Rotax 912S fuel pressure
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Rotax 912S fuel pressure
Can one of you helpful folk tell me something? I want to know if the Rotax supplied 912S mechanical fuel pump (not the peirburg one, the replacement one that was free issued a few years ago), provides a constant pressure (ish) into the carb supply pipe across the engine rpm range. I know that some of you will have a fuel pressure gauge fitted and can see what is going on. I don't have one on mine.
I'm kind of interested for fuel usage and tank balancing (the return line only goes to one of the two tanks) calculations. If the fuel pressure remains constant with rpm then the return flow will be constant. If it is I'll borrow a flow meter and measure it.
Ta.
I'm kind of interested for fuel usage and tank balancing (the return line only goes to one of the two tanks) calculations. If the fuel pressure remains constant with rpm then the return flow will be constant. If it is I'll borrow a flow meter and measure it.
Ta.
I do not have a fuel pressure gauge fitted - though my local guru, a manifest proponent of simplicity, once said "if you really must have a panel full of gauges, the one that could be useful is a fuel pressure gauge".
That said:
* A german engineer, who seemed to know what he was talking about - though one never knows, with engineers - recently told me that the "new" fuel pump is sh!t, delivering some 0,2 less than the original Pierburg (sic!). Not sure what unit, though.
* you seem to imply there is a relation between fuel burn and fuel pressure. I heard this before, but have some trouble understanding it. Surely the carb floats will only admit sufficient fuel to float, at whatever pressure? And even if there are differences, it should be possible to compensate for them by changing carburettor needles?
* I am slightly flabbergasted about your fuel system layout. You do be talking about a Rans S6, surely? Do you have wing tanks only? Or one fuselage tank and one wing tank? The one seems as unlikely as the other, given "the return line only goes to one of the two tanks".
* As a matter of fact, why do you have a return line at all? My own pride and beauty, a high-winger also with a tank in each wing, only has lines from each tank to the header tank, each line including a shut-off valve, a filter, and (I suspect) a non-return valve at the entry of the header tank. From the header tank it is straight to the mechanical pump on the 912, with another shut-off valve and a filter on the way. No return line anywhere, I do recall Rotax recommended these at one point in time but all and sundry laughed their head off and Rotax went rather quiet then.
Mind you mine is a 912 not a 912S but I think that matters little, for this story.
That said:
* A german engineer, who seemed to know what he was talking about - though one never knows, with engineers - recently told me that the "new" fuel pump is sh!t, delivering some 0,2 less than the original Pierburg (sic!). Not sure what unit, though.
* you seem to imply there is a relation between fuel burn and fuel pressure. I heard this before, but have some trouble understanding it. Surely the carb floats will only admit sufficient fuel to float, at whatever pressure? And even if there are differences, it should be possible to compensate for them by changing carburettor needles?
* I am slightly flabbergasted about your fuel system layout. You do be talking about a Rans S6, surely? Do you have wing tanks only? Or one fuselage tank and one wing tank? The one seems as unlikely as the other, given "the return line only goes to one of the two tanks".
* As a matter of fact, why do you have a return line at all? My own pride and beauty, a high-winger also with a tank in each wing, only has lines from each tank to the header tank, each line including a shut-off valve, a filter, and (I suspect) a non-return valve at the entry of the header tank. From the header tank it is straight to the mechanical pump on the 912, with another shut-off valve and a filter on the way. No return line anywhere, I do recall Rotax recommended these at one point in time but all and sundry laughed their head off and Rotax went rather quiet then.
Mind you mine is a 912 not a 912S but I think that matters little, for this story.
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Jan - you are "foreign", so obviously you cannot really understand!
UK air is very complicated, so a Rotax cannot possibly run without a fuel return line. Everybody knows that!
So virtually all Rotax fuel systems on UK aircraft have a return line. Given the plumbing involved it really mans that returning to just one tank, not necessarily the one you are drawing from is pretty common.
