Fuel pressure mystery

[pluessy]

Correct, 0.35mm will give you ~3lph fuel return at the normal fuel pressure.

 

Pre-startup check:

when you test the electric fuel pump, watch the pressure climb slowly and settle at the electric fuel pump pressure (depends on fuel pump)

turn the elelctric pump off and watch the pressure slowly decrease (it decreases to 0 over 5+ seconds) -> orifice is clean

 

This orifice does nothing in flight, you have 7-9lph flowing to each carby and the 3lph of the return line is just change. The purpose of this orifice is to purge vapour and air prior to start, with the electric pump. It will help a bit on decent with low fuel burn but then you also have low power (heat) under the cowl, minimising the vapour problem.

 

The fuel lines should be fire-proofed so they are not going to heat up the fuel by much in flight. Heat comes from the fuel pump, gascolator (not insulated) and any other exposed fitting or gadget.

After shut-down, the temp of the fuel will increase and the orifice will prevent a build-up of pressure and overflowing of the carbys. The downside is that the 0 pressure will encourage vapour formation.

Go back to pre-startup check point "check electric fuel pump"🙂

[RFguy]

I think 2-3 lph is a minimum. Carb use is 6-30 lph

 

For other's benefit...  for 6mm ID hose, that's a volume of 28mL per meter of hose 

 

AT 2 lph  (0.555mL/second)  it would take about  (28mL divided  0.555ml/second )  = 50  seconds to cycle through 1m of hose.

At least its moving. 

Skip, I would bring the T/bleed junction right back as far as possible on the engine. It's hot amongst the water plumbing on top of the crank case... 

 

 

Edited January 12, 2023 by RFguy

[skippydiesel]

16 minutes ago, pluessy said:

Correct, 0.35mm will give you ~3lph fuel return at the normal fuel pressure.

 

Pre-startup check:

when you test the electric fuel pump, watch the pressure climb slowly and settle at the electric fuel pump pressure (depends on fuel pump)

turn the elelctric pump off and watch the pressure slowly decrease (it decreases to 0 over 5+ seconds) -> orifice is clean

 

This orifice does nothing in flight, you have 7-9lph flowing to each carby and the 3lph of the return line is just change. The purpose of this orifice is to purge vapour and air prior to start, with the electric pump. It will help a bit on decent with low fuel burn but then you also have low power (heat) under the cowl, minimising the vapour problem.

 

The fuel lines should be fire-proofed so they are not going to heat up the fuel by much in flight. Heat comes from the fuel pump, gascolator (not insulated) and any other exposed fitting or gadget.

After shut-down, the temp of the fuel will increase and the orifice will prevent a build-up of pressure and overflowing of the carbys. The downside is that the 0 pressure will encourage vapour formation.

Go back to pre-startup check point "check electric fuel pump"🙂

Bit confused Pluessy,

My boost pump performance check is a normal part of my prestart check list - I don't have a problem (that I know of) with this or the mechanical pump. Pressures are well within the Rotax recommended ranges.

"This orifice does nothing in flight"  you may want to rethink this statement - the return fuel line operates,  as long as there is fuel running through the fuel distributer/manifold.

"The purpose of this orifice is to purge vapour and air prior to start, with the electric pump." - again I suggest you rethink this. True, the return lines main functions are - to vent/purge fuel vapour (there should be no air) AND to maintain the flow of fuel within the delivery lines (cooling) thus minimising the risk of fuel vaporisation. These two objectives are achieved with both boost & mechanical (main)fuel pumps.

" It will help a bit on decent with low fuel burn but then you also have low power (heat) under the cowl, minimising the vapour problem." In most carburettor Rotax 9 installations, fuel vaporisation occurs on the ground , after a hot engine has been shut down, not when the aircraft is in flight and has good under cowl air flow.

"After shut-down, the temp of the fuel will increase and the orifice will prevent a build-up of pressure and overflowing of the carbys." True! temperature, in the fuel lines will increase - True! any resultant pressure will be "relived" by the return line - Not true! that the carburettor float bowls will overflow as a result. Fuel into the float bowl is controlled by the float/valves. Only a lowering of fuel level, will open the valves, allowing more fuel to enter.

Note: The Rotax carburettor 9's will run quite happily without a fuel return line.

[pluessy]

Not confused. The 3lph of fuel return are insignificant compared to the 18-26lph that go to the carbys.

