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Boiling fuel due to underpressure?

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frans(at)paardnatuurlijk.
Guest

Posted: Sat Oct 11, 2008 4:41 am Post subject: Boiling fuel due to underpressure?

I guess somebody here can tell me whether this is normal or not.

While testing my just finished fuel system on my 914, I noticed that quite some air was passing through the filters. (I temporarily use the factory supplied "glass" filters, to see what is going on and how much gunk will be collected during the fuel flow tests). I of course expected some air in the system, and also expected it to last some time before the last bubble would be gone.



Today I decided to let the fuel pump run while I was doing some other things, just to see if there were no small leaks and to "flush" the fuel system. After some time I noticed that the "air" still wasn't gone, actually it got worse. It looks like massive amounts of air are sucked in, also regulary I hear the pumps swallowing some air.



Although I have no fuel leaks if I leave the system alone, I suspected to have an air leak upstream the suction side of the filters. Air can pass through smaller holes than fuel I think. It doesn't matter whether I'm on Main or Reserve, so it had to be after the fuel selection valve and before the filters. Happy with that, because that is just one hose, and a Tee to both filters.



Now I'm not too happy anymore, because after spending a few hours chasing this mystery air leak, I'm quite convinced there is no leak. Of course there could be air leaks in both connections before the fuel selection valve, but this would be a dual fault, and not too likely.



If I close the fuel selection valve while the pumps are still running, the fuel in the filters is instantly gone. In fact, this is how I empty the filters before disconnecting them. After test #687 and disconnecting once again one of the filters, a few minutes after I got the filters sucked empty, I noticed how the air got sucked in again when I pulled the hose of it. It indicates that the vacuum is pretty good preserved during these few minutes, and this is in contradiction with the assumption that I have an air leak. At the same time, the amount of air I see through the fllters is increasing all the time, it looks at the moment like the air/fuel ratio is 50%. Still though, the pumps humm happy and don't seem to ingest much of this "air". Something is not right here. As indicated by two fuel flow sensors, there is 130 liters per hour of flow, and the fuel pressure is about 5 psi over the airbox pressure. (The engine is not running, can't do that yet because I'm not finished 

 with the rest of the installation).



Before I go crazy completely, let me ask a question. Is it possible, that while the pumps are pumping 130 liters per hour through the system, that the underpressure on the suction side of the pumps just lowers the boiling point of the fuel so much that the fuel starts "boiling"? Maybe it is just normal to see all these bubbles dancing around in the fuel filters? If anyone has noticed this as well, please let me know, it saves me a lot of time chasing "ghost" air leaks. Temperature at the moment is a nice 20 Celcius and the airplane has been standing in the sun for a while. (Yup, I know that sun isn't good for composite airplanes, but messing with the fuel in a confined space isn't very good for me).



It would explain why today it is much worse than yesterday evening: the temperature is much higher today, It also explains why the amount of "air" apparently sucked in is much more than the occasional burp I hear through the fuel pumps.



If this is not normal, I can only suspect to have air leaks on both tank connections. Tiny enough not to let them drip overnight, but big enough to let massive amounts of air in while the pumps start running...



Frans

Visit -  www.EuropaOwners.org

Gilles.Thesee(at)ac-greno
Guest

Posted: Sat Oct 11, 2008 5:40 am Post subject: Boiling fuel due to underpressure?

Frans Veldman a écrit :

 	  Quote: 	
		   Before I go crazy completely, let me ask a question. Is it possible, that while the pumps are pumping 130 liters per hour through the system, that the underpressure on the suction side of the pumps just lowers the boiling point of the fuel so much that the fuel starts "boiling"? Maybe it is just normal to see all these bubbles dancing around in the fuel filters? If anyone has noticed this as well, please let me know, it saves me a lot of time chasing "ghost" air leaks. Temperature at the moment is a nice 20 Celcius and the airplane has been standing in the sun for a while. (Yup, I know that sun isn't good for composite airplanes, but messing with the fuel in a confined space isn't very good for me).

Frans,
	




You're spot on. And not crazy at all, by the way 

We also noticed those bubbles in the transparent test lines when testing 

our fuel system.

This is normal behaviour for fuel when "sucked" by pumps that are 

*above* fuel level.

Aviation fuels, and to a greater extent autogas are a mix of many 

hydrocarbon substances, several of which have a very low boiling point. 

