, I'm going to change my ways: Instead of turning off the electric pump just after take-off, I'm going to leave it on for more of the climb in future.

 

And that high reading after shut-down, thanks csc, that explains it.

 

Next flow-test will be with the hose disconnected ( the normal way) and I expect a bit more flow. It would be interesting to see what the flow was with the mechanical pump, but of course this is not easy to do. You would need to run the mechanical pump without running the engine.

 

 

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The flow test is done before mech pump, it has big resistance when not running, you need to see 1.4 times max engine usage rate

 

If you have 23 l/hr including mech pump it might be pretty good without it

 

3300 are looking for 60 lph and fall well short. I got from 23 to 40 by new hose routes and reducing fittings. Next would be a larger size hose from header tank to engine. Have to go to rubber hose which brings in timed replacements. Jab red or blue primer hose doesnt have this.

 

Maybe the float valve jumps open and closed and accounts for the pressure variation

 

 

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Thanks jetjr. I did have that detail wrong, about the mechanical pump to not be included for the flow test. I agree that it makes the 23 l/hour look better, but next time I will do the test by the book and see what happens.

 

So the 23.4 l/hour was with every flow impediment there was., including the mech fuel pump and the inlet float-valve.

 

 

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I too was surprised to see fuel pressure remain after engine shutdown. My Sonex/Jabiru/Bing does not have an electric pump, although I did leave the engine pump in the system, contrary to the advice of the installation manual. I do not get an increase in pressure, but the 3 psi normal pressure from the pump is always there. I doubted the reading for a long time, so I disconnected the line and the pressure fell. I guess it means that you have a healthy float needle valve in the carburator. The pressure increase may mean there is a little air in the line somewhere.

 

 

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Id reckon your flow is pretty good Bruce

 

You must be able to achieve free flow without pump running because if it fails in take off climb for some reason thats when fueel flows highest and you are lowest, ie most dangerous

 

Jabiru test method using elect pump on to achieve flows makes the take off depend on the electric pump working

 

 

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Today I did some more flow tests.

 

Test 1. With the pipe removed from the mech fuel pump and hose into the container.. That is, minimum restrictions, no mech pump.

 

Flow=73 l/hr ( Yep, more that the 60 l/hr figure quoted )

 

Test 2. With the mech pump reconnected and pipe from the mech pump to the carby disconnected at the carby and put into the container ( that is with the stationary mech fuel pump impeding flow from the elec pump)

 

Flow = 58.8 l/hr.

 

Test 3. With the container catching fuel from where the carby bowl was removed. That is, maximum impediments of stationary mech fuel pump and float inlet valve present, with the fuel delivered by the elec pump from the tank behind the seats.

 

Flow = 28.8 l/hr ( more than the 23.5 I had measured before, both times were for 30 sec. This time I reckon I primed it better so the first second or two were not filling empty hoses.

 

The latest Jabiru maintenance manual states ( shame on me for not reading this sooner ) that the min fuel pressure is 0.75 psi and the max is 3.0 psi. How much is 0.75 psi? well I tried blowing into the gauge and found that 1.5 psi is about the max I can do. It's more than a balloon needs.

 

So here are my conclusions.

 

1. The fuel system is OK and can be left alone.

 

2. The pressure gauge will not be mounted in the panel, partly because it doesn't give really useful info and partly because it was too cheap to be fuel-tolerant over the years. It will be fine as an annual test instrument.

 

Thanks again for all the helpful comments.

 

 

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Today I did some more flow tests.Test 1. With the pipe removed from the mech fuel pump and hose into the container.. That is, minimum restrictions, no mech pump.

Flow=73 l/hr ( Yep, more that the 60 l/hr figure quoted )

 

Test 2. With the mech pump reconnected and pipe from the mech pump to the carby disconnected at the carby and put into the container ( that is with the stationary mech fuel pump impeding flow from the elec pump)

 

Flow = 58.8 l/hr.

 

Test 3. With the container catching fuel from where the carby bowl was removed. That is, maximum impediments of stationary mech fuel pump and float inlet valve present, with the fuel delivered by the elec pump from the tank behind the seats.

 

Flow = 28.8 l/hr ( more than the 23.5 I had measured before, both times were for 30 sec. This time I reckon I primed it better so the first second or two were not filling empty hoses.

 

The latest Jabiru maintenance manual states ( shame on me for not reading this sooner ) that the min fuel pressure is 0.75 psi and the max is 3.0 psi. How much is 0.75 psi? well I tried blowing into the gauge and found that 1.5 psi is about the max I can do. It's more than a balloon needs.

 

So here are my conclusions.

 

1. The fuel system is OK and can be left alone.

 

2. The pressure gauge will not be mounted in the panel, partly because it doesn't give really useful info and partly because it was too cheap to be fuel-tolerant over the years. It will be fine as an annual test instrument.

 

Thanks again for all the helpful comments.

Hi Bruce, .75psi is three quarters of one psi .

 

 

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A diaphragm fuel pump has a limited life. The likelihood that an impending failure can be reliability detected during periodic maintenance isn’t good. The likelihood that the diaphragm will start leaking and losing pump pressure before its performance affects required engine flow, is good.

 

 

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testing fuel pumps in operation is something to think about and only a gauge can do it

 

Even getting a change in pressure from stopped to engine running is a solid sign.

 

A change too with electric on checks it is ok, along with noise.

 

 

  • Agree 1

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Good point dewil. That diaphragm sure is a flimsy thing. I would like to change it for a new one since this one is old, with its first flight being 17 years ago.

 

 

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Be very cautious, these fuel pumps look simple but have fine tolerances inside. Plunger can bind after rebuild. Can even bind once they get hot.

 

Theres an alignment tool needed to get it right

 

 

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17 yrs!. I think it has done a done fine job. Maybe time to fork out on a new one and enjoy the next 17 trouble free...026_cheers.gif.65c96b31c66a3eb21be04c979f7f1b85.gif

 

 

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I asked Jabiru and they said that they only sold fuel pumps as whole units for $265 and that it is a mandatory replacement at 2000 hour full overhaul. So your point is a good one Downunder. The way I am going, it will be 20 years and I will be 90 before 2000 hours comes up.

 

Is it possible for the diaphragm to tear in such a way that the electric pump puts fuel into the engine sump instead of the carby? I would hope not, but then I have never taken the pump off to have a look at this.

 

 

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