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ROTAX 912 Fuel Pump


IBob

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I just looked through the new article on the operating principal of the Rotax 912 fuel pump:
https://www.rotax-owner.com/en/rotax-blog/item/77-rotax-912-fuel-pumps

This details how the pump delivers varying amounts of fuel, on demand, but at (approximately?) the same pressure: I had assumed it would use a spring loaded overpressure bypass valve with more or less fuel being bypassed (like the oil pressure system) but it's not like that at all.
It may not be new, but it was news to me, and I thought it very clever.

 

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It's pretty much how all stroking fuel pumps work. The spring under the diaphragm determines the fuel pressure and any other pump in tandem upstream (ie Fuel tank) will not add to the pressure unless it's setting exceeds it.. Essential to know (and understand) your fuel system in any aeroplane in detail.  Nev

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This is the standard working principle of most pumps, but unfortunately it is not understood by many who should know better. I recently read an article in a magazine by someone who should be fully conversant with that type of pump, as used in a Jabiru. He said that he had fixed a fuel pressure problem that others could not fix, by changing the stroke length. I never got round to asking how he could explain the theory and the SAAA magazine has gone into storage while I move house.

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7 minutes ago, Yenn said:

.................. I recently read an article in a magazine by someone who should be fully conversant with that type of pump, as used in a Jabiru. He said that he had fixed a fuel pressure problem that others could not fix, by changing the stroke length. I never got round to asking how he could explain the theory and the SAAA magazine has gone into storage while I move house.

Maybe the guy alluded to in the  Rotax article, Yenn?
 

I guess it's a matter of being able to visualise the operation. I'd say it's a good candidate for a simple animation.

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Great, you found one of the best reads on rotax fuel pumps so far... now go find the excellent series of articles by the same author on tuning and servicing the Bing64 carburretor... 👍

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My apologies for going off on a bit of a tangent:

 

I have two, closely related, Rotax fuel pump questions:

 

  • Is it feasible, for the home mechanic/builder, to rotate the fuel pump cap, thus reorienting the fuel in/out spigots? This would allow for different fuel line routing which may reduce the effects of engine heat soak causing fuel vaporisation.

 

  • If the above not within the capacity of the home mechanic/builder - why does Rotax not offer the fuel pump with cap/spigots at different angles to the basic 45 degree up & back (relative to prop)?
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Skippydiesel, looking at the illustration in the article, and assuming the cap screws are equally spaced, it would seem to be feasible to rotate the cap as you suggest.
Three things to consider, that I can think of:
1. We don't have a torque figure for those screws.

2. Opening the pump voids the warranty.

3. Engine heat soak causing vaporisation is best dealt with by

a) Installing the Rotax recommended fuel return line with orifice, which will vent vapor back to the fuel tank/s and

b) implementing engine start and preflight routines that will both expel any vapour and allow sufficient time to cool the engine bay.
 

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4 hours ago, IBob said:

Skippydiesel, looking at the illustration in the article, and assuming the cap screws are equally spaced, it would seem to be feasible to rotate the cap as you suggest.
Three things to consider, that I can think of:
1. We don't have a torque figure for those screws.

2. Opening the pump voids the warranty.

3. Engine heat soak causing vaporisation is best dealt with by

a) Installing the Rotax recommended fuel return line with orifice, which will vent vapor back to the fuel tank/s and

b) implementing engine start and preflight routines that will both expel any vapour and allow sufficient time to cool the engine bay.
 

1. Torque recommendation can usually be found by consulting "generic" charts and even without this common sense (experience) should be sufficient.

2. Yeah! This I know - often this is a manufactures/suppliers way of removing themselves from responsibility rather than a practical/logical barrier.

3. I am well aware of the existing strategies for minimising the effects of fuel vaporisation/lock. Unfortunately rerouting the fuel lines is (at this stage) not an option due to the orientation of the fuel pump spigots.

 

It seems to me, in my aircraft, the option to point the spigots straight down, may enable the fuel lines to be routed below the engine.

 

The only potentially negative, that I foresee, is the proximity of the fuel lines to the exhaust system. This could be easily addressed through targeted shielding/insulation.

 

Fuel vaporisation is generally caused by a hot engine, that has recently been stopped OR protracted ground operations, combined with hot ambient temperatures. The variose fuel lines, running over the top of the engine, are heated by the engine , primarily by convection (rising heat). Fuel lines below the engine may be subject to radiant heat however this may be over a significantly shorter time, as the exhaust system will tend to cool quicker, than the mass of the engine. In addition the hot air leaving the top of the cowling would drag cooler air into the bottom, creating a cooler environment for the fuel lines.

 

The radiant heat from the exhaust may cause in flight heating of the fuel lines but a doubt that this would be any greater than the existing over engine system.