Autogas can even contain some butane and propane solved. So when sucked 

out by the pumps, the most volatile hydrocarbons turn to vapour.



We have flown our airplane up to FL 135/145 with no problem, but I must 

admit that a fuel system with pumps below fuel level would be far better.



FWIW,



Best regards,

-- 

Gilles

http://contrails.free.fr

frans(at)paardnatuurlijk.
Guest

Posted: Sat Oct 11, 2008 6:18 am Post subject: Boiling fuel due to underpressure?

Ok, once I started to suspect that I didn't have an air leak but just boiling fuel, I decided to do a simple test (should have thought about that earlier): I connected one of these glass filters to the fuel inlet of the engine, so I could see what the engine was getting.



While the inlet filter looked like I was pumping foam, the filter at the inlet of the engine showed that the engine was only getting liquid fuel, without a single bubble of air...



Now I have determined that I have no air leak but simply that my fuel is boiling due to underpressure at the suction side of the fuel system, I have some further questions:



1) Is this nornal, or do I have bad fuel, or for some reason too much underpressure that is causing this? With other words, do I have a problem?

2) At altitude, the pressure will be even lower, hence the fuel will boil earlier. Today is a nice fall day, but the temperatures are nowhere as high as I'm going to experience during my flights in the south of Europe. Isn't it likely that some of the vapour bubbles will make it into the carbs once the conditions go "worse"? Once you have vapour, the bubbles tend to grow rather than to dissolve. It just doesn't look good that the fuel is already boiling at this comfortable but not too high a temperature at 10 ft AMSL...



Frans

Visit -  www.EuropaOwners.org

Gilles.Thesee(at)ac-greno
Guest

Posted: Sat Oct 11, 2008 6:45 am Post subject: Boiling fuel due to underpressure?

Franz

 	  Quote: 	
		   Ok, once I started to suspect that I didn't have an air leak but just boiling fuel, I decided to do a simple test (should have thought about that earlier): I connected one of these glass filters to the fuel inlet of the engine, so I could see what the engine was getting.



 While the inlet filter looked like I was pumping foam, the filter at the inlet of the engine showed that the engine was only getting liquid fuel, without a single bubble of air...

   


	


Congratulations for this example of practical engineering sense.

All 914 owners will benefit from such experiments.

 	  Quote: 	
		   Now I have determined that I have no air leak but simply that my fuel is boiling due to underpressure at the suction side of the fuel system, I have some further questions:



 1) Is this nornal, or do I have bad fuel, or for some reason too much underpressure that is causing this? With other words, do I have a problem?

 2) At altitude, the pressure will be even lower, hence the fuel will boil earlier. Today is a nice fall day, but the temperatures are nowhere as high as I'm going to experience during my flights in the south of Europe. Isn't it likely that some of the vapour bubbles will make it into the carbs once the conditions go "worse"? Once you have vapour, the bubbles tend to grow rather than to dissolve. It just doesn't look good that the fuel is already boiling at this comfortable but not too high a temperature at 10 ft AMSL...



   

We regularly fly over the French Alps at FL 130+ with no problem.
	


I imagine you won't have to climb that high to clear terrain every day 

in the Netherlands 



The point is, the fuel pressure at the carbs is higher than airbox 

pressure, so the fuel won't be boiling and any vapour bubble will be 

collapsed by positive pressure.

*But* there is always the risk that the pumps be starved should the 

"boiling" become too intense at the tank pickup. As already said, the 

only real cure is to *push* fuel and not *draw* it up to the pumps.

But this often implies a major redesign of the fuel tank and fuel system.



Best regards,

-- 

Gilles

http://contrails.free.fr

g-iani(at)ntlworld.com
Guest

Posted: Sat Oct 11, 2008 7:07 am Post subject: Boiling fuel due to underpressure?

Frans



The pumps behave as Gilles described.  I have seen bubbles in the pumps at

any temperature from 0C to 35C on a hot day and with various fuels.  From

experience this is nothing to worry about.  The engine gets only what it

needs from the pumps and continues to run very sweetly.



Ian Rickard G-IANI XS Trigear, 200 hours

Europa Club Mods Rep (Trigear)

e-mail mods(at)europaclub.org.uk   

   or direct g-iani(at)ntlworld.com

pjeffers(at)talktalk.net
Guest

Posted: Sat Oct 11, 2008 9:56 am Post subject: Boiling fuel due to underpressure?

