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Air flow Performance purge valve???

pierre smith

pierre smith

Well Known Member
A while back there was a discussion about purge valves and the dangers. What exactly do they do? I looked at an airplane today that has the EM 200 system and suffered an engine failure, blamed on the purge valve.

I need some enlightenment.....Kahuna??

Thanks,
Pierre
 
The purge valve is attached to the distribution block. It essentially allows you to cut all fuel flow to the cylinders instantly. If the purge valve where to open in-flight inadervdantly, the engine would stop.

On the AFP units, the valve has some minor resistance to turning. I have mine spring loaded to remain closed in case the cable were to break.

If I uderstand its use correctly, it is the only way to completed shut off the flow of fuel to the engine. With the throttle completely closed, some fuel still is provided. The other use is to purge the hot fuel in the lines when doing a hot start of the engine. I don't really use it for that since I have the "three fisted start" technique down to an art.
 
f1rocket said:
I don't really use it for that since I have the "three fisted start" technique down to an art.
Click to expand...

Randy, could you explain the technique?

Kent
 
The failure mode Im aware of is from the set screws not being safety wired.
Set screw backs out, valve begins to seperate, fuel begins to go into bypass.

Ive been around the block on this one with Don and he was not interested in a design change. The RV-List has a good discussion on this. Several off fields and injuries that Im aware of from this specific issue.

Best
 
There was one plane around here that lost an engine right on takeoff because of non-aviation stuff controlling the purge valve. Airplane and pilot undamaged. But, there was a lawnmower cable running the purge valve. I used a nice pushbutton aircraft cable with a rod end on mine. The spring is also a good idea.
 
kentb said:
Randy, could you explain the technique?

Kent
Click to expand...
This is my description of the standard technique for doing a hot start on a fuel injected engine. Essentially:

1) Throttle wide open, hit the fuel pump for a few seconds
2) Mixture full lean
3) Hit the starter
4) When the engine catches, similtaneously pull throttle and advance mixture

With hands flying around the cockpit, sometimes you need three hands.
 
Kahuna said:
The failure mode Im aware of is from the set screws not being safety wired.
Set screw backs out, valve begins to seperate, fuel begins to go into bypass.
Click to expand...
This is the biggie. You absolutely gotta check those screws to make sure they're safety wired. The AFP installation manual is very clear about this (big bold note).

The purge valve is your friend but can quickly become your enemy if you or your engine builder overlook an important step during installation.
 
AlexPeterson said:
There was one plane around here that lost an engine right on takeoff because of non-aviation stuff controlling the purge valve. Airplane and pilot undamaged. But, there was a lawnmower cable running the purge valve. I used a nice pushbutton aircraft cable with a rod end on mine. The spring is also a good idea.
Click to expand...

Take a look at this thread, read post 6 from George, looks like exactly what is needed for the purge valve control,--------except the hinge of course.

http://www.vansairforce.com/community/showthread.php?t=17938

Mike
 
f1rocket said:
This is my description of the standard technique for doing a hot start on a fuel injected engine. Essentially:

1) Throttle wide open, hit the fuel pump for a few seconds
2) Mixture full lean
3) Hit the starter
4) When the engine catches, similtaneously pull throttle and advance mixture

With hands flying around the cockpit, sometimes you need three hands.
Click to expand...

For hot starts on my IO-360A1A I've been doing it this way with a lot of success:

1) Mixture - full rich
2) Throttle - cracked 1/4"
3) Fuel pump - on for a split second, then off
4) Mixture - full lean
5) When the engine catches, advance mixture

The advantage of doing it this way is I only have to move one lever. It works, because I operate the the pump only long enough to get fuel back in the injector lines so therefore the engine doesn't receive enough fuel to get flooded. I'm not saying this is the right way or the method above is the wrong way. It's just what happens to work for me and I like it. In addition I have my starter button on my Infinity stick grip so there are less hands flying around. I too don't have three hands. :)

Jerry
 
pierre smith said:
A I looked at an airplane today that has the EM 200 system and suffered an engine failure, blamed on the purge valve.

I need some enlightenment.....Kahuna??

Thanks,
Pierre
Click to expand...