The carbys will overflow if the fuel pressure exceeds the float force. With the engine shut down and temps increasing, the fuel pressure will increase and over-power the float force (fuel pump valve prevents venting backwards) if there is no return line vent. The longer the fuel lines, the more fuel it will force into the carb bowls.

Reading a pressure gauge is not just the value, it is also the way the needle moves (fast/slow/steady etc).

I have been flying behind a 912 for over 500h and do my own maintenance, so know a thing or two.

 

The return orifice is not required for flying, just makes starting easier and helps with extended ground taxiing times (low fuel flow to the carbys and high under-cowl temps).

[Thruster88]

Flow rate through an orifice will be dependent on diameter and to a great extent the length of orifice. 

 

The 0.35mm carb jet would have minimal length and could have a similar flow to a 0.5mm drilled hole that is say 2 or 3mm "long".

 

Only a flow test will will determine actual performance of any orifice in the system.

  

 

 

[RFguy]

Thruster88 should know, he's got ag sprayers with hundred of jets.

 

I can see the 2-3 lph being useful for a hot conditions idle RPM taxi down to the end of the runway before TO roll.  Minimal flow rates present and airplane not into the wind.  aux fuel pump on to push up line pressure should deal with most heat issues.  (aside from carb float bowl overflowing which is a IMO a real fire risk with the rotax carb locations without overflow pipes and heat shields. )

[IBob]

Skippy, Alan recently posted to his thread 'CHT sensor Temperature versus original below sparkplug thermocouple' regarding the amount of trouble and expense he went to, only to discover that his instrumentation was not reading correctly: he was chasing false hi temperature indications.

It seems to me you may be in a similar position, in that we are still not sure whether you have poor fuel pressure, or just poor fuel pressure readings.
You stated early on that the aircraft has never shown any signs of fuel starvation.
You may install a smaller return line jet, but I doubt this is your problem.
If at all possible, I would beg/borrow/steal a steam gauge, hook it up temporarily, and establish which it is. Because if it is something wrong with the readings, you could be mucking around forever with fuel system components without seeing any improvement.

[IBob]

There was a reference earlier to other owners having similar problems and just 'learning to live with it.' It did not say if these were other owners with glass cockpits, but if so I would begin to suspect either the equipment or the way it is being installed. And while I can see the logic in keeping the fuel lines out of the cockpit, where possible, I am not sure about siting the fuel pressure transducer/sender in the engine bay. Quiet simply, if there is any pressure (over static pressure) in the engine bay in flight, that additional pressure will be subtracted from the fuel pressure reading.

 

[IBob]

On 12/01/2023 at 11:07 AM, IBob said:

Hi Skippy, following this with interest.
Here is a pic of the (spare) return jet setup from my aircraft. (It as 2 identical jets, the second one goes to the fuel pressure gauge).
My measure of the hole is a best guess: I can get a .35mm strand of wire into the hole, but certainly not 2 strands.

I would guess the hole is 0.5mm

 

I should add that this is mounted where the fuel line splits to the two carbs, and works fine for me: I have had vapour lock after short shutdowns: I always use the booster pump pre-start, and with vapour it runs fast  for 5 to 15secs before settling down once the vapour is gone, at which point I get a fuel pressure reading.  (Obviously, vapour will pass through the jet much faster than liquid.)

 

 

Skippy, I have misinformed you:
I just remeasured the return jet/s on my (spare) fuel splitter, using a fine needle cut to a long taper with emery paper and vernier callipers: I am seeing a measure of approx 0.38 -  0.4mm, certainly not the 0.5mm I previously estimated.
 

[skippydiesel]

8 hours ago, IBob said:

Skippy, Alan recently posted to his thread 'CHT sensor Temperature versus original below sparkplug thermocouple' regarding the amount of trouble and expense he went to, only to discover that his instrumentation was not reading correctly: he was chasing false hi temperature indications.

It seems to me you may be in a similar position, in that we are still not sure whether you have poor fuel pressure, or just poor fuel pressure readings.
You stated early on that the aircraft has never shown any signs of fuel starvation.
You may install a smaller return line jet, but I doubt this is your problem.
If at all possible, I would beg/borrow/steal a steam gauge, hook it up temporarily, and establish which it is. Because if it is something wrong with the readings, you could be mucking around forever with fuel system components without seeing any improvement.

All true my friend but I do have one factual, indisputable, reading - my fuel return is delivering over 7 litres per hour (Hobbs). Most correspondents think this is higher flow rat than it should be. Some go so far as to suggest 2-3 L/hr is "normal" -  I have yet to hear where this flow rate comes from.

