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Afp purge valve

rvdave

Well Known Member
I'm on the fence about installing the purge valve control although I do have the purge valve which came mounted to the engine. I know that there are methods of starting a hot engine other than the purge valve but wondered if anyone else has been in the same position but installed the control later? Or is the afp system designed around the purge valve? Looking for opinions.
 
I had AFP on my RV-8A and installed the purge valve and single 1/4" return to the left tank (procedure was to have the left tank selected when purging). 800 hours and I think I used the purge valve for a hot start twice - just because I could. I did however use the purge valve on all engine shutdowns to bleed pressure off the spider.

When building the RV-10 I added a 1/4" fitting to the left tank for a purge return. The engine came from Van's so it came with Precision injection instead of AFP. My thought was if I had a issue with hot starts I'd add a AFP purge valve. 350 hours now and no issue with hot starts. The fitting on the left tank has a cap on it.

If however the engine came with AFP injection I would add the purge return to use for engine shutdown.

If I had to do it all over again I'd probably still add the return fitting. It is simple to do when building the tank.

Carl
 
Older AFP servos used the purge valve to help kill the engine since idle cutoff didn't completely kill the flow. I don't know if that's changed since I got my circa 2001 servo.
 
Purge valve

You probably have an FM-200 if the engine is a 360 or 540. If so it is recommended to use the purge valve for starting and stopping the engine. Since the mixture control in these fuel controls uses a rotary mixture control valve, ICO fuel flow is not zero, therefore there will be leakage of fuel out the nozzles at shut down which causes oil and gook to drain into the air filter box. By using the purge valve the fuel pump pressure is dumped at shut down and there is no leakage of fuel after shut down. The added benefit is that you can circulate fuel in through the fuel system to eliminate hot fuel and vapor from the system before a hot restart and not put any fuel in the engine.

If you do not want this feature, the FM-200 can be converted to the FM-200A fuel control which does not require a purge valve. The cost to convert this unit is around $1500.00 parts and labor.
 
purge valve

AFP system can be set up with or without the purge valve. I am very pleased with the purge valve in my -6 (O-360). Hot starts are a heck of a lot cleaner than with the other fuel injected engines I've flown.

Plumbed the purge line back into the right tank. Push pull cable with a locking button.
 
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Purge valve

Thanks Don,
That gives me more understanding of the system so I will go ahead and hook up the valve, will have to figure out whether to go directly to a tank or tee into a line. I'm assuming when purging select opposite tank on fuel selector that the valve is teed into?
Is a standard push pull cable normally used? Or locking cable?
 
Locking push/pull cable

Probably best to install a good quality push pull cable with a lock function.
Mine is from McFarlane and seems to be just right.
It is a .063 wire and push to unlock feature, not the cheapest version and still not overkill for the job.
The termination of an unthreaded cable will be your next challenge and below you'll see how I solved that problem.
A return spring to the open position is optional too.

I've had a purge valve on 2 airplanes and accumulated about 800 hours total
between the two.
Shut down with a purge valve is quick and crisp and helps avoid the "wet dog"
shaking of the engine. Hot starts are essentially cold starts after you purge
your fuel system of heat saturated gas and vapors.

Where you route the purged fuel is really of little concern and best to follow Don's advice.
If you prefer to route it to a tank, you are dealing with much less than a quart of fuel per purge and most of us leave just a little room in the tank to keep fuel from spilling out if you happen to purge it into a "full" tank.

IMG_2568.JPG


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Nice!

Ernst has a really nice setup here, and while he mentions that the spring is optional, I would recommend that it should always be installed, especially when using the solid wire controls.

Vic
 
Ernst has a really nice setup here, and while he mentions that the spring is optional, I would recommend that it should always be installed, especially when using the solid wire controls.

Vic

Did the same thing and also recommend it. The spring was found in the aviation department of Ace Hardware.

Carl
 
Ernst, I like the size of that control, do you have a part number I can get from you? Didn't see it at acs.
 
Purge Valve

I installed the purge valve on initial assembly back in 2001 on IO-360. Yes return spring should be installed in case cable breaks. 1970 hours later, I use it every start, hot or cold. Makes starts predictable. Master On, Ignition On, Purge valve handle out, mixture control half way, throttle slightly forward, boost pump on for 3-4 secs and off, mixture idle, PURGE VALVE BACK IN and LOCKED. Crank a few blades and it is running.
 
Any sense of the typical length of one of these cables for an RV-10? IOW, do I need, the 6', 8', or 10' cable? I'm guessing a 6' would be fine...
 
The purge valve provides better control over fuel to distribution lines.
There are differing techniques to use it, but invariably start hot or cold and shut down are more predictable.
In 245 hours I developed a technique of priming for 7 seconds cold, zero for hot but circulate for two minutes, both with WOT. Purge valve knob in and out for cold prime, no prime hot just circulate.