As for the initial request - well who cares? Plumb the lines so you use the fuel from the pilots side. Burn an hours worth, change tanks for half an hour and then equal times.
Realistically the return line Tee piece hole is usually around 0.1mm diameter so the return amount is pretty small, enough to overfill a full tank but, but, but,
Take the average fuel pressure and work it out, my fuel pressure seemed pretty constant, unfortunately I cannot remember at what pressure!
UK air is very complicated, so a Rotax cannot possibly run without a fuel return line. Everybody knows that!
So virtually all Rotax fuel systems on UK aircraft have a return line. Given the plumbing involved it really mans that returning to just one tank, not necessarily the one you are drawing from is pretty common.
As for the initial request - well who cares? Plumb the lines so you use the fuel from the pilots side. Burn an hours worth, change tanks for half an hour and then equal times.
Realistically the return line Tee piece hole is usually around 0.1mm diameter so the return amount is pretty small, enough to overfill a full tank but, but, but,
Take the average fuel pressure and work it out, my fuel pressure seemed pretty constant, unfortunately I cannot remember at what pressure!
UK air is very complicated, so a Rotax cannot possibly run without a fuel return line. Everybody knows that!
So perhaps BE is just a tiny corridor of "normal" air between those two major imperfections? Or perhaps it is an outlier from France, where many things seem to be more relaxed.
Seriously though: on a high-winger, does it mean there is sufficient pressure available to push the fuel back up into the wing tanks? That requires some believing - or is THAT perhaps the reason one sees those big vent pipes on the S6 fuel caps? To create a bit of vacuum to assist in sucking the fuel up?
Thread Starter
the aircraft, a microlight, is a high winger with tanks in the wings but it is not a Rans S6. Nor a Skyranger, nor an Escapade, nor a Kitfox, nor an Avid, nor a C42, nor a Thruster, nor an X'air, nor an AX2000, nor a Kolb, nor a CT, nor a Dragon ........
I have not had a 912 fuel pump to bits to see how it works but I think it may have a spring pushing the plunger and fuel forwards. The cam follower on the gearbox pushes the plunger backwards, the length of the stroke will depend upon the volume of fuel used since the last time the cam came around. The pressure will depend upon the strength of the spring?
It may be very different.
I have not had a 912 fuel pump to bits to see how it works but I think it may have a spring pushing the plunger and fuel forwards. The cam follower on the gearbox pushes the plunger backwards, the length of the stroke will depend upon the volume of fuel used since the last time the cam came around. The pressure will depend upon the strength of the spring?
It may be very different.
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I fly an a/c with a 912S and fuel pressure gauge. Might be a week or so before I fly it again but I'll have a look next time I do if you don't need an urgent answer.
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I don't have direct experience of the exact carbs used on Rotax engines, but aren't these "Bing" motorcycle type or similar, i.e. Constant velocity? Unless they are of a very unusual design, they don't need fuel "pressure" to work, only a fuel supply sufficient to keep the float chamber filled to the designed level. So a lower maximum fuel pressure probably wouldn't cause a problem.
The engine driven pumps I've dealt with over many years simply consist of a diaphragm and two simple non-return valves, which work in opposite directions, one opening to "suck in, close out" and the other vice-versa. The operating arm on the rotating cam pushes a circular metal plate to move the diaphragm against a return spring. There isn't a way of regulating fuel pressure in that design, so it would probably vary depending on engine rpm and fuel demand. At full power and max revs, the pump would move the maximum amount of fuel, but the pressure might be lower in the supply line to the carb. The size of the pump would be chosen to provide slightly more fuel flow than the engine could consume at maximum output. With the throttle closed but with high engine rpm, the fuel flow would be lowest whereas the pressure would be highest. The output pressure must not be sufficient to overcome the float valve in the carburettor, for obvious reasons.
Sometimes a separate fuel pressure regulator is fitted after a mechanical pump. This may use a return pipe to bleed off excess pressure.