Hi guys,

 I now have 1250 hours and since new there have always been lots of bubbles in the filters.when the pump is running.  All i can do is reassure anyone that this is completely normal.  Don't know why but is OK.

 

 Pete Jeffers

 

   

 

   

 

 --

frans(at)paardnatuurlijk.
Guest

Posted: Sat Oct 11, 2008 10:14 am Post subject: Boiling fuel due to underpressure?

Gilles Thesee wrote:



 	  Quote: 	
		   We regularly fly over the French Alps at FL 130+ with no problem.

 I imagine you won't have to climb that high to clear terrain every day

 in the Netherlands 


	


We won't stay with the Europa in the Netherlands. Quite a substantial

amount of flying will be done in the French Alps, as my wifes parents

live close to Barcelonnette, and we love to fly over there (she has a

PPL also). We have been there twice this year, with a Cessna with

Thielert conversion. The bad news with a Cessna is its poor climb rate,

but the good news with this Cessna is that it is turbo-charged, and it

always climbs with 500 ft per minute, on sea level, but also on 12.500

feet. It is one of the reasons we wanted to have a 914 (turbo) in the

Europa as well. We are looking forward to start flying over there with

our Europa once it is finished. These plans are one of the reasons I

want to make sure the fuel system is working correctly, also at

altitude, in the mountains where you won't find acceptable emergency

landing spots everywhere.



Frans

air.guerner(at)orange.fr
Guest

Posted: Sun Oct 12, 2008 2:33 am Post subject: Boiling fuel due to underpressure?

I fully agree that this is normal. This phenomenon  is caused by the loss of pressure across the filter. In spite of the fuel tank  level being higher than the pumps, the pressure between the filter and the  pump becomes low enough so that the liquid fuel turns partially into vapor. As  soon as the fuel/vapor mixture exits the pump, the much higher pressure  causes vapor to turn back to liquid.

 The higher the fuel flow, the higher the pressure  loss across the filter. On the 914, when the engine is not running, there is of  course no boost pressure and therefore the fully open fuel pressure regulator is  sending all the fuel back to the tank. This is when the fuel flow  across the filters is maximum. However, when the engine is running at max boost,  more fuel pressure is required and the regulator sends a lot less fuel back  to the tank. Therefore, at high power, the flow through the filters is greatly  reduced and bubbles in the filters is less likely and possibly totally  eliminated. 

 Regarding the Europa supplied glass fuel filters, I  have always thought that they are too small for the task. The high number of  accidents or incidents due to clogged  filters proves it. I strongly  recommand the use of higher capacity filters especially with the 914.  Higher capacity will also reduce the pressure loss and the likelihood of vapor  bubbles in the filters.

  Regards

 Remi Guerner 



  

  

  

  Hi  guys,

I now have 1250 hours and since new there have always been lots of  bubbles in the

filters.when the pump is running.? All i can do is reassure  anyone that this

is completely normal.? Don't know why but is OK.



Pete  Jeffers

--

rampil

Joined: 04 May 2007
Posts: 870

Posted: Sun Oct 12, 2008 6:03 am Post subject: Re: Boiling fuel due to underpressure?

Hi Remi,



Can you point me in the direction of documentation of the "high number"

of accidents or incidents involving fuel filters?



In the US NTSB files, only one of 7 total Europa incidents was tied to the

fuel system, and that one was due to blockage of the filter inlet by

a chunk of rubber hose.



The size of the filter area within the filter was not a factor in that case.



FWIW, the most common accident factor was loss of directional control

or poor pilot technique.

_________________
Ira N224XS

topglock(at)cox.net
Guest

Posted: Sun Oct 12, 2008 6:12 am Post subject: Boiling fuel due to underpressure?

Frans, While the air bubble in the filter is not uncommon, you might 

want to check the selector valve.  Slight chance that you have a leaking 

"O" ring.  Other than that, get a hand operated suction pump and check 

the entire suction side of the system.



Jeff - Baby Blue



Frans Veldman wrote:

 	  Quote: 	
		   

 

 I guess somebody here can tell me whether this is normal or not.