 

The combination of removing the fuel lines from the hot over engine location and the cooling air entering the bottom of the cowling should markedly reduce the possibility of fuel vaporisation - I font understand why this option has not been tried/offered..

Edited by skippydiesel
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Skippydiesel, since we don't seem to have a problem with the fuel lines located where Rotax usually put them, I'm not sure what you are trying to achieve that has not already been achieved.

Convection, which you mention, is only a factor with the engine stopped: for most engine installations, the cooler place will be on top of the engine once it is running.
Sure, there may be a better routing to reduce gassing as a result of a recently stopped engine...but since you say you are well aware of existing strategies to deal with that, what is the issue here?

Edited by IBob
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Simple IBob;

 

There is no "we don't seem to have a problem"  - No Rotax 9 (carburetted) operator (flying in high ambient temperatures) is immune from the potential for fuel vaporisation.  

 

The existing strategies do not completely remove the possibility. Its possible that routing the fuel lines  below the engine would make little difference but on the other hand it might just add a further/incremental  improvement - unless its been tried you don't know. 

 

For engines prone to carbi ice, would you be so dismissive of a reduction in the potential for this to occur?

 

- who, if anyone,  has explored this possible option for further reducing, the very real, the chance of fuel vaporisation ?.

 

- can anyone see any problems (asides from the illogical warranty matter) with this idea?

 

- does anyone have a fully functioning  Rotax fuel pump, replaced for no other reason than 5 years in service, that they might like to donate to me, so that I can explore this concept (or go for it themselves)?

 

Any improvement in safety, especially where there is little or no cost involved, should be explored.

Edited by skippydiesel
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Aye, well, I've said my bit.

I'll only add this: in the carburettor function description, also linked above, the authors spell out how they go about troubleshooting carburettor problems. Specifically, they go looking for what has changed since the factory assembled the engine, and they then put it back as per the factory assembly.

That seems like solid advice to me.

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Bob;

 

I have never proposed changing the fuel pumps function or internal settings (I do know how a diaphragm pump works  & am satisfied with the pumps performance) - what I am interested in is the orientation of the spigot's and the potential (or not) to change that orientation. 

 

Once it has been established that the orientation can be changed (either by the purchaser or the Rotax factory/supplier) I would like to see a discussion on the merits of routing the fuel lines under (rather than over) the engine.

 

Pretty simple idea - why is it such a taboo subject? 

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2 hours ago, skippydiesel said:

Simple IBob;

 

There is no "we don't seem to have a problem"  - No Rotax 9 (carburetted) operator (flying in high ambient temperatures) is immune from the potential for fuel vaporisation.  

 

The existing strategies do not completely remove the possibility. Its possible that routing the fuel lines  below the engine would make little difference but on the other hand it might just add a further/incremental  improvement - unless its been tried you don't know. 

 

For engines prone to carbi ice, would you be so dismissive of a reduction in the potential for this to occur?

 

- who, if anyone,  has explored this possible option for further reducing, the very real, the chance of fuel vaporisation ?.

 

- can anyone see any problems (asides from the illogical warranty matter) with this idea?

 

- does anyone have a fully functioning  Rotax fuel pump, replaced for no other reason than 5 years in service, that they might like to donate to me, so that I can explore this concept (or go for it themselves)?

 

Any improvement in safety, especially where there is little or no cost involved, should be explored.

Routing the fuel lines below the cylinders will only exacerbate any vapour lock. Think about what happens when you go to depart, engine starts, top of cowl fills with cool air, vapour exits via the orifice or enters the carburetors and exits the vent line. Remember cool air enters  the top of cowl and hot air exits the bottom.

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You also have the radiated heat from the exhaust system low down. Baffling and lagging fuel lines help that . The fuel cooling effect is not very significant when no evaporation takes place so you can't rely entirely on that. where an engine is baffled , cool air blast tubes can be effective if directed appropriately. Nev

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2 hours ago, skippydiesel said:

Bob;

 

I have never proposed changing the fuel pumps function or internal settings (I do know how a diaphragm pump works  & am satisfied with the pumps performance) - what I am interested in is the orientation of the spigot's and the potential (or not) to change that orientation. 

 

Once it has been established that the orientation can be changed (either by the purchaser or the Rotax factory/supplier) I would like to see a discussion on the merits of routing the fuel lines under (rather than over) the engine.

 

Pretty simple idea - why is it such a taboo subject? 

As the fuel supply from tank to pump is on top and if you must have the feed coming up from below; perhaps get some soft aluminium tube 5/16" and form it to 'U' towards the front then turn down and put a rolled bead on both ends and that will route the feed to your desired location for the design and just route the 1/4" feeds to carbs as normal.  This way pump is as assembled at factory spec.

