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sidsel.svein(at)oslo.onli
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 Posted: Mon Jul 30, 2012 11:14 pm Post subject: SV: Vapour lock? Why? |
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Frans,
If vapour is formed anywhere in the fuel line, the bubbles will move towards the carbs as soon as a pump is switched on. I do not know the length of the fuel line from the pump to the carbs, but let us for argument's sake assume 3 m. This mean an internal volume of 0.15 liter, plus say 0.05 liter for one filter and a gascolator (if fitted). This means that when a total volume of 0.2 liter has been pumped through the line, there should be no more vapour bubbles (with one exception, see below).
I do not know how much fuel is returned to the tank in a 914 installation during ground operation. Engine consumption is also unknown, but I am sure it is at least "some liters" per hour. At consumption of 1 liter/hour it takes 12 minutes for 0,2 liter to flow through; at 5 liter/hour it takes 2.4 minutes. Any fuel return flow reduces this time. With some idling for warm-up, taxi to holding, run-up and line-up, I believe that much more than 0.2 liter is pumped through the line before you begin the take-off roll.
Theoretically, bubbles may get "stuck" in the filter or gascolator (if fitted) at low flow, and then "torn loose" when the flow increases at full throttle. My 912ULS consumes abt. 33 liters/hour at full take-off RPM/power. At this flow, it would take 24 seconds for a bubble to pass through the entire 3 meter long system, less if it had been trapped somewhere closer to the engine.
What does the transient pressure drop that you observed indicate? There will be a pressure drop just when a bubble coming from upstream of the pump passes through it, and when a bubble passes through the pressure regulator and into the fuel return line (I do not know the details of the 914 installation, so I may be mistaken here). Both drops will be of very short duration, however.
A pressure drop in itself will not cause any change in engine performance, but reduced pressure may cause too low fuel flow to the carb bowls. A fuel vapour bubble reaching a carb has of course the same effect - too little fuel.
What speaks against vapour lock as the cause is your observation that the engine afterwards ran fine at low/medium throttle but not at high, and that the fuel pressure then dropped. This would indicate that the fuel flow is restricted (as already suggested on this forum). Later on all was fine, you report. Has the restriction migrated to the filter, where it does not do so much harm? Hmmmmm………
Your question regarding AVGAS vs. MOGAS: Wikipedia states that "Avgas has a lower and more uniform vapor pressure than automotive gasoline so it remains in the liquid state despite the reduced atmospheric pressure at high altitude, thus preventing vapor lock."
Regards,
Svein
LN-SKJ
[quote][b]
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tennant
Joined: 19 Apr 2011
Posts: 121
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| Posted: Tue Jul 31, 2012 12:11 am Post subject: Re: SV: Vapour lock? Why? |
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Hi Franz,
I had a very scary episode about 12 years ago with my Europa taking off from a glider site in Germany.
1500 ft elevation & 36 deg C temps. I had to taxi through long grass at quite high power to get to a short tarmac runway. Take off roll was fine and only when I was in the air did the stuttering and power loss begin. Luckily the glider site is on a hill so I could get up some speed descending into the valley.
She stuttered her way up to about 1000 ft above the field and I waited to see if the "vapor lock" would clear but after 5 mins landed again to change my pants!! There was no pressure loss, or at least my warning lamp did not come on which is quite sensitive.
I took the cowling off straight away and removed one of the fuel lines from the carbs. Fuel spit me in the face so I was sure at that time that it was a vapor lock. I had had enough for one day and called someone to bring my trailer.
The next day I tried a run up on the trailer. Everything was OK until I tried full power then the stuttering began again, no vapor lock this time!!
I searched for the problem for a month before in frustration I wanted to measure the fuel flow from the pumps and found that about 50% of what was coming out of the fuel line at a carb was air!!
I had no obvious fuel leaks but air was getting into the system.
I changed all the fuel hoses (Europa originals), re-routed them with insulation, changed the clips and no more air.
I am still not sure that it was a vapor lock on that day but it certainly made me re think my fueling strategy. I now always fill with Avgas when the temperature is approaching 30 deg C or bellow 0 deg C.
No idea and not suggesting that it ha anything to do with your case. Just an experience.