Pierre what was the purge valve failure mode in this case?
Was anyone hurt?
 
I don't have the exact spec in front of me, but it is an ACS control cable (which can be ordered from Aircraft Spruce, or perhaps directly from ACS) similar to the A-700, only with a 10-32 threaded end. I put a clevis on the threaded end to connect to the AFP purge valve lever. There is a crummy little picture in Aircraft Spruce's website.
 
Where?

Mornin' guys,
Where is the purge valve located? Is it on the rear of the unit that's mounted under the oil sump or somewhere else?

Thanks,
Pierre
 
Allen Barrett said:
The purge valve is mounted with the flow divider on top of the engine, as one unit. Allen
Click to expand...

Allen, I have this system on my airplane, and just by looking at it, I am concerned that the purge valve is putting a lot of stress on the fitting it mounts to (short nipple), as it is cantilevered off that fitting.

Question here is ----------is this mounting adequate, or should there be some kind of bracket in addition to the nipple???

Thanks, Mike
 
OOPS

Well, my memory aint what it used to be.

There is a bracket there already, I just went out to get a couple of photos of the purge valve, and there it was!!!





Mike
 
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More photos, maybe a little clearer:
20030620_purge_valve.jpg

20030620_purge_valve2.jpg
 
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Also notice in my last post that all the screws are SAFETY WIRED? That's the issue. Specifically the screw holding the STOP for the purge valve arm. That sucker HAS TO BE SAFETIED. Why on earth wouldn't you safety it?!
 
Purg cable

I just complete this and used from Aircraft Spruce part no. 05-16272 a-770 control red 6' at 50.75. Works great. I also live about 5 miles from AFP and work with Don on this. :)
 
Yes, that is the cable I used. Mounting the cable is not a no-brainer, due to the design. But, I built a couple little plates which mount to the crankcase bolts to mount the cable. Think of two plates with holes, which are positioned a little out of alignment with one another. And no, I do not have a picture at the moment...

I wouldn't use anything less than a cable like this for anything important, like the AFP purge valve. This is no place for lawnmower cables...
 
Purge Valve Spring

Anyone know what I need to set up my purge valve to be spring loaded to the "engine keep running" position? A part number and a picture of installation would be appreciated.

Thanks.
 
Personally I think I would throw the purge valve away after reading all this. I have been working on FI Lycs and Conts for years with out purge valves and one can learn to start a hot FI engine. The above starting suggestions work real good. I have noticed that if the engine is left sitting for about 15 minutes hot on a hot 110 degree day that is when it is the hardest to start. I guess it "perculates" or vapor locks. When it is in this condition I turn on the fuel pump and open both the mixture and throttle wide then close them both leaving the fuel pump on start the engine. I do this to circulate the fuel in the injector lines. Typically it will start just as normal. Hands flying all over the place is normal. The reason I leave the fuel pump on is it takes longer for the engine to quit if you "miss it". When I shut off an engine to make a small adjustment then go to re start it I don't prime it at all and they start normally. I seem to have the most trouble starting a geared engine. Or a carbruated engine in the winter than a FI engine. With a FI engine it is easier for me to start it somtimes by deliberatly flooding it. With a carbed engine I can't tell when it is flooded. That is when the plugs freeze over.
 
Yes, I installed a spring from the arm on the purge valve to an adel clamp around one of the pushrod tubes. Works great.

Vic
 
pierre smith said:
A while back there was a discussion about purge valves and the dangers. What exactly do they do? I looked at an airplane today that has the EM 200 system and suffered an engine failure, blamed on the purge valve.

I need some enlightenment.....Kahuna??

Thanks,
Pierre
Click to expand...

Pierre,

I don't think anyone actually answered your question.

Don Rivera invented the device to solve the hot day, hot engine, vapor lock issue with the Lycoming FI system. There is no way to relieve a vapor lock in the spider lines except go full rich, wide open throttle and flood the engine to get fuel moving. The purge valve relieves that pressure and gets fuel flowing - but it also will shut the engine down in this mode, no fuel will reach the cylinders. If the purge side of the valve is opened in flight for any reason, the engine will quit just like pulling the mixture.

