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Fuel system

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europa(at)pstewart.f2s.co
Guest

Posted: Tue Jun 30, 2009 12:40 pm Post subject: Fuel system

Can anyone tell me why rotax swapped from parallel to series arrangement for

the 914 fuel pumps . Also is it mandatory in the UK to have them in series.



Regards



Paul

G-GIDY

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rparigor(at)suffolk.lib.n
Guest

Posted: Tue Jun 30, 2009 1:12 pm Post subject: Fuel system

Hi Paul



" Can anyone tell me why rotax swapped from parallel to series arrangement

for the 914 fuel pumps."



To keep differential fuel pressure (pressure to carb float bowls over

airbox pressure) less than I think 5 PSI.



Parallel will blow over that number and has chance of overcoming ability

of needle valve and seat to hold it back.



Also when operating on two pumps in parallel, it is harder on each pump

compared to operating two pumps series with bypass.



Note Europa has modified Rotax recommendation by installing a second

filter, great idea IMHO.



I have details of my system here:

http://www.europaowners.org/modules.php?set_albumName=album231&op=modload&name=gallery&file=index&include=view_album.php

Gilles was kind enough to put up fuel pump testing on his website, link

provided in above link.

There is a diagram on Gilles website that is easier to follow than what

Europa has in their FWF manual, but it is the same. I only substituted two

Andair 375 gasculators for supplied filters.



Ron Parigoris

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tonyk(at)kaon.co.nz
Guest

Posted: Tue Jun 30, 2009 1:32 pm Post subject: Fuel system

 	  Quote: 	
		  >Can anyone tell me why rotax swapped from parallel to series

arrangement for
	


 	  Quote: 	
		  > the 914 fuel pumps . Also is it mandatory in the UK to have them in

series.
	


Rotax always had them in series - it was Europa who put them in

parallel. During testing of the UMA differential fuel pressure gauge it

was discovered that the parallel pumps were actually over pressurising

the system so the change was made to swing them back to series. 



Incidentally, the UMA fuel pressure gauge mod is very easy to fit and

does work well. For those who are interested, the output of the UMA

differential fuel pressure sensor is 0-5v so should be compatible with

most glass panels capable of accepting a 0-5v input from fuel pressure

sensors.



Tony

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budyerly(at)msn.com
Guest

Posted: Tue Jun 30, 2009 4:11 pm Post subject: Fuel system

<?xml:namespace prefix="v" /><?xml:namespace prefix="o" /><![endif]-->  Tony and others, 

 The original Rotax installation documentation for the 914 does have a  parallel setup, Ref:  Rotax installation manual dated 1996 05 10.   Europa did not come up with this...

  

 As seems usual for Rotax, they give almost no heads up to why they do  things.  Refer to Contrails comments http://contrails.free.fr/engine_pierburg_en.php. I  have a number of clients flying with the parallel system, without  failures.  The original documentation shows that a pump failure will not  allow fuel to bypass in a parallel setup, but it could.  As Rotax said in  1996 "Note:  There is no need for a checkvalve for the operation of the two  electric fuel pumps since the pump itself won't allow any passage against  correct direction of flow.  Attention:  Employ  GENUINE-BOMBARDIER-ROTAX fuel pumps only.  Non compliance will release  BOMBARDIER -ROTAX from any liability."  The new installation manual  drops all but the attention step) comment, necessitating the flurry of  emails and comments in Contrails.  John Hurst in high altitude flying saw  the need for the differential to keep the engine from flaming out above 15,000  (if memory serves) when running full boost.  Of course turning on the other  pump clears the problem.   The manual says to keep the fuel  pressure below manifold Pressure by 5 psi and fuel pressure min at airbox  pressure plus 0.25  bar (3.5 psi ish).

  

 Glad to hear that your having no problem with the sender, as the original  UMA differentials did not work with the GRT EIS 4000, which is quite popular in  the states.  I personally do not use one, but they are handy.  

  

  

 In my opinion, I can't do anything about the fuel pressure differential in  flight.  The operations manual gives the min and max, but you can't do  anything about it.  A parallel system adds volume, not pressure in a quick  glance.  Series can add pressure, but has redundancy problems if a pump  clogs.  So we go with a series parallel system.  My head hurts.   I'll go with Piersburg analysis that MRC did in the link above and perhaps add  check valves as prescribed, but not change my old system.  However, don't  get slack as new systems should be plumbed by the book.

  

 On a more lighthearted note:

 As for flying,  I monitor fuel pressure so as to monitor my filters  and delivery system.  If I go above 15,000 in full turbo (haven't yet) I'll  be sure to turn on the other pump.  Rotax says that the fuel  pressure of the electric fuel pump must not exceed the manifold pressure by  more than 0.35 bar (5psi) and the airbox boost can't exceed the fuel  pressure.  In that case I'll do the math  34" MP less outside  pressure from my standard tables (everyone is not a geek and memorized the  tables) of 17"  is 13 psi ish differential (34-17=27/2= 13.5 psi)  and my fuel pressure is 3 psi because of the lower atmospheric pressure  aiding the pump, OH, that's a big gap TURN ON THE OTHER PUMP as the  pressure in the airbox is running high and pushing the fuel out of the carbs.  (Tongue in cheek of course as the engine would have failed before I got  this far.)

  

 Seriously,

 Do it my way above and get a head ach or install the differential  gauge, do less math. The differential only gives you an idea of your fuel  pressure regulator health, and if you can't maintain the min and max, land and  investigate.  Without the device, monitor your fuel pressure and manifold  pressure.  If the engine acts up, select reserve (other filter if plumbed  that way) and turn on the other pump.  Note MP and fuel pressure.  If  it runs OK, continue and land as soon as practical, if not land as soon as  possible and investigate.   

  

 Bud Yerly

  

  

 [quote]   ---

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