Barry
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Barry Tennant
D-EHBT
At EDLM - Germany |
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frans(at)privatepilots.nl
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| Posted: Tue Jul 31, 2012 12:37 am Post subject: SV: Vapour lock? Why? |
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Hi Svein,
| Quote: | I do not know how much fuel is returned to the tank in a 914
installation during ground operation.
|
The 914 has no mechanical fuel pump, only two electrical pumps. These
pumps pump at least 60 liters per hour through the system. Always,
including during idle. So the vast majority of the fuel returns to the
tank after passing the pressure regulator which is mounted on top of the
air box.
| Quote: | consumption of 1 liter/hour it takes 12 minutes for 0,2 liter to flow
through; at 5 liter/hour it takes 2.4 minutes.
|
For a 914 we are talking about seconds here.
| Quote: | A pressure drop in itself will not cause any change in engine
performance, but reduced pressure may cause too low fuel flow to the
carb bowls.
|
On a 914 the fuel pressure is regulated over the airbox pressure. So, if
at full power the turbo kicks in and the carbs are pressurised, the fuel
pressure has to go up with it. On this particular mountain field the
ambient pressure was about 24" (this was indicated by my manifold
pressure gauge before starting the engine). The 914 has 40 inch of
manyfold pressure on take off power, slightly more in the airbox. The
fuel pressure has to remain 6 psi above this. See where this is going?
What I think has happened is that the mogas vaporised in the fuel
filters, due to the pressure drop, caused by the flow resistance of the
filters. The filters are before the fuel pumps. So any vaopur forming in
the fuel filters will flow to the pumps, which then turn into a void.
At lining up, I switched on both fuel pumps. Now two pumps where sucking
on the fuel filters, probably releasing a lot of bubbles at the instant
I switched on the second fuel pump. About 5 to 10 seconds later, the
engine hesistated. I think this was the moment where the vapour bubbles
reached the pressure regulator, at the very same time the turbo reached
maximum pressure and the fuel pressure had to go up in a similar fashion.
The only way to prevent this from happening (except for using avgas or
avoiding high and hot take offs) is to use a fuel pump mounted inside
the fuel tank, to get the pressure up before the fuel even leaves the
tank and avoiding the pressure drop at the suction side and filters
altogether.
| Quote: | What speaks against vapour lock as the cause is your observation that
the engine afterwards ran fine at low/medium throttle but not at high,
and that the fuel pressure then dropped.
|
My fuel pressure gauge indicates relative fuel pressure, i.e. fuel
pressure on top of the airbox pressure. The pressure regulator keeps the
pressure at 6 psi above the airbox pressure at all times. So, regardless
of airbox pressure, turbo pressure or throttle setting, all I should see
is 6 psi of fuel pressure (which is the same as the carbs are seeing
because they live in a pressurised world; their vent lines are connected
to the airbox). In reality though the absolute fuel pressure has to go
up with the turbo pressure to maintain this relative pressure. This
relative fuel pressure could no longer be maintained when the turbo
pressure went up, hence the indicated pressure drop on the gauge. The
absolute pressure probably remained the same because it was during idle
already on the max that the pumps could deliver in this particular
circumstance. The vent line of the carbs is connected to the airbox, so
if the airbox pressure goes up but the fuel pressure doesn't follow, the
fuel will stop flowing into the carbs.
Hmm, the explanation is a bit chaotic but I think you will understand it
anyway.
If I had become airborn, I could probably have saved the day by
throttling back to a throttle setting where the airbox pressure would
fall below the fuel pressure. At worst this would have been 24" because
this was the ambient pressure at that altitude. I think I can maintain
altitude with 24" and probably even climb a bit, but sure it won't have
been fun. Provided of course that I had enough common sense to throttle
back against instinct before the engine stalled due to lack of fuel.
| Quote: | Your question regarding AVGAS vs. MOGAS: Wikipedia states that /"Avgas
has a lower and more uniform vapor pressure than automotive gasoline so
it remains in the liquid state despite the reduced atmospheric pressure
at high altitude, thus preventing vapor lock."/
|
Yep. I have seen that too. Lesson learned.