 While testing my just finished fuel system on my 914, I noticed that quite some air was passing through the filters. (I temporarily use the factory supplied "glass" filters, to see what is going on and how much gunk will be collected during the fuel flow tests). I of course expected some air in the system, and also expected it to last some time before the last bubble would be gone.

 

 Today I decided to let the fuel pump run while I was doing some other things, just to see if there were no small leaks and to "flush" the fuel system. After some time I noticed that the "air" still wasn't gone, actually it got worse. It looks like massive amounts of air are sucked in, also regulary I hear the pumps swallowing some air.

 

 Although I have no fuel leaks if I leave the system alone, I suspected to have an air leak upstream the suction side of the filters. Air can pass through smaller holes than fuel I think. It doesn't matter whether I'm on Main or Reserve, so it had to be after the fuel selection valve and before the filters. Happy with that, because that is just one hose, and a Tee to both filters.

 

 Now I'm not too happy anymore, because after spending a few hours chasing this mystery air leak, I'm quite convinced there is no leak. Of course there could be air leaks in both connections before the fuel selection valve, but this would be a dual fault, and not too likely.

 

 If I close the fuel selection valve while the pumps are still running, the fuel in the filters is instantly gone. In fact, this is how I empty the filters before disconnecting them. After test #687 and disconnecting once again one of the filters, a few minutes after I got the filters sucked empty, I noticed how the air got sucked in again when I pulled the hose of it. It indicates that the vacuum is pretty good preserved during these few minutes, and this is in contradiction with the assumption that I have an air leak. At the same time, the amount of air I see through the fllters is increasing all the time, it looks at the moment like the air/fuel ratio is 50%. Still though, the pumps humm happy and don't seem to ingest much of this "air". Something is not right here. As indicated by two fuel flow sensors, there is 130 liters per hour of flow, and the fuel pressure is about 5 psi over the airbox pressure. (The engine is not running, can't do that yet because I'm not finish

 
	


ed 

 	  Quote: 	
		    with the rest of the installation).

 

 Before I go crazy completely, let me ask a question. Is it possible, that while the pumps are pumping 130 liters per hour through the system, that the underpressure on the suction side of the pumps just lowers the boiling point of the fuel so much that the fuel starts "boiling"? Maybe it is just normal to see all these bubbles dancing around in the fuel filters? If anyone has noticed this as well, please let me know, it saves me a lot of time chasing "ghost" air leaks. Temperature at the moment is a nice 20 Celcius and the airplane has been standing in the sun for a while. (Yup, I know that sun isn't good for composite airplanes, but messing with the fuel in a confined space isn't very good for me).

 

 It would explain why today it is much worse than yesterday evening: the temperature is much higher today, It also explains why the amount of "air" apparently sucked in is much more than the occasional burp I hear through the fuel pumps.

 

 If this is not normal, I can only suspect to have air leaks on both tank connections. Tiny enough not to let them drip overnight, but big enough to let massive amounts of air in while the pumps start running...

 

 Frans

 

 

 

 

 

 

 Visit -  www.EuropaOwners.org

 

 

 

 

 

 

 

 

 

 ------------------------------------------------------------------------

 

 

 Checked by AVG - http://www.avg.com

grahamsingleton(at)btinte
Guest

Posted: Sun Oct 12, 2008 3:19 pm Post subject: Boiling fuel due to underpressure?

Remi Guerner wrote:

 	  Quote: 	
		   Regarding the Europa supplied glass fuel filters, I have always 

 thought that they are too small for the task. The high number of 

 accidents or incidents due to clogged  filters proves it. I strongly 

 recommand the use of higher capacity filters especially with the 914. 

 Higher capacity will also reduce the pressure loss and the likelihood 

 of vapor bubbles in the filters.

 Regards

 Remi Guerner 

Remi
	


I think you are right, I recommend the Andair GAS 375, stops water and 

anything bigger than 70micron

Graham

frans(at)paardnatuurlijk.
Guest

Posted: Sun Oct 12, 2008 3:22 pm Post subject: Boiling fuel due to underpressure?

[quote:0fd8912cfa="air.guerner at orange.fr"]

The higher the fuel flow, the higher the pressure loss across the filter. On the 914, when the engine is not running, there is of course no boost pressure and therefore the fully open fuel pressure regulator is sending all the fuel back to the tank. This is when the fuel flow across the filters is maximum. However, when the engine is running at max boost, more fuel pressure is required and the regulator sends a lot less fuel back to the tank. Therefore, at high power, the flow through the filters is greatly reduced and bubbles in the filters is less likely and possibly totally eliminated.