 

From the known flow rate, the next known, 27L/hr which is Rotax specified fuel consumption, at full power. Add these two figures together , 34L/hr and we are very close to Rotax specification for its max pump fuel delivery of 35L/hr (nominal).

 

I would suggest that the symptoms of a pump nearing/exceeding its pumping capacity are;

• Low fuel pressure during periods of high demand
• As the systems ages/wears, a further symptom may be fuel exhaustion - the engine may not deliver full power, could hesitate, etc

So its no longer just about pressure - its now about fuel flow (volume) (pressure is but indicator).

[skippydiesel]

5 hours ago, IBob said:

Skippy, I have misinformed you:
I just remeasured the return jet/s on my (spare) fuel splitter, using a fine needle cut to a long taper with emery paper and vernier callipers: I am seeing a measure of approx 0.38 -  0.4mm, certainly not the 0.5mm I previously estimated.
 

Very interesting - I wonder what difference a jet of your diameter would make in my system. 

 

Are you able to give me the suppliers name/details?

[IBob]

26 minutes ago, skippydiesel said:

Very interesting - I wonder what difference a jet of your diameter would make in my system. 

 

Are you able to give me the suppliers name/details?

Skippy, I used a very fine sewing needle, then tapered it by hand, then inserted it in the (slightly recessed) aluminium 'jet', marking the place with my thumbnail, then extracted it and used vernier callipers.
Easy to write, but in practise not so easy to be confident of an exact measurement. Which is why I wrote 0.38 - 0.4mm
I have remeasured a number of times now, I would say it is around the lower figure.
( But I do know it is bigger than 0.35mm, as I have a wire of that size, and it fits with some additional wiggle room.)
 

If so, then your current jet at 0.5mm is 42% larger in cross section than mine.
And if the recommended jet size is 0.35mm, your current jet is over twice that size.
So, yes, I agree that the next step is to source and fit a 0.35mm jet.

To answer your question: mine is not a jet, as such. It comes as part of the ICP Savannah kit and is a fuel coupling or spigot, with the end blanked (probably not fully drilled out) and the hole drilled in the end. Possibly manufactured in house.

 

If you get really stuck, I can send you one, though you would need to figure out thread size and how to mount it.
However, it would probably be easier and also better to source a genuine 0.35mm jet, if you can.

As you say, reducing the jet size may well sort your problem. If not THEN might be a time to check whether it is poor pressure or a poor reading.

 

[Blueadventures]

14 minutes ago, IBob said:

Skippy, I used a very fine sewing needle, then tapered it by hand, then inserted it in the (slightly recessed) aluminium 'jet', marking the place with my thumbnail, then extracted it and used vernier callipers.
Easy to write, but in practise not so easy to be confident of an exact measurement. Which is why I wrote 0.38 - 0.4mm
I have remeasured a number of times now, I would say it is around the lower figure.
( But I do know it is bigger than 0.35mm, as I have a wire of that size, and it fits with some additional wiggle room.)
 

If so, then your current jet at 0.5mm is 42% larger in cross section than mine.
And if the recommended jet size is 0.35mm, your current jet is over twice that size.
So, yes, I agree that the next step is to source and fit a 0.35mm jet.

To answer your question: mine is not a jet, as such. It comes as part of the ICP Savannah kit and is a fuel coupling or spigot, with the end blanked (probably not fully drilled out) and the hole drilled in the end. Possibly manufactured in house.

 

If you get really stuck, I can send you one, though you would need to figure out thread size and how to mount it.
However, it would probably be easier and also better to source a genuine 0.35mm jet, if you can.

As you say, reducing the jet size may well sort your problem. If not THEN might be a time to check whether it is poor pressure or a poor reading.

 

I put my restricter into a 3/16” brass tail so as not to ever mix up 1/4” fuel line connections to carbs that form part of the 4 way junction.  Then 3/16” I’d hose to fuel tank.

[aro]

The restrictor sizing is confusing. The Rotax installation manual specifies 0.5mm and also #35 jet.

 

It seems that jet numbers can be either size or flow rate so #35 jet could be 0.35mm, or 35cc/min = 2.1 l/hr. My guess would be flow rate since there is a separate 0.5mm size specification. The flow rate is influenced by the shape of the passage, not just the size.

 

How did you measure the return flow rate? Timed flow into a container?