The engine driven pumps I've dealt with over many years simply consist of a diaphragm and two simple non-return valves, which work in opposite directions, one opening to "suck in, close out" and the other vice-versa. The operating arm on the rotating cam pushes a circular metal plate to move the diaphragm against a return spring. There isn't a way of regulating fuel pressure in that design, so it would probably vary depending on engine rpm and fuel demand. At full power and max revs, the pump would move the maximum amount of fuel, but the pressure might be lower in the supply line to the carb. The size of the pump would be chosen to provide slightly more fuel flow than the engine could consume at maximum output. With the throttle closed but with high engine rpm, the fuel flow would be lowest whereas the pressure would be highest. The output pressure must not be sufficient to overcome the float valve in the carburettor, for obvious reasons.
Sometimes a separate fuel pressure regulator is fitted after a mechanical pump. This may use a return pipe to bleed off excess pressure.
Last edited by ShyTorque; 5th Nov 2016 at 09:05.
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The Corona pump keeps the fuel pressure constant at just under 0.4 bar on my 912UL. The vapour return line orifice is 0.35mm bore, as specified by Skydrive.
Correction: (From LAA Safety Spot) One thing that has been discovered since the December edition is that the diameter of the return restrictor in the continuous flow system is critical to system performance; these restrictors are essentially metered carburettor jets. Skydrive, the UK agent for Rotax, manufactures a clever ‘T’ piece which contains one of these jets and, as 0.35 was stamped on the side of the jet, it was thought that this number related to the jet’s size, 0.35 mm. Well, we now know that this number relates to a fluid flow number which, whilst related to size, isn’t a dimension in the normal sense. Nigel’s measured a few as far as it is possible to do so and a 0.35 jet appears to be about 0.7 mm
Correction: (From LAA Safety Spot) One thing that has been discovered since the December edition is that the diameter of the return restrictor in the continuous flow system is critical to system performance; these restrictors are essentially metered carburettor jets. Skydrive, the UK agent for Rotax, manufactures a clever ‘T’ piece which contains one of these jets and, as 0.35 was stamped on the side of the jet, it was thought that this number related to the jet’s size, 0.35 mm. Well, we now know that this number relates to a fluid flow number which, whilst related to size, isn’t a dimension in the normal sense. Nigel’s measured a few as far as it is possible to do so and a 0.35 jet appears to be about 0.7 mm
Last edited by patowalker; 5th Nov 2016 at 17:20.
@ST: agree about the carbs (they are indeed Bing, type 54 I think) and they have a membrane operated sleeve regulating the airflow over the nozzles, so yes they are "constant something". The goal is to avoid the need for a mixture control.
And agree again, the fuel pump uses the simple diaphragm technology, one reason for this is that it will only pump as much fuel as the carbs will take up.
Lastly: I recently replaced the Pierburg fuel pump (it was seriously damaged and the repair was only so so) with the newer Italian one (is that the one called "Corona"?). The new pump does have an overflow exit tube so it must have some kind of over-pressure relief indeed.
And agree again, the fuel pump uses the simple diaphragm technology, one reason for this is that it will only pump as much fuel as the carbs will take up.
Lastly: I recently replaced the Pierburg fuel pump (it was seriously damaged and the repair was only so so) with the newer Italian one (is that the one called "Corona"?). The new pump does have an overflow exit tube so it must have some kind of over-pressure relief indeed.
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Jan, out of my curiosity, where does the return pipe go? It must run either back to a fuel tank or simply back to the inlet side of the pump.
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Its a Vapour return line
Hi,
I have read this post with interest, as an owner of a 912 UL in my Europa
As I understand it, on my aircraft the return line is a vapour return line. This is to prevent vapour lock in the fuel lines. After the mech fuel pump, the lines to the pair of Bing CV carbs is brought over the top of the engine, and although sleeved in heat protective shield, is clearly prone to heat soak ( especially true on a hot day, with a line up delay)
There is a "T" piece inserted at the highest piont on the fuel line, this then returns vapour to the fuel tank. As there is no vapour separator, to prevent the carbs being bypassed by the fuel, a restriction is included in this return line, that being a spare carb main jet orifice inserted into the return line.
