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  #1  
Old 09-15-2019, 01:09 PM
bkervaski's Avatar
bkervaski bkervaski is offline
 
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Default Lean of Peak ... I smell fuel?

So getting my head around Lean of Peak, I got the basics.

I've gone LoP twice, once on the way to Airventure very briefly, thought I smelled fuel so I went back RoP. My passenger didn't smell anything.

Today, went LoP (7500 feet) and smelled fuel again, this time the same passenger did smell it.

I never smell fuel any other time.

I'm assuming that it's just the overflow from the engine driven fuel pump vent line making its way back into the airplane from suddenly going rich to lean?

My fuel pressure usually stabilizes around 28psi (without the boost pump).

At any rate, scared me off trying again until I figure it out.

Thoughts?
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Last edited by bkervaski : 09-15-2019 at 09:22 PM.
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  #2  
Old 09-15-2019, 01:40 PM
mturnerb mturnerb is offline
 
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I would pull inspection covers and look for fuel stains in the tunnel and along the route of all the fuel lines. If you have the standard fuel system running LOP shouldn't have any effect on presence of fuel in cabin IMHO.
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  #3  
Old 09-15-2019, 02:10 PM
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Mark Dickens Mark Dickens is offline
 
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I'm hardly an expert, but the engine driven pump shouldn't be pumping gas back into the plane...there's an internal bypass valve built into the pump so that excess pressure is bypassed but not pumped back into the plane. Are you sure that the pump is not pumping gas out of the overflow line and overboard? I'd think if you have leaks in the system, you'd know it by now, so I discount that. I'd look at your overflow line and see if you have any blue stains. Just a guess!
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  #4  
Old 09-15-2019, 02:23 PM
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bkervaski bkervaski is offline
 
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Quote:
Are you sure that the pump is not pumping gas out of the overflow line and overboard?
Yes, this is what I think is happening .. and the fuel is somehow making its way to the air vents .. anyone ever experience something similar?

Quote:
I would pull inspection covers and look for fuel stains in the tunnel and along the route of all the fuel lines. If you have the standard fuel system running LOP shouldn't have any effect on presence of fuel in cabin IMHO.
I never smell fuel any other time, only the two times I've gone LoP and those were 20 hours apart from each other. Once I went back RoP the smell went away.

Curious.
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  #5  
Old 09-15-2019, 02:29 PM
Cannon Cannon is offline
 
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Pull the cowl and look for staining around the injectors.

Having fuel burp out of the injectors when LOP is not unheard of. There’s a whole thread over at the BeechTalk forums about this. It’s a long thread, but might be what is happening here.

Basically, the local pressures both inside the intake and around the cylinders is very chaotic. The pressure spikes caused by the intake valve closing can be higher than the upper deck pressure, and when fuel pressure at the injector is low enough (this lowering the venturi effect in the injector) the fuel can back up out the injector air vents. Sometimes happens to Mooney guys (Lycoming powered) with the ram air intake open, and apparently it’s quite common amongst the Reno racers as well. Maybe one of them will chime in.

If there’s no other sources of fuel leaks found, try doing running LOP again, but this time pull the throttle back by 1/2” of MP and/or change the RPM. That will often stop the burping.

If this is what’s happening and you can smell it, you might also have some firewall sealing issues as well, though it could just as easily be working its way out of the cowl vs through the firewall.

Someone actually filmed it happening in his Bonanza.

https://youtu.be/cvepzhj_OLE
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Last edited by Cannon : 09-15-2019 at 03:20 PM.
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  #6  
Old 09-15-2019, 05:13 PM
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DanH DanH is offline
 
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Quote:
Originally Posted by bkervaski View Post
Today, went LoP (7500 feet) and smelled fuel again, this time the same passenger did smell it.
There is a phenomenon called "auto rough", proven to occur when flying over bodies of water, right after spotting a shark.

Quote:
I'm assuming that it's just the overflow from the engine driven pump making its way back into the airplane from suddenly going rich to lean?
There is no overflow from the engine driven pump, and no path back into the airplane.

Quote:
My fuel pressure usually stabilizes around 28psi (without the boost pump).
Right. It's determined by diaphragm spring pressure, which is the same rich or lean.

Quote:
Originally Posted by Mark Dickens View Post
...there's an internal bypass valve built into the pump so that excess pressure is bypassed but not pumped back into the plane.
No, there is not. See above.

Quote:
Originally Posted by bkervaski View Post
Yes, this is what I think is happening .. and the fuel is somehow making its way to the air vents .. anyone ever experience something similar?
The RV-8's rear seat air vent intake under the right wing will pick up some fuel smell if the right tank is full, and burps a little out the tank vent during climbout.