A quality locking control cable to open and close the valve is essential. I don't think the idea of a spring to keep it closed is necessary anymore than having one on the throttle or mixture arms. If the arm screws are safety wired as per instructions along with a good cable, the device is near bullet proof. I like it.

I wonder if the engine you looked at had a problem with the "sniffle" valve which is located on the sump.
 
David-aviator said:
If the purge side of the valve is opened in flight for any reason, the engine will quit just like pulling the mixture.
Click to expand...

On the AFP system, pulling the mixture "may" not shut down the engine as the fuel controller is designed to continue to flow fuel even at the full lean stop. In reality, this flow is usually not enough to keep the engine running but it's important to understand that you are not fully closing off the fuel and Don recommends shutting down the engine with the purge valve.

David-aviator said:
A quality locking control cable to open and close the valve is essential. I don't think the idea of a spring to keep it closed is necessary anymore than having one on the throttle or mixture arms. If the arm screws are safety wired as per instructions along with a good cable, the device is near bullet proof. I like it.
Click to expand...

I think the spring is a good idea. Much of it depends on how you actually connect the cable to the arm of the purge valve. This area will vibrate pretty good so it's always possible that something might break over time. A spring is very cheap and easy insurance IMO. I put a fairly light spring on the arm so that if the cable breaks, it will pull it full open. Probably cost me $.70 at Ace hardware.

David-aviator said:
I wonder if the engine you looked at had a problem with the "sniffle" valve which is located on the sump.
Click to expand...

I can't view pictures at work so if this is a redundant comment, I apologize. If you have a purge valve installed there is no need for the sniffle valve. If you remove or don't install the purge valve, I think a sniffle valve is almost mandatory to get the pooling fuel out of the intake elbow. The purge valve prevents the fuel from pooling here, hence no need for the sniffle.
 
...hot day, hot engine, vapor lock issue with the Lycoming FI system. There is no way to relieve a vapor lock in the spider lines except go full rich, wide open throttle and flood the engine to get fuel moving. The purge valve relieves that pressure and gets fuel flowing
Click to expand...

With all due respect, the above is a common misconception.

"Vapor lock" is simply a pump full of air, often combined with air volume in the line leading to the pump. Given a flow restriction downstream from the engine-driven diaphragm pump (notably the vertical head from the TB up to the flow divider,and the spring loaded spool in the flow divider), the pump is unable to move fuel. It just varies the volume of the compressible air in its chamber and inlet line.

The spider lines are after the purge valve and flow divider. They can't "vapor lock"; fuel (and air) simply flows in one end and out the other through the injector's always-open orifice. And opening the purge valve doesn't create any flow through the spider lines anyway. Opening the purge valve diverts liquid fuel and vapor bubbles back to a fuel tank before it ever gets to the flow divider and spider lines; there is no flow in these components with the purge valve open to the tank. Circulation via an open purge valve (with throttle open and mixture rich) clears vapor bubbles from the engine-driven pump, the line to the TB, the TB internals, and the metered line up to the purge valve.

With a Bendix system, or with the AFP purge valve in the no-bypass (run) position, priming merely pushes the bubbles out through the injector nozzles instead of back to a fuel tank. The catch is that you also inject an unknown quantity of fuel; there was no way to know how much of the system volume was trapped vapor and how much was liquid fuel. Thus the classic hot start method; dump a for-sure excess of fuel and vapor into the intake ports, then go to ICO. Open the throttle wide and crank. At first the fuel-air proportion is too rich to ignite, but each intake stroke evaporates some fuel from the excess. Eventually the fuel-air proportion reaching the cylinder gets into the combustible range and you get ignition. The alternate method is to crack throttle, start cranking, then ease the mixture forward slowly to start fuel flow. At some point delivered fuel will bring fuel-air proportion into the combustible range. Here's the difference between the two methods: in the first, the mixture delivered to the cylinder starts out rich and moves toward lean. In the second, mixture starts out lean and moves toward rich. Either way, when you get to the sweet spot it should fire.
 
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Dan,

That's a great explanation, very clear and well written.