Frans
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frans(at)privatepilots.nl
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| Posted: Tue Jul 31, 2012 12:56 am Post subject: SV: Vapour lock? Why? |
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Hi Barry,
| Quote: | 1500 ft elevation & 36 deg C temps. I had to taxi through long grass
at quite high power to get to a short tarmac runway. Take off roll
was fine and only when I was in the air did the stuttering and power
loss begin. Luckily the glider site is on a hill so I could get up
some speed descending into the valley. She stuttered her way up to
about 1000 ft above the field and I waited to see if the "vapor lock"
would clear but after 5 mins landed again to change my pants!! There
was no pressure loss, or at least my warning lamp did not come on
which is quite sensitive.
|
Must have been a scary experience!
Do you have a 914? The only correct way to measure fuel pressure in that
case is to measure relative fuel pressure, i.e. fuel pressure compared
to airbox pressure. The 914 has no "static" fuel pressure. Sufficient
fuel pressure for a low throttle setting may be insufficient for higher
throttle settings.
| Quote: | time!! I searched for the problem for a month before in frustration I
wanted to measure the fuel flow from the pumps and found that about
50% of what was coming out of the fuel line at a carb was air!! I had
no obvious fuel leaks but air was getting into the system.
|
Are you sure it was air? I had the very same problem and spent almost a
whole frustrating day searching for the air leak. There wasn't any, the
bubbles where fuel vapor bubbles, which form inside the suction line and
fuel filter due to the pressure drop caused by the fuel pump.
| Quote: | I changed
all the fuel hoses (Europa originals), re-routed them with
insulation, changed the clips and no more air.
|
Maybe it was a colder day, or different fuel...
| Quote: | I am still not sure
that it was a vapor lock on that day but it certainly made me re
think my fueling strategy. I now always fill with Avgas when the
temperature is approaching 30 deg C or bellow 0 deg C.
|
Well, most of my flying is at higher temperatures or at higher
altitudes, so this pretty much bans the use of mogas unless I solve this
problem.
Frans
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tennant
Joined: 19 Apr 2011
Posts: 121
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| Posted: Tue Jul 31, 2012 2:01 am Post subject: Re: SV: Vapour lock? Why? |
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Hi Frans,
As I said previously, my case is surely different to your experience.
I have a humble 912 - 80 hp.
It was definitely air and since I changed the hoses it has gone. The original Europa hoses in the mid 90's were not compatible with Mogas and they split at the ends after 2 or 3 years on Mogas.
I think that my problem was a combination of vapor and porous hoses, it is only strange that it showed itself at the same time, or one caused the other.
Barry
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_________________
Barry Tennant
D-EHBT
At EDLM - Germany |
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sidsel.svein(at)oslo.onli
Guest
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| Posted: Tue Jul 31, 2012 10:58 am Post subject: SV: Vapour lock? Why? |
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Frans,
In the tank bottom there are no bubbles at the temperature in question (otherwise it would be boiling). Fuel pressure here is atmospheric pressure plus static pressure (negligible) from the fuel column above. At the fuel pump inlet, suction means lower pressure than in the tank, and here you could get bubbles if the fuel temperature is not too far from the phase shift point liquid-to-vapour, especially as the filter is upstream of the pump (between the tank and the pump) since the filter restricts the flow a little.
If the fuel pressure after the pump is equal to or higher than the pressure in the bottom of the tank, and there is no heating of the fuel on its way so far, the bubbles will disappear. The transition from liquid to vapour or from vapour to liquid is virtually instantaneous. This phase shift follows a certain temperature/pressure curve - there is no hysteresis (no difference going from liquid to vapour, compared to from vapour to liquid). With the pressure you state in one of your postings, meaning considerably higher pressure after the pump than at the bottom of the tank, I cannot see how a gas bubble created due to suction can "survive" the passage through the pump, unless heated up on its way through the pump/hose.
My posting earlier today described the movement of a vapour bubble that exists downstream of (after) the pump. If the heat downstream is such that gas bubbles will form despite the relatively high pump pressure, then gas bubbles will of course reach the carbs unless vented back through the return line.
Given the composition/characteristic of the fuel on board and given the fuel pressure, the only variable we can do anything with is the fuel temperature.