Throttle setting matters as fuel flow varies from about 1 gph to 4, this is factor with cold priming.

To start hot or cold, throttle open slightly, simultaneously engage starter and purge valve knob in...engin starts.

I like the purge valve, had it on 7 and 8 with BPE IO-360, 180 hp.
 
I like the purge valve and always use it for shut down. Be sure to use steel fittings especially the fitting between the purge valve and the divider.

It would be nice if it came with a place to connect a return spring or better yet if it came with an integral return spring.

Bevan.
 
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More information

Please help me understand. I thought the AFP pump was designed with an internal purge.
Why would you need the second purge valve?
Where does it get plumbed into the system?
Where does it purge into?
 
Please help me understand. I thought the AFP pump was designed with an internal purge.
Why would you need the second purge valve?
Where does it get plumbed into the system?
Where does it purge into?

The purge valve is an optional feature. It isn't (to the best of my knowledge) a second purge valve. When used, it returns fuel (generally warm fuel from a quick turn-around) back to one of the tanks. You can either plumb it directly to a tank or to one of the fuel lines between the tank and the selector valve, in which case it'll push fuel back to that tank, assuming you're using fuel from the other one.
 
Please help me understand. I thought the AFP pump was designed with an internal purge.
Why would you need the second purge valve?
Where does it get plumbed into the system?
Where does it purge into?

I don't have the AFP pump so can't comment on that--I have the EFII pump, which has internal pressure relief bypass. Anyway the purge valve is mounted right next to spider and when closed bypasses the spider and returns fuel to somewhere in the "non-selected" fuel tank/fuel line (usually the right). Personally mine is plumbed into the the right tank line right below the fuel selector valve.

So for starting with the valve closed (control pulled out), I select the left tank, turn on the boost pump and recirculate fuel from the left to the right until I'm ready to prime and/or start, which is when the control gets pushed all the way in opening the valve for normal fuel flow to the spider.

For shutdown, I pull the control out closing the valve and starving the engine of fuel. It happens very quickly.
 
Relief valve vs purge valve

I don't have the AFP pump so can't comment on that--I have the EFII pump, which has internal pressure relief bypass. Anyway the purge valve is mounted right next to spider and when closed bypasses the spider and returns fuel to somewhere in the "non-selected" fuel tank/fuel line (usually the right). Personally mine is plumbed into the the right tank line right below the fuel selector valve.

So for starting with the valve closed (control pulled out), I select the left tank, turn on the boost pump and recirculate fuel from the left to the right until I'm ready to prime and/or start, which is when the control gets pushed all the way in opening the valve for normal fuel flow to the spider.

For shutdown, I pull the control out closing the valve and starving the engine of fuel. It happens very quickly.

What's the difference?
"This type of pump requires a relief valve to bypass fuel when the pump is
running, which keeps the pump cool and sets the pressure to the system. The relief valve (25 PSI) is built into the boost pump package. Normal operating pressure for pumps with this relief valve is 26-32 PSI. The relief valve is not adjustable. A bypass valve is incorporated into the relief bypass housing, so that when the boost pump is not running, fuel will bypass with low restriction and flow directly to the engine driven fuel pump."
 
What's the difference?
"This type of pump requires a relief valve to bypass fuel when the pump is
running, which keeps the pump cool and sets the pressure to the system. The relief valve (25 PSI) is built into the boost pump package. Normal operating pressure for pumps with this relief valve is 26-32 PSI. The relief valve is not adjustable. A bypass valve is incorporated into the relief bypass housing, so that when the boost pump is not running, fuel will bypass with low restriction and flow directly to the engine driven fuel pump."

The purge valve is not a bypass valve for the pump nor does it replace one. It's purpose is to allow you to push cool fuel all the way to the spider to assist in starting (eliminates any fuel vapor in line from the mechanical fuel pump to the spider due to boil off/evaporation) and to provide a means to completely shutoff fuel to the spider as the AFP FM-200 throttle body uses a rotary mixture control valve which doesn't completely shutoff fuel flow at ICO.
 
The AFP fuel pump itself has a pressure relief device on it that you may be thinking of as the "first" purge valve, but it's just a pressure relief. It's purpose is to keep a set fuel pressure on the system FWF and allow all the excess fuel the pump is moving to dump back to the tank via the return line. The actual purge valve is a separate valve mounting between the servo and divider, usually right at the divider, that is just a three-way valve. When open it allows fuel from the servo to the divider, when closed it isolates the divider and flows from the servo back to a return line.

This arrangement gives a crisper cutoff than ICO mixture, and allows for easier hotstarts by purging the hot fuel/vapor back to the tanks prior to cranking, without flooding the engine. For those like myself that are running injection and mogas, especially with high-vapor-pressure winter blend 91E10, it's VERY handy for hot starts.
 