-----[/quote:0fd8912cfa]



Sorry, I don't get it. As far as I understand the fuel pump delivers a constant rate to the engine, and all the fuel not used by the engine is going back to the tank. At high power, still the same is being pumped to the engine, but as the engine is consuming more, less is being sent back to the tank. The fuel filters are however not in the return line, but in the line TO the engine, and they see a constant flow, independant of the actual fuel consumption of the engine.



Correct me if I'm wrong.



Frans

Visit -  www.EuropaOwners.org

air.guerner(at)orange.fr
Guest

Posted: Mon Oct 13, 2008 11:25 am Post subject: Boiling fuel due to underpressure?

Frans,

  

 Let's take this  example:

 You are flying at full power at 16000 feet, ISA  conditions. According to the Rotax Manuals, the air box pressure (absolute  pressure) is 1,2 bars.    The fuel pressure at the carbs must be 0.25 bars  above airbox pressure, that is 1,45 bars absolute. Ambient pressure at 16000ft  is 0,53 bars, so the fuel pressure must be 0,92 bars above ambient. The fuel  pressure regulator adjusts the flow back to the tank so that the pump is able to  supply the required pressure. The Rotax Install Manual provides the pressure  curve of the fuel pumps. According this curve, the pump is able to supply 120  liters/hour at zero bar. This is what you get when the engine is not  running. However, if you need 0,92 bars (relative pressure), the out put flow  has to be restricted to 96 liters /hour.   

 I must admit that the 96 vs 120 liters per hour is not a so big difference,  but the fact is that the fuel flow through the filters is reduced when you  increase the power.

  

 Remi    

  

  

  

  

  



 [quote:0fd8912cfa="air.guerner at orange.fr"]

The higher the fuel  flow, the higher the pressure loss across the filter. On the

914, when the  engine is not running, there is of course no boost pressure and

therefore the  fully open fuel pressure regulator is sending all the fuel back

to the tank.  This is when the fuel flow across the filters is maximum. However,

when the  engine is running at max boost, more fuel pressure is required and

the  regulator sends a lot less fuel back to the tank. Therefore, at high  power,

the flow through the filters is greatly reduced and bubbles in the  filters

is less likely and possibly totally  eliminated.

-----[/quote:0fd8912cfa]



Sorry, I don't get it. As far as  I understand the fuel pump delivers a constant

rate to the engine, and all  the fuel not used by the engine is going back to

the tank. At high power,  still the same is being pumped to the engine, but as

the engine is consuming  more, less is being sent back to the tank. The fuel filters

are however not  in the return line, but in the line TO the engine, and they

see a constant  flow, independant of the actual fuel consumption of the engine.



Correct  me if I'm wrong.



Frans



    [quote][b]

frans(at)paardnatuurlijk.
Guest

Posted: Mon Oct 13, 2008 12:36 pm Post subject: Boiling fuel due to underpressure?

[quote:6ba7d7472f="air.guerner at orange.fr"]Frans,



Let's take this example:

You are flying at full power at 16000 feet, ISA conditions. According to the Rotax Manuals, the air box pressure (absolute pressure) is 1,2 bars. The fuel pressure at the carbs must be 0.25 bars above airbox pressure, that is 1,45 bars absolute. Ambient pressure at 16000ft is 0,53 bars, so the fuel pressure must be 0,92 bars above ambient. The fuel pressure regulator adjusts the flow back to the tank so that the pump is able to supply the required pressure.

[/quote:6ba7d7472f]



I don't agree with that. The fuel pump delivers 120 liters, with 15 psi or something like that. The fuel pressure regulator simply opens a valve when the pressure exceeds a limit, so the excess fuel flows back to the tank.

If the engine consumes more, the pressure drops a bit, so the valve closes a bit, and less fuel is flowing back to the tank. Let's say that 96 liters flow back (your example). This means that the engine is using 24 liters per hour. The flow through the filter is 96 + 24 liters, still 120 liters per hour. The ratio may differ, but the total remains constant.

You can also see it like this: the pressure regulator does not communicate with the pump, so the pump does not know what is going on and always delivers 120 liters per hour, regardless whether the engine is running at full power, or not even running at all.