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1 hour ago, Thruster88 said:

Routing the fuel lines below the cylinders will only exacerbate any vapour lock. Think about what happens when you go to depart, engine starts, top of cowl fills with cool air, vapour exits via the orifice or enters the carburetors and exits the vent line. Remember cool air enters  the top of cowl and hot air exits the bottom.

Thruster - No offence intended but I do not think you have a grasp of the issue.

 

Please Note:  I am limiting my comments to Rotax 9 series carburettor engines.

 

My experience with fuel vaporisation/lock (FVL) and the understanding I have from commentary by others, is that this is principally a phenomena of aircraft ground operations, during periods of high ambient temperature.

 

Those flying in cooler climates and or confining their flying to cooler days or times of day (a management strategy) rarely if ever experience the problem.

 

I suggest (without any data) that temperatures from 30C will see an increasing rise in the potential for FVL (not suggesting it could not occur below this temp.).

 

So high ambient temperatures AND a hot (recently operated) engine are the precursor's for FVL

 

Susceptibility will likely be influenced by engine cowl design, parking into wind, duration of heat soak (engine off), degree of fuel pipe insulation and  fuel type (ULP being more susceptible than AvGas ).

 

Mitigation usually takes the form of;

  • Avoiding flying on the hot days or times of day (in my view the best option).
  • Allowing the engine to cool to or near ambient. 
  • Using "fire" sleeves on fuel lines - fire sleeves having some insulating properties.
  • Using AvGas (the Americans seem to be particularly fond of this expensive partial solution).
  • Parking into wind in the hope of increased cooling air flow when engine off.
  • Improving cowl air flow design for ground operations (including engine off)

I have never experienced taxi induced fuel vaporisation however there are many anecdotal reports that this can occur with prolonged ground running on hot days.

 

It would seem that few, if any, pilots have reported FVL in cruise or even well established (late) climb out. I speculate that this is due to enhanced cooling (air flow/engine rpm) greater fuel flow  and cooling with altitude.

 

Symptoms;

 

Pilots may experience difficulty with "hot" engine start but this is more of an annoyance/inconvenience than a safety issue.

 

Once started there may be short periods of rough running, as in line fuel gas/vapour interrupts the liquid delivery to the carbi's, again more of an annoyance than a safety issue. A very strong signal that the pilots should be alert for this to occur during take off & climb out

 

I suggest that the real problem (safety) is the engine that appears to run normally during taxi & run up and then looses power during, the critical periods of, take off ground role and early climb out. 

 

No matter the timing of the problem, I believe it all starts on the ground.

 

So speculation about in flight FVL is  misplaced - the problem is on the ground  - this is why I am interested in the possibility of fuel lines being routed  under, rather than over, the engine as this simple change has the potential to add to the existing mitigation strategies.

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1 hour ago, Blueadventures said:

As the fuel supply from tank to pump is on top and if you must have the feed coming up from below; perhaps get some soft aluminium tube 5/16" and form it to 'U' towards the front then turn down and put a rolled bead on both ends and that will route the feed to your desired location for the design and just route the 1/4" feeds to carbs as normal.  This way pump is as assembled at factory spec.

Thanks Blueadventure - might work but more connections means more weight and potential for failure. 

 

I just cant see why the fuel pump cap/spigots cannot or should no be rotated/oriented to the builders desired position.

 

If the concern is Rotax QA then why doesn't Rotax offer pumps with spigots pointing in at least two directions eg the existing set up and a straight down ????.

 

I presume the robot/person who assemble the pump, could be programmed/asked to rotate the cap to the desired position and the whole shebang still meet Rotax QA & specifications.

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2 minutes ago, skippydiesel said:

Thanks Blueadventure - might work but more connections means more weight and potential for failure. 

 

I just cant see why the fuel pump cap/spigots cannot or should no be rotated/oriented to the builders desired position.

 

If the concern is Rotax QA then why doesn't Rotax offer pumps with spigots pointing in at least two directions eg the existing set up and a straight down ????.

 

I presume the robot/person who assemble the pump, could be programmed/asked to rotate the cap to the desired position and the whole shebang still meet Rotax QA & specifications.

You then have to get up to the carbys each side.

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Mitigation usually takes the form of;

 

My list was short two important mitigation tools ;

  • A metered (small orifice) fuel return line, may encourage vaporised fuel to exit the up stream side of the fuel delivery.
  • The use of a fuel "boost" pump may assist the expulsion of gases fuel from the system and assist in the suppression of further vaporisation.

 

I left out a symptom:

 

  • If aircraft fitted with a fuel flow and or pressure gauge, the pilot may notice sharp oscillations in fuel delivery/pressure.

 

Edited by skippydiesel
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