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frans(at)privatepilots.nl
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 Posted: Wed Aug 01, 2012 12:48 pm Post subject: Vapour lock? Pressure regulator? |
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Hallo Jim & Heather,
| Quote: | So we're pretty confident the problem is not MOGAS.
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Ok, I got multiple messages like this.
Because of these messages I set aside my vapor lock assumption and I did
some additional investigations.
Fuel filters look fine.
Fuel flow looks OK (on both individual pumps).
Fuel pressure holds ok on full power.
However...
During flight testing I could reproduce the problem ((at)4000 ft, 25C), if
I go to idle, wait a few seconds, and then (smoothly) open the throttle
up to 115% power. What happens then is that the fuel pressure drops for
half a second, the engine falls back in power, and then the pressure
recovers and the engine runs fine, and keeps running fine at full power
as long as I want, even on just one fuel pump. Pressure is then rock
stable at 3.8 psi, exactly the Rotax recommended value.
The short drop in fuel pressure is odd. I can't recall that I have seen
that before. (Well, not as much. I have always seen a slight movement of
the needle with significant power changes, but can't remember having
seen such a sharp drop, but maybe I don't remember it correctly).
It doesn't matter whether I have both pumps on or only one. On idle it
is 3.8 psi. on full power it is 3.8 psi, but during the transition I see
it fall back to 2 psi and then come up again.
If it would be a blocked filter then the fuel pressure should
permanently drop I think. And it should make a difference with one or
both pumps on. But it doesn't.
For the 914, the fuel pumps always have full flow. What the engine
doesn't consume is returned to the tank. From the pumps perspective,
nothing changes with different power settings.
The only plausible thing I can come up with, is that the fuel pressure
regulator is "sticky", i.e. needs to overcome some friction befor its
valve adapts to a changed pressure demand. So, if the turbo kicks in,
the airbox pressure increases so the fuel pressure has to follow. If the
fuel pressure doesn't follow I see a drop on my differential gauge and
the carbs starve for fuel. Once the pressure is restored the engine runs
fine on full power.
What I would expect though is a similar (but reversed) excursion from
full power to idle, i.e. a momentary overshoot of fuel pressure. But
this doesn't happen. I don't understand this.
So, what I would like to know:
1) Does anyone recognise these symptoms?
2) Has anyone ever had a faulty fuel pressure regulator?
3) Is it normal do see a momentary drop in fuel pressure when going from
idle to full power? (I'm not slamming the throttle open, but take one
second or so, to give carbs, prop, etc. some time to adapt).
4) If indeed this is not normal but it is not the pressure regulator,
what else could it be? Any ideas?
At least the problem seems altitude/temperature related, getting worse
with higher altitudes/higher temperatures.
Thanks everyone for convincing me that it could be something else than
vapor lock!
Frans
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rparigoris
Joined: 24 Nov 2009
Posts: 805
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| Posted: Wed Aug 01, 2012 1:12 pm Post subject: Vapour lock? Pressure regulator? |
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Hi Frans
You mention you checked fuel flow. Did you check fuel flow to the carbs with the fuel pressure regulator cheated to thinking it's at ~ 18K?
Did you follow Rotax and replace the float pivot pins> Did you replace the fuel pressure regulator if it was one of suspect performance as per Rotax?
Did you check your fuel tank vent for being free and clear?
Ron Parigoris
[quote][b]
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Remi Guerner
Joined: 14 Dec 2010
Posts: 284
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| Posted: Thu Aug 02, 2012 10:10 am Post subject: Re: Vapour lock? Pressure regulator? |
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Hi Frans,
I am following this thread from the beginning with interest. It is also my opinion that your problem is not caused by vapor lock. I do not think it is a fuel pressure problem either as lack of fuel makes the engine sputter, not just loosing power as you described.
If your engine lost power while still running smoothly, it is more likely caused by something wrong with the control system of the turbo. Could be any component of the system : waste gate, waste gate bowden cable, waste gate actuator, TCU (hardware or software), sensors or the associated wiring and plumbing.
