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<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
----- Original Message -----
From: Tony Krzyzewski<mailto:tonyk@kaon.co.nz>
To: europa-list@matronics.com<mailto:europa-list@matronics.com>
Sent: Tuesday, June 30, 2009 5:24 PM
Subject: RE: Europa-List: Fuel system
<tonyk@kaon.co.nz<mailto:tonyk@kaon.co.nz>>
>>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.
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
http://www.matronics.com/Navigator?Europa-List<http://www.matronics.com/N
avigator?Europa-List>
http://www.matronics.com/contribution<http://www.matronics.com/contributi
on>
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