I have never had a vapour lock problen in a 912 engined Europa... and I've flown many hundreds of hours in them, on various different airframes.
I'm sure some small quantity of fuel will come back down the return line, on my aircraft it goes back to the single fuel tank, but I do not know how much this is. Does the OP's twin tank set up have a balance line between the tanks?
I have a pressure gauge fitted, the pressure is more or less constant in flight, as the pump is capable of delivering many more times the volume of fuel than the carbs can use.
I tend to use it more to check the initial operation of the backup electric pump in my preflight checks. I then turn the electric pump off ( having primed the carbs by testing it) and check the fuel pressure returns to zero -via the orifice, I presume- and then check the mechanical fuel pump pressure on starting the engine. As part of my line up checks the electric pump goes back on, and I have now ensured, as far a practicable, continuous fuel delivery for take off.
I hope this is of some help. FlyingApe.
I have read this post with interest, as an owner of a 912 UL in my Europa
As I understand it, on my aircraft the return line is a vapour return line. This is to prevent vapour lock in the fuel lines. After the mech fuel pump, the lines to the pair of Bing CV carbs is brought over the top of the engine, and although sleeved in heat protective shield, is clearly prone to heat soak ( especially true on a hot day, with a line up delay)
There is a "T" piece inserted at the highest piont on the fuel line, this then returns vapour to the fuel tank. As there is no vapour separator, to prevent the carbs being bypassed by the fuel, a restriction is included in this return line, that being a spare carb main jet orifice inserted into the return line.
I have never had a vapour lock problen in a 912 engined Europa... and I've flown many hundreds of hours in them, on various different airframes.
I'm sure some small quantity of fuel will come back down the return line, on my aircraft it goes back to the single fuel tank, but I do not know how much this is. Does the OP's twin tank set up have a balance line between the tanks?
I have a pressure gauge fitted, the pressure is more or less constant in flight, as the pump is capable of delivering many more times the volume of fuel than the carbs can use.
I tend to use it more to check the initial operation of the backup electric pump in my preflight checks. I then turn the electric pump off ( having primed the carbs by testing it) and check the fuel pressure returns to zero -via the orifice, I presume- and then check the mechanical fuel pump pressure on starting the engine. As part of my line up checks the electric pump goes back on, and I have now ensured, as far a practicable, continuous fuel delivery for take off.
I hope this is of some help. FlyingApe.
Jan is talking about the drain pipe on the Corona pump, not the vapour return line.
If "Corona" is really the term for the "new" Italian-made mechanical fuel pump, than it does have a drain outlet, pointing downwards.Now to answer the question: as of now it leads nowhere though I am not really happy about that. I am even less happy about the overflow lines from the carb float chambers, they dingle around loosely with all the risks that involves. So I was thinking of some common solution for all three, only it seems the carb overflow lines are rather delicate and better not extended.I heard of some people sticking them into the air filter, sounds queer to me but I am no engineer.
And I am reluctant to install a fuel return line, it would have to go all the way to the header tank which is located behind and slightly below the pilot's seat. Plus I would have to tinker with the plumbing near the aluminium header tank and that's a scary idea to me. A vent line would have to go to one wing tank, with a separate "tap" so as to be above the fuel level - even less attractive.
Jan,
Perhaps instead of beginning from a state of your own inventive logic you should consult your own version of the LAA.
Certainly in England long before we volunteered to move away to float her further off the Continental shores, the LAA following (I suppose) Rotax' advice have demanded a small 35 jet bleed off the main post pump fuel pressure piping. And to return it to a handy fuel tank & not back to the pump inlet, or recirculate to the inline fuel filter/gascolator either.