 

 

[pluessy]

I used the fuel flow gauge, engine off, electric pump on and running it for 10-15min to get a stable reading. Then I used that to re-calibrate the fuel flow meter so it reads correctly at full fuel pressure.

I have suggested to JP Instruments to add a calibration routine to the FS-450 so that the instrument can compensate for the return fuel and show true fuel flow.

[IBob]

15 minutes ago, pluessy said:

I used the fuel flow gauge, engine off, electric pump on and running it for 10-15min to get a stable reading. Then I used that to re-calibrate the fuel flow meter so it reads correctly at full fuel pressure.

I have suggested to JP Instruments to add a calibration routine to the FS-450 so that the instrument can compensate for the return fuel and show true fuel flow.

Nice to see someone come up with figures from practical testing.
The figure of 2L/hr gets bandied around a lot, but I have never been able to find where it comes from. And I have suspected my own return flow is higher than that while observing it raising the level in a valved off near-full tank while in flight.

[pluessy]

That 3lph is correct for 0.4bar fuel pressure and reduces proportionally with lower fuel pressure. In flight at 4,800 and higher rpm, I do have 0.4bar. At lower rpms, it can be less, at WOT in climb (5,300rpm), it is just below 0.4bar.
I did have a few episodes where the fuel pressure fluctuated a bit in cruise. It is usually happening with fuel level at the lower end and possibly short unporting of the pickup. My Tecnam has wide but shallow tanks with no internal baffles, so fuel sloshing around will unport the pickup at low levels. I deliberately ran the LH tank (with fuel return) dry once to see when I run out and what happens. I picked it up on the fuel pressure, watching it slowly dropping to 0 and then switched tanks before the engine stumbled.

[old man emu]

WARNING:

How accurate is your measuring device? When you are measuring very small diameters such as those you need to here, it doesn't take much of a mechanical error in the device to return a reading several poofteenths out.

 

To help you decimalise the sizes of numbered drills, here's a link. https://vermontamerican.com/drill-bit-decimal-equivalency-chart/

[IBob]

7 minutes ago, old man emu said:

WARNING:

How accurate is your measuring device? When you are measuring very small diameters such as those you need to here, it doesn't take much of a mechanical error in the device to return a reading several poofteenths out.

 

To help you decimalise the sizes of numbered drills, here's a link. https://vermontamerican.com/drill-bit-decimal-equivalency-chart/

Yep, as I found trying to measure my jet size.
We had suggestions like 'stick a pin in it' (the hole is too small and the taper on a pin and even a small needle is far too abrupt to get any accurate reading) and 'file a taper' (anyone here ever tried filing a round taper on a 0.4mm needle?).......

[facthunter]

The length  and finish of the dimensioned hole affect flow rate which is what we are talking about.  Fuel is not a great dissipator of heat in any case..  Nev

[skippydiesel]

8 hours ago, aro said:

How did you measure the return flow rate? Timed flow into a container?

 

8 hours ago, aro said:

 

 

Did it the most accurate real world way I could - Directed return line fuel into its own collection tank. Flew the aircraft (several TO/Landing & cruise over several hours). Drained & measured fuel from dedicated tank. Fuel quantity divided by Hobbs hours.  

 

Just rechecked my notes - the actual flow rate is 7.6L/hr (Hobbs)

[BRL]

Don't know if this is related to your fuel pump problem but l have just been flying and noticed my fuel pressure was barely keeping above the min pressure and had to keep the electric pump on to maintain pressure.

On landing pump was removed and dismantled ( not expecting warranty pump 3yr old) by the resident L2 and he found the the discharge side the valve loose in its seat the 3rd of which he has seen in recent times. l too see the fluctuation in the pressure going from electric to mechanical but deemed it one of those things, to prove the pump was right or wrong was proven by refitting the old pump that was changed out on the 5yr rule and everything returned to normal and with less fluctuation than the new pump in regards to switching from electric to mechanical.Will be speaking to Floods about this.

[facthunter]

It's pretty much the first thing to look at.  IMHO.Nev

[skippydiesel]

Some more information:

 

The Rotax fuel lune return restrictor jet is almost certainly a Mikuni  VM 22/120 #35.

 

These slow idle jets come is a wide range of flow rates # 10 - #140

 

I would like to reduce my flow rate of 7+l/hr down to 3-4 L/hr . Anyone on the Forum able to sugest which # jet is most likly to meet my needs?

 

(Mikuni person cant/wont help)

[skippydiesel]

What no past/present hot rodders ??

 

Fluid dynamic oficiendo's armature/professional???