Anyway, as soon as I have the installation ready I can tell you for sure, as I have a fuel flow sensor in series with both the feed line and the return line. I will tell you how much flows through the filter with various power settings.



Frans

Visit -  www.EuropaOwners.org

frans(at)paardnatuurlijk.
Guest

Posted: Mon Oct 13, 2008 12:43 pm Post subject: Boiling fuel due to underpressure?

[quote:4e544db179="grahamsingleton at bti..."]



I think you are right, I recommend the Andair GAS 375, stops water and 

anything bigger than 70micron

Graham[/quote:4e544db179]



I'm using an Andair GAS 375 for the main fuel pump. For the backup pump I will use a simple filter (at the moment the supplied glass filter). The Andair filter has noticably less fuel boiling problems than the smaller glass filter, so it seems it can handle the flow better, and causes less underpressure and hence less boiling.



BTW, the filter for the secondary fuel pump will be hardly used: when both pumps are running then all the fuel will still go through the Andair filter. The glass filter only sees fuel when the main pump stops, or when the Andair is clogged. This is the reason I think to have two Andairs is a bit overkill.



Frans

Visit -  www.EuropaOwners.org

grahamsingleton(at)btinte
Guest

Posted: Mon Oct 13, 2008 3:03 pm Post subject: Boiling fuel due to underpressure?

Frans Veldman wrote:

 	  Quote: 	
		   



 [quote:4e544db179="grahamsingleton at bti..."]



 I think you are right, I recommend the Andair GAS 375, stops water and 

 anything bigger than 70micron

 Graham[/quote:4e544db179]



 I'm using an Andair GAS 375 for the main fuel pump. For the backup pump I will use a simple filter (at the moment the supplied glass filter). The Andair filter has noticably less fuel boiling problems than the smaller glass filter, so it seems it can handle the flow better, and causes less underpressure and hence less boiling.



 BTW, the filter for the secondary fuel pump will be hardly used: when both pumps are running then all the fuel will still go through the Andair filter. The glass filter only sees fuel when the main pump stops, or when the Andair is clogged. This is the reason I think to have two Andairs is a bit overkill.



 Frans

   

the Andair is designed so that any entrained air is bled out of the 
	


filter, the awful glass thing doesn't, so it sits there threatening you 

and masking any fine contamination..

I recommend pumping the fuel into a can through a clear tube from just 

before the carbs and looking for bubbles. If there are any it implies 

air being sucked in through minor leaks upstream of the pump. Certainly 

when everything is cold during testing. Test at low fuel level is more 

searching.

Graham

air.guerner(at)orange.fr
Guest

Posted: Tue Oct 14, 2008 1:15 am Post subject: Boiling fuel due to underpressure?

Frans,

  

 Sorry to confirm you are wrong! 

 The fuel pump is unable to supply  120 liters  per hour and 15 PSI at the same time. The pump performance curve suplied by  Rotax (914 Install Manual, paragraph 14.4.1, page 50) shows that the pump  is able to deliver only about 90 liters per hour at 15 PSI (relative pressure).  Should you need 1,5 bars/22 PSI (relative) then the flow would have to be  restricted to 55 liters per hour. Should you need 1,75 bars/26 PSI  (relative) then the flow would have to be restricted to 20 liters per hour.  120 liters per hour can be delivered only with zero out put pressure. Again this  comes from the pump performance figures provided by Rotax and this behaviour  could be verified with any similar fuel pump. 

  In my previous example, assuming you are  running on one pump,  the flow through the pump and the filter would  be 96 liters per hour.  The engine would use about 30 liters per hour.  The remaining 66 liters would go back to the tank. Again, the  conclusion is that the fuel flow through the pump and filter is reduced when you  push the power lever. I am pretty sure you will be able to confirm  this whenever you are ready to run your engine. I would be curious to know your  measured figures.

  

 Remi

  

  

  

  

  

 "You are flying at full power at 16000 feet, ISA conditions. According to  the Rotax

Manuals, the air box pressure (absolute pressure) is 1,2 bars. The  fuel pressure

at the carbs must be 0.25 bars above airbox pressure, that is  1,45 bars

absolute. Ambient pressure at 16000ft is 0,53 bars, so the fuel  pressure must

be 0,92 bars above ambient. The fuel pressure regulator adjusts  the flow back

to the tank so that the pump is able to supply the required  pressure.

