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Purge valve return spring

AX-O

Well Known Member
This thread is not intended to be a discussion of if a purge valve is a bad idea or the best thing since sliced bread. It is simply to document one way of adding a spring to keep the purge valve in the position that allows fuel to flow to the spider in the case that the control cable fails. I looked for examples of how people have done this in the past and was not able to find much. Only a few pics of set ups not similar to mine. If you wish to post your set up, please do so. Thanks.

Fuel flows to the spider
IMG_2433.JPG


Fuel get diverted to right fuel tank
IMG_2434.JPG


IMG_2435.JPG
 
Before this start digressing, please read the first sentence in post 1. Thanks.
 
Nice clean approach. The two failures that I know of where not the cable, they where the lock nut, or lack of safety of the nut that holds the arm on.
Good job AC. Check that arm and make sure it is properly safetied.
 
The purpose of the spring is to return the arm to full fuel flow if there is a failure in the cable. I see three possible failure modes
1. Cable breaks
2. bolt holding cable comes loose and falls from arm
3. nut holding arm comes off.

The spring, as installed will look after the 1st failure mode. If you were to drill a small hole in the arm and put the spring directly on the arm you would eliminate two of the three failure modes.

Nice work!
 
Belt and suspenders!

It is interesting to note that the manufacturer of the purge valve says no return spring is necessary since it has a built in provision to keep the valve in the open position.
Unlike a throttle arm which on its own generally settle in the closed position and no springs are attached to any throttle cables as far as I know.
Anyway, nice installation and FWIW I too attached a return springs to the purge valve arm.
 
It is interesting to note that the manufacturer of the purge valve says no return spring is necessary since it has a built in provision to keep the valve in the open position.

I am currently working on a spring confuiguration that covers all the bases. I have not seen any statement about the purge valve having a "built in provison" to keep the valve open. Can anyone elaborate/confirm this? If so, how is it done without a spring?

Bevan
 
It is interesting to note that the manufacturer of the purge valve says no return spring is necessary since it has a built in provision to keep the valve in the open position.

No disrespect intended, but this isnt completely true. The AFP purge valve for example has no built in provision to keep it open. It is true that the manufacturer believes a return spring isnt necessary, but their reasoning is that a high quality control cable should be used that is very unlikely to fail if installed correctly. Installation of a spring to keep the valve open is left to the discretion of the customer.

erich
 
...and the failure point of that cable is where the solid wire exits the little pin vice end attachment assembly. 'been there, done that, wore the pointy cap.
 
For my edification, could someone explain what the purge valve does? Please.

Purge valve is intended for fuel injected engines and is installed at the fuel inlet to the fuel divider. It has the capabuility to redirect fuel (away from the divider) back to the tank. It has two positions, RUN and ICO. It has several uses.

A) Is an alternate means to shut down the engine. Leaving the purge valve in the ICO position after shut down leaves the injector lines depressurized. This prevents the remaining fuel in the divider and injector lines to perculate into the cylinders, contributing to hot start issues and washing the oil off cylinder walls.

B) Prior to engine start (particularly with a hot engine) using the purge valve allows you to push all the hot fuel out of the lines and engine diven fuel pump (everything except the fuel divider and injector lines) and back to the tank. Cold fuel now ready in the system. Again this may aid in hot start issues.

C) If you run with one tank of 100LL and the other Mogas or combo, you may not want to park the plane with mogas sitting in the lines, filters and pumps. With the purge valve, you can flush the Mogas and replace with 100LL for storage.

Bevan
RV7A not flying yet. My opinions only. I may have missed some uses of the optional purge valve.
 
...and the failure point of that cable is where the solid wire exits the little pin vice end attachment assembly. 'been there, done that, wore the pointy cap.

The purge valve has a control arm with a hole sized for a -3 bolt for push-pull control cable similar to engine control cables. A pull type cable would absolutely need a return spring.

The purge valve control arm is cast aluminum. I would prefer a control arm more beefy looking like the certified throttle arms.

Bevan
 
.and the failure point of that cable is where the solid wire exits the little pin vice end attachment assembly.

You are right about that neat looking pin vise, come to think of it it failed on my airplane at exactly that same spot.
On my 10 I used a .60 McFarlane lockable push pull cable with hefty attachment hardware able to accomodate some flex on the cable.
The slight curve in the cable linkage is the shut down position "no vibration".
The assembly aligns perfectly in the run position so as not to induce stress on any part of the cable attachment.



IMG_2572.JPG


IMG_2573.JPG

No disrespect intended, but this isnt completely true. The AFP purge valve for example has no built in provision to keep it open. It is true that the manufacturer believes a return spring isnt necessary, but their reasoning is that a high quality control cable should be used that is very unlikely to fail if installed correctly. Installation of a spring to keep the valve open is left to the discretion of the customer.

I stand corrected :eek:
 
Purge valve

Purge valve is intended for fuel injected engines and is installed at the fuel inlet to the fuel divider. It has the capabuility to redirect fuel (away from the divider) back to the tank. It has two positions, RUN and ICO. It has several uses.

A) Is an alternate means to shut down the engine. Leaving the purge valve in the ICO position after shut down leaves the injector lines depressurized. This prevents the remaining fuel in the divider and injector lines to perculate into the cylinders, contributing to hot start issues and washing the oil off cylinder walls.

B) Prior to engine start (particularly with a hot engine) using the purge valve allows you to push all the hot fuel out of the lines and engine diven fuel pump (everything except the fuel divider and injector lines) and back to the tank. Cold fuel now ready in the system. Again this may aid in hot start issues.

C) If you run with one tank of 100LL and the other Mogas or combo, you may not want to park the plane with mogas sitting in the lines, filters and pumps. With the purge valve, you can flush the Mogas and replace with 100LL for storage.

Bevan
RV7A not flying yet. My opinions only. I may have missed some uses of the optional purge valve.

Not to hijack the thread but would you also have return lines to both tanks or would the purge valve be the preferred solution to hot starts and vapor lock?
I like the purge valve solution. I assume there is only one return to one tank.
 
...and the failure point of that cable is where the solid wire exits the little pin vice end attachment assembly. 'been there, done that, wore the pointy cap.

I agree... depending on the particular installation the wire can be very vulnerable to fatigue failure at the point where it exits the pin vice.
This is because in a lot of installations the wire is loaded in compression (not the ideal situation for a piano wire control anyway)when in its static state. The pin vise connected to even the smallest rod end has a lot of mass compared to the stiffness of the wire, so engine vibration can easily induce a constant vibration mode. But in some installations they do fine. Probably instances where the cable is working in tension at all times, like the one the OP has presented, because of the use of the return spring.
I think the spring is a very good idea on this installation. It's insurance if the cable does break, and it will probably go a long way towards helping reduce the chance of it breaking in the first place.
 
Looks like you should also safety wire the spring? Or secure it so it can't pop off.

...or just remove it, and put it back in the lawnmower parts drawer.

Install a locking push-pull cable suitable for engine control, something equal to what would be used for a throttle.
 
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