As folk above ave explained it does have a minor function and removes air bubbles or vapour whilst having no effect of the carb's feed pressure. This bleed back also allows pressurised fuel, when static with engine off & hot to escape back to the tank and this helps stop carb float chambers perhaps temporarily overflowing with excess pressure and the consequent mess plus fire hazard.
In my experience the return quantity with the original PIerburg pump during flight is in the order of 2 to 3 litres per hour.
In my a/c a Rans S6-116 which is fitted with the current bog standard overhead 2 X 35 litre wing tanks it's common sense when both tanks are full to run off the fuel returned tank till it's gone down enough to accept the bleed feed 'topping it up'. Easy to see by the pilot either from the simple clear pipe gauge or easier through the translucent tank wall on view.
Lastly those "annoying" pipes from the carb's ! They have a serious duty.
You really ought to re-read the Rotax free on-line manuals which explain quite lucidly their purpose. They are to monitor the ambient pressure and deliver that to the float chamber. Pressure above or below this do affect the 'push' on the fuel up & through the carb jets to either enrich or weaken the mixture.
That's how some of the common 'leaning' devices work , because they insert a bleed valve - screw adjustable - suction source to these pipes and by the P1 juggling the screw against EGT's he may save a little petrol/gasoline/benzine per hour.
Most of us are happy to go with whatever Mr Bing delivers when set up to Mr Rotax' spec's.
In fact I'm surprised your Inspection authority didn't ensure the return bleed from pump outlet to a tank wasn't done when the a/c was first assembled. It's not difficult - a standard Tee piece is put in with three 13 mm pipe clips with a 35 jet on the return leg and a pipe following the same route back to the tank as the regular feed. To get it into the tank is another question, but either a Tee in the sight gauge pipe (like mine) or at a guess in via a tee into the tanks top breather/overflow right as it goes in at the neck ?
Better solution is please ask your Inspector.
Regards,
mike hallam
Perhaps instead of beginning from a state of your own inventive logic you should consult your own version of the LAA.
Certainly in England long before we volunteered to move away to float her further off the Continental shores, the LAA following (I suppose) Rotax' advice have demanded a small 35 jet bleed off the main post pump fuel pressure piping. And to return it to a handy fuel tank & not back to the pump inlet, or recirculate to the inline fuel filter/gascolator either.
As folk above ave explained it does have a minor function and removes air bubbles or vapour whilst having no effect of the carb's feed pressure. This bleed back also allows pressurised fuel, when static with engine off & hot to escape back to the tank and this helps stop carb float chambers perhaps temporarily overflowing with excess pressure and the consequent mess plus fire hazard.
In my experience the return quantity with the original PIerburg pump during flight is in the order of 2 to 3 litres per hour.
In my a/c a Rans S6-116 which is fitted with the current bog standard overhead 2 X 35 litre wing tanks it's common sense when both tanks are full to run off the fuel returned tank till it's gone down enough to accept the bleed feed 'topping it up'. Easy to see by the pilot either from the simple clear pipe gauge or easier through the translucent tank wall on view.
Lastly those "annoying" pipes from the carb's ! They have a serious duty.
You really ought to re-read the Rotax free on-line manuals which explain quite lucidly their purpose. They are to monitor the ambient pressure and deliver that to the float chamber. Pressure above or below this do affect the 'push' on the fuel up & through the carb jets to either enrich or weaken the mixture.
That's how some of the common 'leaning' devices work , because they insert a bleed valve - screw adjustable - suction source to these pipes and by the P1 juggling the screw against EGT's he may save a little petrol/gasoline/benzine per hour.
Most of us are happy to go with whatever Mr Bing delivers when set up to Mr Rotax' spec's.
In fact I'm surprised your Inspection authority didn't ensure the return bleed from pump outlet to a tank wasn't done when the a/c was first assembled. It's not difficult - a standard Tee piece is put in with three 13 mm pipe clips with a 35 jet on the return leg and a pipe following the same route back to the tank as the regular feed. To get it into the tank is another question, but either a Tee in the sight gauge pipe (like mine) or at a guess in via a tee into the tanks top breather/overflow right as it goes in at the neck ?