To determine if this is the cause, I suggest you try to run the engine with the waste gate in a frozen position. Stabilize the aircraft at the Barcelonnette 4000 ft at full 115% power. Turn off the TCU switch. Then the waste gate will remain in a fixed position. Reduce power to idle. Increase power to 115% as you would do for taking off and see what happens. Repeat power variations several times. If the engine delivers the power every time without showing the problem that you experienced, then the cause is somewere in the turbo control system.
Note that the process of taking off with an inactive TCU is described in the 914 Operator Manual, paragraph 3.6.2.
Good luck !
Remi Guerner
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frans(at)privatepilots.nl
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| Posted: Thu Aug 02, 2012 11:10 am Post subject: Vapour lock? Pressure regulator? |
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Hi Remi,
| Quote: | I am following this thread from the beginning with interest. It is
also my opinion that your problem is not caused by vapor lock. I do
not think it is a fuel pressure problem either as lack of fuel makes
the engine sputter, not just loosing power as you described.
|
Well, the fuel pressure dropped. I can't argue around that. I also
closely monitored the manifold pressure, but it didn't surge in any way.
| Quote: | If your
engine lost power while still running smoothly,
|
It didn't run exactly smoothly, it didn't sputter either, it was more a
rough running.
| Quote: | To determine if this is the cause, I
suggest you try to run the engine with the waste gate in a frozen
position. Stabilize the aircraft at the Barcelonnette 4000 ft at full
115% power.
|
Good idea, although I'm not at Barcelonnette anymore. But I think I can
repeat the procedure while flying at altitude. (Problem at Barcelonnette
was that the brakes didn't hold full power and there wasn't anything to
attach the plane to, except for the runway parallel tie down steel cable
but it had some gliders attached to it as well. So experiments were
limited and I definitely didn't want to land in the Ubaye, which is fine
for rafting but not for landing.)
| Quote: | Turn off the TCU switch. Then the waste gate will remain
in a fixed position. Reduce power to idle. Increase power to 115% as
you would do for taking off and see what happens.
|
Sounds like a good experiment. I thougth already that maybe the waste
gate is sticky, and while opening after reaching enough pressure, it
gives a tiny boost peak, too fast to show up on my manifold pressure
gauge, but enough to trigger the fuel pressure regulator.
I will also lock the prop in the max power setting, to rule that out
too. So, with the waste gate locked and the prop also locked it is
interesting to see what happens to my little problem.
Thanks,
Frans
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h&jeuropa
Joined: 07 Nov 2006
Posts: 650
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| Posted: Fri Aug 03, 2012 3:52 am Post subject: Re: Vapour lock? Pressure regulator? |
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Hi Frans,
| Quote: | So, what I would like to know:
1) Does anyone recognise these symptoms?
2) Has anyone ever had a faulty fuel pressure regulator?
3) Is it normal do see a momentary drop in fuel pressure when going from
idle to full power? (I'm not slamming the throttle open, but take one
second or so, to give carbs, prop, etc. some time to adapt).
4) If indeed this is not normal but it is not the pressure regulator,
what else could it be? Any ideas?
|
We replaced our fuel regulator when trying to figure out why our pressure varied about +/- .5 psi. The old regulator diaphragm had a funny appearance that the Rotax tech hadn't seen before. It's a pretty simple device and you can open it and clean the valve inside.
We do not see a change in fuel pressure when going from idle to 100% power (we haven't found a need to use 115% power even at high density altitude) and our EFIS is set to alarm at +/- .5 psi from nominal.
We think Remi is on the right track.
Jim & Heather
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Remi Guerner
Joined: 14 Dec 2010
Posts: 284
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| Posted: Fri Aug 03, 2012 4:22 am Post subject: Re: Vapour lock? Pressure regulator? |
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Hi Frans,
A word of caution when running the engine with the TCU inactive: do not overboost the engine. Let say you turn off the TCU at full power at 4000 ft; then at a lower altitude the fixed wastegate will allow the engine at full throttle to produce more than 115% power. I also suggest you reduce power before switching the TCU back on.
Hope that helps.
Remi
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