Quote:
Originally Posted by Cannon View Post
Pull the cowl and look for staining around the injectors.
Having fuel burp out of the injectors when LOP is not unheard of. There’s a whole thread over at the BeechTalk forums about this. It’s a long thread, but might be what is happening here.
Just signed up with Beechtalk, as I'd like to hear the arguments. It's a recent interest. I made two sets of deltaP measurements, upper plenum next to the nozzle vs MP gauge. Then I went to a much more detailed method, using a fast differential pressure sensor recording intake tract pressure (at the primer port) vs nozzle bleed pressure in a turbo injector rail.

The first two sets are really overall averages, slow analog stuff. They did not indicate any circumstance in which the 390 suffered from a pressure reversal in the nozzle...bleed was always higher than manifold.

However, averages might easily miss some period in each 720 degree 4-stroke cycle where there could be a flow reversal in the nozzle bleed, thus the fast sensor recording on my laptop. The detailed record shows one very short period of pressure reversal, and the amplitude is quite small. I very much doubt there is any significant fuel escaping from the nozzle bleed.

The data is on my laptop, which is at the office today. I'll try to post a plot later.

Quote:
Basically, the local pressures both inside the intake and around the cylinders is very chaotic.
I might buy into the idea of turbulent upper plenum pressure, thus highly variable bleed source pressure. I have not attempted to record 720 degree cycles using an open static source near a nozzle. Probably would not do any good, as it might vary a lot from airplane model to airplane model. I can demonstrate that the deltaP across the nozzle is very stable, cycle to cycle, when fed with a turbo rail.

Quote:
The pressure spikes caused by the intake valve closing can be higher than the upper deck pressure, and when fuel pressure at the injector is low enough (this lowering the venturi effect in the injector) the fuel can back up out the injector air vents.
The assumption here is that air is drawn into the nozzle by a venturi effect. It is not, something pretty clear from the fast pressure measurements. It enters because there is a pressure difference between the bleed source and the intake port.

Quote:
Someone actually filmed it happening in his Bonanza.

https://youtu.be/cvepzhj_OLE
Fascinating. That guy is losing a lot of fuel. Got a link to the story?
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Last edited by DanH : 09-15-2019 at 05:16 PM.
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  #7  
Old 09-15-2019, 05:40 PM
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bkervaski bkervaski is offline
 
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Quote:
There is a phenomenon called "auto rough", proven to occur when flying over bodies of water, right after spotting a shark.


Real fuel smell in this case, not faint or brief, pronounced. No leaks as far as I can tell. Baffled.
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  #8  
Old 09-15-2019, 05:55 PM
Cannon Cannon is offline
 
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Sorry, Dan. I don?t have any more info on the video, other than that he was having issues with this phenomenon and decided to film it.

Is it possible that the less-optimized cooling intake from a certified Bonanza would allow the delta-p to be closer than your RV-8? There are several Bonanzas exhibiting this behavior, but most apparently do not. I wonder if the Bonanza cowl design creates conditions that, on average, fall just outside where this can happen, but there?s enough airplane-to-airplane variability that a small percentage of them do. 🤷🏼

In the few cases I?ve read about where it was tried, a slight reduction in MP prior to leaning prevented the fuel from burping out of the injectors. This makes me think that it?s probably a delta-p issue. I?ve been wrong many times before, though...

It would be nice to hear from some of the Reno racers who run turbo injectors with ram air feeds into them to know for sure if this is the reason.

Chris
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  #9  
Old 09-15-2019, 06:03 PM
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Mark Dickens Mark Dickens is offline
 
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I'm sure I'm wrong, but I may have misinterpreted the following from EAA's "Basic Fuel Pump System" by Tony Bingelis:

"When the correct engine driven fuel pump is installed, it will discharge (pump) more fuel than your engine needs . . . actually the pump should be capable of providing a minimum fuel flow of 125% of that required for maximum take-off power. This excess capacity will not be a problem in the operation of your engine as an internal relief valve-factory adjusted to deliver the fuel at the correct pressure for a particular carburetor or fuel injector installation - prevents the development of excessive fuel pressure at the fuel inlet."

That seems to imply that there's a bypass valve. Now I'm very confused. Dan, please explain.
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  #10  
Old 09-15-2019, 06:10 PM
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bkervaski bkervaski is offline
 
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There's a fuel vent line coming out of the engine driven fuel pump that dumps below the cowl .. my best guess so far is that is the source of the fuel smell.

Also, I have my two firewall penetration wraps off due to my pmag install, wonder if this is how vent fumes making their way into the cabin.
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Last edited by bkervaski : 09-15-2019 at 09:22 PM.
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