Better solution is please ask your Inspector.
Regards,
mike hallam
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Hi Jan,
Yes, the new pumps are made in Italy by BCD Corona, but you won't find them in their catalogue, because the rights are owned by Rotax.
The drain outlet should have a line attached and routed into the airflow below the engine, just like the coolant overflow.
When you fit a vapour return line, you can do this inyour Eurofox https://www.youtube.com/watch?v=JifpxUFgJa4 :-)
Yes, the new pumps are made in Italy by BCD Corona, but you won't find them in their catalogue, because the rights are owned by Rotax.
The drain outlet should have a line attached and routed into the airflow below the engine, just like the coolant overflow.
When you fit a vapour return line, you can do this inyour Eurofox https://www.youtube.com/watch?v=JifpxUFgJa4 :-)
Last edited by patowalker; 5th Nov 2016 at 17:27.
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Like Mike Hallam, I have found that the vapour return line on my 912S returns 2-3 litres per hour to one of my Kitfox 5 wing-tanks (which can be a nuisance on long flights).
As to the original question, I do not know in practice because I am just about to fit a fuel pressure gauge! However, the Skydrive instructions state "At all speeds within the normal range (say from 2000 to 5000rpm} the pressure should remain in the range 0.15 to 0.5 bar. The normal pressure with a 912 engine is around 0.25 - 0.4 bar."
Hope this helps.
As to the original question, I do not know in practice because I am just about to fit a fuel pressure gauge! However, the Skydrive instructions state "At all speeds within the normal range (say from 2000 to 5000rpm} the pressure should remain in the range 0.15 to 0.5 bar. The normal pressure with a 912 engine is around 0.25 - 0.4 bar."
Hope this helps.
@PW: thanks, there's really no end to learning. As for that movie, such flying would be doubly illegal for me: spinning an ultralight is forbidden anyway, and, a bit more subtle, there is a general rule to disallow any action that might disturb or distract the pilot. Even watching this video disturbs me, almost physically, I'll never even think of flying this!
@mike & @ forfoxake: those 2-3 litres per hour that go through your vapour bleed line, where do they go in my plane that has none such? I am slightly bewildered there.
@forfoxake: what fuel pressure gauge are you going to fit? I looked around but found prices rather hefty.
@mike: no need for surprise, we do not have any kind of inspection authority, neither are ultralights periodically verified (in Germany they call it JahresNeuPrüfung, JNP for short, and some have told me it is a bit of a farce, not much more than a pre-take-off walk-around). Neither is there a vast amount of technological know-how in our national federation, or I should have missed it. OTOH there are a lot of knowledgeable people around, I do will ask them.
PS apologies to threadstarter for hijacking the thread!
@mike & @ forfoxake: those 2-3 litres per hour that go through your vapour bleed line, where do they go in my plane that has none such? I am slightly bewildered there.
@forfoxake: what fuel pressure gauge are you going to fit? I looked around but found prices rather hefty.
@mike: no need for surprise, we do not have any kind of inspection authority, neither are ultralights periodically verified (in Germany they call it JahresNeuPrüfung, JNP for short, and some have told me it is a bit of a farce, not much more than a pre-take-off walk-around). Neither is there a vast amount of technological know-how in our national federation, or I should have missed it. OTOH there are a lot of knowledgeable people around, I do will ask them.
PS apologies to threadstarter for hijacking the thread!
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More info on the Skydrive fuel pressure kit:
http://www.speedcomfly.com/sito-ecom...info/00128.pdf
SKYDRIVE :: Product Details
http://www.speedcomfly.com/sito-ecom...info/00128.pdf
SKYDRIVE :: Product Details
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The instructions that patowalker has helpfully provided are for the older version of the kit. The newer version is Skydrive part FPK 912/12.I think the only difference is that the upper red line on the gauge has been increased from 0.4 to 0.5 bar.