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Fuel return line attachment point with 912U

 
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mz(at)cariama.nl
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PostPosted: Thu Apr 23, 2009 6:32 am    Post subject: Fuel return line attachment point with 912U Reply with quote
[quote:08f86472ee="skaajo"]If you use the standard Europa supplied restrictor in the fuel return line from 912S, the return flow is an amazingly high 10 liters (approx) / a little less than 3 USG per hour even with full fuel tank pressure head on the return line.[/quote:08f86472ee]
Hi Svein,

You are the first to mention hard return flow numbers for a NON-914 engine, thanks! I would never have expected 10 ltrs/hr fuel running through the return line (I thought it would be MUCH less)

We know that return flow varies with RPM and power setting. Including Svein's return flow number to this piece of knowledge, things seems to implicate that also the VARIATION in return-flow might be considerably larger than previously assumed. If this indeed is the case, correcting the K-factor for the single sender with return flow numbers (established by doing some fuel flow measurements) doesn't cut it either (that is simply too inaccurate to my likings).

This strengthens my believe that for obtaining accurate FF numbers, one either has to go with 2 senders (in feed AND return line), or with one sender in a setup as laid out in the schema that I posted (or is my reasoning incorrect anywhere?)

Since my engine is a 912[b:08f86472ee]UL[/b:08f86472ee], do you or anybody else know if the return flow for the 912S and the 912UL are about the same? (I would expect them to be close)

Marcel
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sidsel.svein(at)oslo.onli
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PostPosted: Thu Apr 23, 2009 7:36 am    Post subject: Fuel return line attachment point with 912U Reply with quote
Marcel,

<We know that return flow varies with RPM and power setting. Including
Svein's return flow number to this piece of knowledge, things seems to
implicate <that also the VARIATION in return-flow might be considerably
larger than previously assumed. If this indeed is the case, correcting the
K-factor for the <single sender with return flow numbers (established by
doing some fuel flow measurements) doesn't cut it either (that is simply too
inaccurate to my <likings).

The return flow is a function only of the fuel pressure in the return line
ahead of the restrictor. The fuel pressure sensor is in the main fuel line
between the carburetors, I assume, but even so the fuel flow rate's effect
on the pressure in the return is probably rather small. Therefore, the
return flow is (mainly) a function of the fuel pressure that you read off
the gauge. I simply do not remember whether the fuel pressure reading
differs much with RPM and load, but I will check that next time I fly. My
understanding is that the engine driven fuel pump on 912ULS is of the
constant pressure type, i.e. the return flow would not show much variation
(but I have been mistaken before!)

<This strengthens my believe that for obtaining accurate FF numbers, one
either has to go with 2 senders (in feed AND return line), or with one
sender in <a setup as laid out in the schema that I posted (or is my
reasoning incorrect anywhere?)

I am writing this from another PC than at home, so I don't have your
proposal at hand, or the purpose.

<Since my engine is a 912[b:08f86472ee]UL[/b:08f86472ee], do you or anybody
else know if the return flow for the 912S and the 912UL are about the same?
<(I would expect them to be close)

I don't know the answer to this, but it will depend on the restrictor and
the fuel pressure. You can easily check whether the fuel pump is the same
on the two engines by looking at their part numbers (Rotax web pages will
give you access to spare parts lists) and the Builders Manual (firewall
forward) on Europa's web pages should tell you whether the two engine
installations have the same restrictor - or an e-mail or phone call to Roger
at Europa should clarify it.

Regards
Svein
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