In addition to fire sleeves on all engine compartment fuel and oil hoses and good shielding from direct exhaust pipe heat, I also use heat protection sleeves (silvery outer layer over insulation layer) placed outside the fire sleeve forward of the fire wall. Also T and X metal joints are insulated. Aft of the fire wall I have used the heat sleeve several places as wear protection, but it is so lightweight that one may use it all over. You can buy these at auto supply stores.
Given a choice of fuel, Avgas in hot weather/high altitude seems to be the best option to avoid vapour lock, as also confirmed by the very interesting posting today regarding LAA’s restriction on Mogas use. Replacing engine oil and filter more frequently is an inexpensive consequence of using Avgas. And never use Mogas purchased in the winter during the warmer period of the year.
Regards,
Svein
LN-SKJ
[quote][b]
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frans(at)privatepilots.nl
Guest
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| Posted: Tue Jul 31, 2012 11:54 am Post subject: SV: Vapour lock? Why? |
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Svein,
| Quote: | If the fuel pressure after the pump is equal to or higher than the
pressure in the bottom of the tank, and there is no heating of the fuel
on its way so far, the bubbles will disappear. The transition from
liquid to vapour or from vapour to liquid is virtually instantaneous.
|
I thought (but I'm in no way an expert) that for a fluid to boil you
need some contamination which acts as some sort of trigger. Just like
bubbles in a glass of cola seem to form at certain spots. Once just a
bit of vapour has accumulated, other molecules hook in and the bubble
keeps growing.
So if the fuel temp is on the edge, once vapour bubbles exist it is
easier for other molecules to take the same route.
On the other hand, I see what you are saying. If there is no hysteresis
then bubbles should disappear under pressure. Then again, it would be
impossible to get vapor lock unless you heat the fuel. (Which is what
you are saying).
| Quote: | My posting earlier today described the movement of a vapour bubble that
exists downstream of (after) the pump.
|
Ok, I believe you. Then the problem would only be at the pump inlet and
not beyond. Still, the pump would then turn into a void. With a bubble
inside the pump the pump can not maintain pressure, so the transition
from gas to fluid doesn't take place.
| Quote: | Given the composition/characteristic of the fuel on board and given the
fuel pressure, the only variable we can do anything with is the *fuel
temperature*.
|
The engine was just started. The oil temp was about 60 degrees. We where
anxious to take off because of the heat in the cockpit. There was no
heat anywhere in the fuel system except for heat created by the sun.
Again, this is a 914 with a very high fuel return flow. Any heat
somewhere in the system will be carried away quickly.
The only thing remaining is the fuel pressure drop inside the suction
area, i.e. the fuel filters.
| Quote: | In addition to fire sleeves on all engine compartment fuel and oil hoses
and good shielding from direct exhaust pipe heat, I also use heat
protection sleeves (silvery outer layer over insulation layer) placed
outside the fire sleeve forward of the fire wall.
|
I have this too. Keep in mind that the 914 has no mechanical fuel pump.
The fuel enters the engine compartment to the airbox where the pressure
regulator is, and then returns back to the firewall. It is a very short
route, and fuel is flowing there with a rate of 1 liter per minute.
Vapor lock between the regulator and carbs is of no consequence, as the
needle valves of the carb just let any gaseous fuel through until some
fluid matter raises the floats.
| Quote: | Given a choice of fuel, Avgas in hot weather/high altitude seems to be
the best option to avoid vapour lock, as also confirmed by the very
interesting posting today regarding LAA’s restriction on Mogas use.
Replacing engine oil and filter more frequently is an inexpensive
consequence of using Avgas.
|
Despite changing the oil and filter, the engine contaminates
nevertheless. There are some interesting documents with pictures on the
internet from the British Rotax dealer. Avgas is just bad for the Rotax.
And it is more expensive. Enough reasons to investigate how we can
operate more reliably on mogas.
Thanks for sharing your insight.
I guess it is time to wait for an exceptional hot day, pour some mogas
in the poor thing, switch on the electric pumps, and then make some
pictures at various places in the fuel system about what is travelling
through the fuel lines. With our insights combined we would see a lot of
bubbles in the fuel pump inlet, and less (or none) after the fuel pump.