[/quote:6ba7d7472f]



I don't agree with that. The fuel pump  delivers 120 liters, with 15 psi or something

like that. The fuel pressure  regulator simply opens a valve when the pressure

exceeds a limit, so the  excess fuel flows back to the tank.

If the engine consumes more, the pressure  drops a bit, so the valve closes a bit,

and less fuel is flowing back to the  tank. Let's say that 96 liters flow back

(your example). This means that the  engine is using 24 liters per hour. The

flow through the filter is 96 + 24  liters, still 120 liters per hour. The ratio

may differ, but the total  remains constant.

You can also see it like this: the pressure regulator does  not communicate with

the pump, so the pump does not know what is going on and  always delivers 120

liters per hour, regardless whether the engine is running  at full power, or not

even running at all.

Anyway, as soon as I have the  installation ready I can tell you for sure, as I

have a fuel flow sensor in  series with both the feed line and the return line.

I will tell you how much  flows through the filter with various power  settings.



Frans"



    [quote][b]

rparigor(at)suffolk.lib.n
Guest

Posted: Tue Oct 14, 2008 8:58 am Post subject: Boiling fuel due to underpressure?

For what its worth, I fooled with 914 fuel system on bench.



Gilles was kind enough to put on his site:

http://contrails.free.fr/engine_914_fuel_syst_test_en.php



I commented on bubbling.



Note if the drain on Andair 375 is opened just a tiny little bit, bubbling

goes out of control and yuo can hear pumps injesting air.



Ron Parigoris

frans(at)paardnatuurlijk.
Guest

Posted: Tue Oct 14, 2008 11:43 pm Post subject: Boiling fuel due to underpressure?

[quote:08514aa99f="air.guerner at orange.fr"]Frans,



Sorry to confirm you are wrong! 

The fuel pump is unable to supply 120 liters per hour and 15 PSI at the same time. The pump performance curve suplied by Rotax (914 Install Manual, paragraph 14.4.1, page 50) shows that the pump is able to deliver only about 90 liters per hour at 15 PSI (relative pressure). Should you need 1,5 bars/22 PSI (relative) then the flow would have to be restricted to 55 liters per hour. Should you need 1,75 bars/26 PSI (relative) then the flow would have to be restricted to 20 liters per hour.



[/quote:08514aa99f]



Somehow you make the assumption that giving more power causes somehow the regulator to change its pressure setting?

I agree that the turbo changes the pressure somewhat if you get over the 100% stop, and then you may be right, the fuel pump needs to work harder. However, up to the 100% level, the pressure in the airbox should hardly change. In this range, the power lever should have no influence on the fuel pressure needed, and hence not affect flow.



Of course I will let you know what happens once I'm able to start the engine.



Frans

Visit -  www.EuropaOwners.org

air.guerner(at)orange.fr
Guest

Posted: Thu Oct 16, 2008 11:48 am Post subject: Boiling fuel due to underpressure?

Frans,<?xml:namespace prefix = o ns =  "urn:schemas-microsoft-com:office:office" /> 

  

Have  a look at the curve on paragraph 9.5 of the 914 Operator’s Manual which shows  the correlation between throttle position and airbox pressure (absolute  pressure). This curve shows that airbox pressure increases from 45 percent  throttle position and beyond . So as long as you are running beyond the 45  percent throttle position,  when you  push the throttle forward you increase airbox pressure. This causes the fuel  pressure regulator to increase the fuel pressure, and, as demonstrated before,  the flow through the pump is reduced. 

  

There  is a common misconception that the turbo is providing boost beyond the 100  percent throttle position only. This is far from the truth. The airbox pressure  curve shows that the turbo is boosting any time you are running beyond 45  percent throttle position. And this is at sea level. At altitude, the turbo is  providing boost all the time, whatever the throttle  position. 

  

Remi   

  

  

  

[/quote:08514aa99f]



Somehow you make the  assumption that giving more power causes somehow the regulator

to change its  pressure setting?

I agree that the turbo changes the pressure somewhat if you  get over the 100% stop,

and then you may be right, the fuel pump needs to  work harder. However, up

to the 100% level, the pressure in the airbox should  hardly change. In this

range, the power lever should have no influence on the  fuel pressure needed, and

hence not affect  flow.

    [quote][b]

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