Return

The AFP fuel pump itself has a pressure relief device on it that you may be thinking of as the "first" purge valve, but it's just a pressure relief. It's purpose is to keep a set fuel pressure on the system FWF and allow all the excess fuel the pump is moving to dump back to the tank via the return line. The actual purge valve is a separate valve mounting between the servo and divider, usually right at the divider, that is just a three-way valve. When open it allows fuel from the servo to the divider, when closed it isolates the divider and flows from the servo back to a return line.

This arrangement gives a crisper cutoff than ICO mixture, and allows for easier hotstarts by purging the hot fuel/vapor back to the tanks prior to cranking, without flooding the engine. For those like myself that are running injection and mogas, especially with high-vapor-pressure winter blend 91E10, it's VERY handy for hot starts.

Ok. What are the options for the return? My tanks are built and plumbing installed to FWF.
 
Ok. What are the options for the return? My tanks are built and plumbing installed to FWF.

For those of us who dont have EFII plumbing, the "poor-boy" method, which I'm using, is to T into the line going to the gascolator. Not as theoretically cool as tank fuel, but it is a reservoir of fuel that is cooler than the fuel boiling out in the spider at the top of a hot motor.

In my case (stock Bendix inj) I'm also using the Flyboys flush mounted gascolator which is intended to be mounted in a floor board ie outside the engine cowl area, so its always got cool fuel in it and theres enough volume to get the job done. https://www.flyboyaccessories.com/product-p/72rr02.htm
 
Ok. What are the options for the return? My tanks are built and plumbing installed to FWF.

You tee into one of the lines between the wing and fuel selector. Let's say you choose the left one. So you never top that tank prior to a hot start, leaving room for some (say 1/2 gallon) returned fuel. Prior to starting, you select the other tank, turn on the boost pump, open the purge valve, and the hot fuel is pushed into the left tank, while cool fuel from the right tank is pushed to the engine.

Close the purge valve, and do a normal start.

Make sense?
 
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Purge plumbing

You tee into one of the lines between the wing and fuel selector. Let's say you choose the left one. So you never top that tank prior to a hot start, leaving room for some (say 1/2 gallon) of returned fuel. Prior to starting, you select the other tank, turn the boost pump, open the purge valve, and the hot fuel is pushed into the left tank, while cool fuel from the right tank is pushed to the engine.

Close the purge valve, and do a normal start.

Make sense?

Totally. Thanks. Now I need to look for a "T".
 
Don't worry so much about keeping an air gap in the return tank. The injector servo, even with full throttle and rich mix set, will only move about 6gph without the engine running. You're going to purge for about 2 minutes tops, so you're moving just under a quart of fuel through the system. Not enough to worry about even if it all vents out of a full tank.
 
You tee into one of the lines between the wing and fuel selector. Let's say you choose the left one. So you never top that tank prior to a hot start, leaving room for some (say 1/2 gallon) returned fuel. Prior to starting, you select the other tank, turn on the boost pump, open the purge valve, and the hot fuel is pushed into the left tank, while cool fuel from the right tank is pushed to the engine.

Close the purge valve, and do a normal start.

Make sense?

Yes, now next question. Would a simple open/close valve on the return line located in the cockpit work as well as the AFP purge valve? That would eliminate the cable and the expensive AFP purge valve. The only disadvantage I can think of would be an additional pressurized fuel line in the cockpit.
 
Yes, now next question. Would a simple open/close valve on the return line located in the cockpit work as well as the AFP purge valve? That would eliminate the cable and the expensive AFP purge valve. The only disadvantage I can think of would be an additional pressurized fuel line in the cockpit.

Short answer - no.

If you did this, you open a parallel path for fuel flow, so fuel would still be going into the spider and then out to the cylinders when you did the purge. Not what you want.

As a couple of data points, I had the AFP injection on my 8A. While I liked pulling the purge valve at shutdown, over 10 years I used the purge valve for a hot start exactly twice. On the RV-10 I had the standard Precision injection and never missed having a purge valve. For the new RV-8 I did order the engine via Van?s to come with the AFP system (same price as the Precision system) so I will again have the purge valve. When I built the tanks I added a 1/4? purge line return to the left tank. I also added a 3/8? fitting on the right tank if I ever want to add a ferry tank - it would feed into the right tank.

Carl
 
Purge valve knob in and out for cold prime

Can you elaborate on your procedure for priming?

Engine needs fuel to start. WOT, electric pump on, purge valve in for 7 seconds, purge valve knob out, engine primed for start. Engage starter, purge valve in to keep fuel coming, engine will start, at least it works in Missouri.
Don?t know about other parts of country...:)
 
Engine needs fuel to start. WOT, electric pump on, purge valve in for 7 seconds, purge valve knob out, engine primed for start. Engage starter, purge valve in to keep fuel coming, engine will start, at least it works in Missouri.
Don?t know about other parts of country...:)

7 seconds? I do the same thing for no more than 2 seconds on my IO360, get consistently great cold starts - are you using the Bendix or AFP servo?
 
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