Frans
| | - The Matronics Europa-List Email Forum - | | | Use the List Feature Navigator to browse the many List utilities available such as the Email Subscriptions page, Archive Search & Download, 7-Day Browse, Chat, FAQ, Photoshare, and much more:
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grahamsingleton(at)btinte
Guest
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| Posted: Tue Jul 31, 2012 12:15 pm Post subject: SV: Vapour lock? Why? |
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Frans
if the fuel boils in the carb bowls where does the vapour go? On a 912 it goes into the inlet manifold anyway so will cause rich mixture.
And displace some air. In the 914 it will be at manifold pressure so may force its way into the needle jet and again cause rich mixture.
Graham
From: Frans Veldman <frans(at)privatepilots.nl>
To: europa-list(at)matronics.com
Sent: Tuesday, 31 July 2012, 21:47
Subject: Re: SV: Vapour lock? Why?
--> Europa-List message posted by: Frans Veldman <frans(at)privatepilots.nl (frans(at)privatepilots.nl)>
Svein,
| Quote: | If the fuel pressure after the pump is equal to or higher than the
pressure in the bottom of the tank, and there is no heating of the fuel
on its way so far, the bubbles will disappear. The transition from
liquid to vapour or from vapour to liquid is virtually instantaneous.
|
I thought (but I'm in no way an expert) that for a fluid to boil you
need some contamination which acts as some sort of trigger. Just like
bubbles in a glass of cola seem to form at certain spots. Once just a
bit of vapour has accumulated, other molecules hook in and the bubble
keeps growing.
So if the fuel temp is on the edge, once vapour bubbles exist it is
easier for other molecules to take the same route.
On the other hand, I see what you are saying. If there is no hysteresis
then bubbles should disappear under pressure. Then again, it would be
impossible to get vapor lock unless you heat the fuel. (Which is what
you are saying).
| Quote: | My posting earlier today described the movement of a vapour bubble that
exists downstream of (after) the pump.
|
Ok, I believe you. Then the problem would only be at the pump inlet and
not beyond. Still, the pump would then turn into a void. With a bubble
inside the pump the pump can not maintain pressure, so the transition
from gas to fluid doesn't take place.
| Quote: | Given the composition/characteristic of the fuel on board and given the
fuel pressure, the only variable we can do anything with is the *fuel
temperature*.
|
The engine was just started. The oil temp was about 60 degrees. We where
anxious to take off because of the heat in the cockpit. There was no
heat anywhere in the fuel system except for heat created by the sun.
Again, this is a 914 with a very high fuel return flow. Any heat
somewhere in the system will be carried away quickly.
The only thing remaining is the fuel pressure drop inside the suction
area, i.e. the fuel filters.
| Quote: | In addition to fire sleeves on all engine compartment fuel and oil hoses
and good shielding from direct exhaust pipe heat, I also use heat
protection sleeves (silvery outer layer over insulation layer) placed
outside the fire sleeve forward of the fire wall.
|
I have this too. Keep in mind that the 914 has no mechanical fuel pump.
The fuel enters the engine compartment to the airbox where the pressure
regulator is, and then returns back to the firewall. It is a very short
route, and fuel is flowing there with a rate of 1 liter per minute.
Vapor lock between the regulator and carbs is of no consequence, as the
needle valves of the carb just let any gaseous fuel through until some
fluid matter raises the floats.
| Quote: | Given a choice of fuel, Avgas in hot weather/high altitude seems to be
the best option to avoid vapour lock, as also confirmed by the very
interesting posting today regarding LAA’s restriction on Mogas use.
Replacing engine oil and filter more frequently is an inexpensive
consequence of using Avgas.
|
Despite changing the oil and filter, the engine contaminates
nevertheless. There are some interesting documents with pictures on the
internet from the British Rotax dealer. Avgas is just bad for the Rotax.
And it is more expensive. Enough reasons to investigate how we can
operate more reliably on mogas.
Thanks for sharing your insight.
I guess it is time to wait for an exceptional hot day, pour some mogas
in the poor thing, switch on the electric pumps, and then make some
pictures at various places in the fuel system about what is travelling
through the fuel lines. With our insights combined we would see a lot of
bubbles in the fuel pump tp://www.matronics.com/Navigator?Europa-List" ="http://forums.matronics.com/" target="_blank">http://forums.matronics  tronics.com/contribution" target="_blank">http://www.matronics.com/contri================
[quote][b]
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