I've never use my purge valve for hot starting, just engine shut down. When I hot start, I prime as normal and open the throttle about 1/4" of the way. I move mixture to full lean and then enrichen just as it catches.

I used to open the throttle much further but when that baby catches, you better be on the throttle or it's going over on her nose. I eventually found (through comments on this forum) that WOT wasn't needed so now I can avoid the "three-handed flying" start.
 
DanH said:
With all due respect, the above is a common misconception.

"Vapor lock" is simply a pump full of air, often combined with air volume in the line leading to the pump. Given a flow restriction downstream from the engine-driven diaphragm pump (notably the vertical head from the TB up to the flow divider,and the spring loaded spool in the flow divider), the pump is unable to move fuel. It just varies the volume of the compressible air in its chamber and inlet line.

The spider lines are after the purge valve and flow divider. They can't "vapor lock"; fuel (and air) simply flows in one end and out the other through the injector's always-open orifice. And opening the purge valve doesn't create any flow through the spider lines anyway. Opening the purge valve diverts liquid fuel and vapor bubbles back to a fuel tank before it ever gets to the flow divider and spider lines; there is no flow in these components with the purge valve open to the tank. Circulation via an open purge valve (with throttle open and mixture rich) clears vapor bubbles from the engine-driven pump, the line to the TB, the TB internals, and the metered line up to the purge valve.

With a Bendix system, or with the AFP purge valve in the no-bypass (run) position, priming merely pushes the bubbles out through the injector nozzles instead of back to a fuel tank. The catch is that you also inject an unknown quantity of fuel; there was no way to know how much of the system volume was trapped vapor and how much was liquid fuel. Thus the classic hot start method; dump a for-sure excess of fuel and vapor into the intake ports, then go to ICO. Open the throttle wide and crank. At first the fuel-air proportion is too rich to ignite, but each intake stroke evaporates some fuel from the excess. Eventually the fuel-air proportion reaching the cylinder gets into the combustible range and you get ignition. The alternate method is to crack throttle, start cranking, then ease the mixture forward slowly to start fuel flow. At some point delivered fuel will bring fuel-air proportion into the combustible range. Here's the difference between the two methods: in the first, the mixture delivered to the cylinder starts out rich and moves toward lean. In the second, mixture starts out lean and moves toward rich. Either way, when you get to the sweet spot it should fire.
Click to expand...

Thanks for the education on the Lycoming system. My brain is still focused on the pulse injector with the Subby which is always closed except when fuel is called for by the ECU. (That problem was solved by Jan Eggenfellner with a .040 by pass off the rail. That fuel simply joined the return line to the tank source and would bleed down in about 3 seconds after shut down.)

When you think about the physics of the Lycoming system, vapor lock could occur in a spider line. Increased pressure due to heat after shut down has no where to go but into the top cylinder area and if the exhaust valve is closed, no where to escape. When the engine is cranked, though, it should push through.

I am still on a learning curve with the AFP system. My last Lycoming had a single point injection system which was quite different and the one before that had a carb. I've flown the with Bendix and a couple airplanes but never found the system very friendly on a hot day. For that reason, I do like the AFP purge valve.
 
Erm no

David-aviator said:
When you think about the physics of the Lycoming system, vapor lock could occur in a spider line. Increased pressure due to heat after shut down has no where to go but into the top cylinder area and if the exhaust valve is closed, no where to escape. When the engine is cranked, though, it should push through.

I am still on a learning curve with the AFP system. My last Lycoming had a single point injection system which was quite different and the one before that had a carb. I've flown the with Bendix and a couple airplanes but never found the system very friendly on a hot day. For that reason, I do like the AFP purge valve.
Click to expand...

Vapour lock by definition is always at the inlet to a pump...Fuel vapourising in a line DOWNSTREAM of the pump is not VL...And simply gets cranked out on the next start....There is no incresed back pressure on the injector anyway..its simply an open hole pointing at the back of the inlet valve...whether the valve is open or not it cannot back up the injector lines..and certainly not as far as the inlet to the pump.

The AFP purge valve flushes cool fuel up to the spider..Then this cool fuel chills the injector lines..I never had a hot start issue with the AFP system even with autofuel and/or ethanol mix

Frank
 
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