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Purge valve

olyolson

Well Known Member
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My airplane came with a Purge Valve already installed on the engine when I got it. It does make a hot start pretty easy but reading all the posts lately about hot starts it seems like DH’s sweep technique is working well. Just had the 2,200 hr engine overhauled and debating whether to keep the Purge Valve installed.

One more bit of complexity that could be a possible failure point. Would appreciate everyone’s thoughts on whether I should keep the Purge Valve or not. Most of the mechanics I talk to are not fans of the Purge Valve
 
Oly,
I was on the wing of a **** ton of starts of your airplane with Falcon at the controls since about 2008, and I can assure you that it always fired. I don't know exactly what his procedure was, but it always fired on the hack.
 
As time passes we may see fewer purge valves installed, because the original reason for their existence is slowly going away.

As introduced, all Airflow Performance fuel controls had drum mixture valves. The business plan was fuel injection for any high performance application; the drum valve is easy to machine, not fussy about fuel type, and shrugs off dirt. However, it also had a small leak rate at ICO, and didn't always shut down cleanly when the operator pulled the mixture knob. So, the purge valve dumped fuel pressure at the divider for instant shutdown. The secondary (but better known) use, circulating fuel prior to hot start, was serendipity.

In recent times AFP introduced the FM-150, a fuel control matching the physical dimensions and flanges common to a Bendix RSA5, as well as the Precision and Avstar clones. Although it's bore and venturi are quite different, it included a disk-type mixture valve like the others. And since then, AFP has been developing new valve sections for older designs (like the popular FM-200) which include a disk valve.

Here's the thing. The disk valve has a very low leak rate, so it shuts off clean in ICO unless damaged. You can still order a purge valve if you want it for circulation prior to start, but it's not standard with the disk valve fuel controls.

Best I know, there are only two potential risks to installing a purge valve. All installations should be inspected for the use of an aluminum nipple between the purge valve and the divider body (it should be stainless steel). The other is the cable and links to operate the valve. Poor hardware choices, notably breakage, may inadvertently move the valve arm in flight.
 
To reiterate what Dan said, if you have the FM200 I would suggest you keep the valve, if you remove it all kinds of fuel gets dumped in the cylinders at shutdown, makes a mess on the floor and makes hot starts harder. If you have the 150 you can go either way.

On my old engine I had the 200 with purge valve, I liked the purge valve system enough to install one on my new engine with the FM150 as well, even though it's much better than the 200 at shutting off fuel there is still a benefit at shutdown by dumping the fuel pressure off the system.

Purging hot fuel out of the system after a quick stop makes for a better running engine (and safer as no vapor lock issues).
 
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Valve bodies, and valves, drum left, disk right.
 

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One other thing that may be related - completely shutting off the fuel with the newer style AFP servo might be a factor in the fuel pressure (Kavlico) sender leaks. No where for the warming, and thus expanding, fuel to go after shutdown. I'm sure this has been vetted in other threads that I've not followed and is maybe irrelevant to this.
 
Drum Valve

On the drum valve, you can rotate the control arm either way. So, if you need more clearance, the control arm can be on the top or the bottom.
 
...Best I know, there are only two potential risks to installing a purge valve. All installations should be inspected for the use of an aluminum nipple between the purge valve and the divider body (it should be stainless steel). The other is the cable and links to operate the valve. Poor hardware choices, notably breakage, may inadvertently move the valve arm in flight.

I wonder why we don’t see an integrated spring that would put the purge valve in the run position should the control cable break. Do properly installed threaded push-pull style cables like the type used for throttles and mixture break occasionally?

Bevan
 
I wonder why we don’t see an integrated spring that would put the purge valve in the run position should the control cable break. Do properly installed threaded push-pull style cables like the type used for throttles and mixture break occasionally?

Bevan

I fabricated a torsion spring from music wire to keep the purge valve in the run position in the event the simple Bowden cable :)eek:) breaks.
 
I wonder why we don’t see an integrated spring that would put the purge valve in the run position should the control cable break. Do properly installed threaded push-pull style cables like the type used for throttles and mixture break occasionally?

Bevan

Good question - I installed a spring return on mine to keep it in the run position in case of a cable break.
 
McFarland has a very nice locking Bowden style cable with std pressed on ends, definitively better than the std Bowden style cable.
 
Here's a write up with pics of the torsion spring I fabricated for the AFP purge valve. Includes pics of the spring as installed on the valve.

View attachment Torsion.pdf

I didn't describe fabricating the spring: Chucked up a shaft in my mini lathe having a dia a little smaller than the metal 1/4-28 locknut securing the purge valve's lever arm to the shaft. There was a cross-drilled hole at the end of the shaft, tapped for a pan-head screw (probably was 8-32 or thereabouts). The music wire was held under the screw head (I think I added a washer with a vee-groove filed into into it for better purchase) with a couple inches sticking out that would become on leg of the torsion spring. With the wire perpendicular to the axis of the shaft, the screw was tightened down. I clamped two small wood blocks on either side of the wire, and affixed the blocks to the toolpost to provide tension on the wire as I turned the chuck by hand, coiling the wire around the shaft for several turns.

The diameter of the wire coil sprang back, of course, once the tension was released. I turned down the diameter of the shaft on the lathe a bit at a time until the coil's inside diameter fit around the valve shaft locknut in the purge valve, and also figured out far to take the final coil around the shaft until there was approximately 90 degrees between the straight ends of the wire.

The write up describes the bends made in the straight sections to anchor the spring to the purge valve bracket and lever arm, and the "keeper" bushing that retains the coil on the valve shaft.
 
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Can you post a picture of it?

Unfortunately no - I didn't take a pic of it at the time, and it's since been removed when I installed the SDS system.

I don't think my spring return was a good one though - I used a linear spring anchored to the front of the engine to pull the lever arm forward if the cable broke. I think the torsional spring arrangement described above is a more favorable approach.
 
Cable quality

I used a high quality cable to avoid an issue with failure and a return spring. I'm not sure I'm 100% comfortable with no spring, but it's what I've done so far. Here's my installation on the valve.

IMG_1639.jpg
 
I think the spring design is great, wish AFP would provide those with the valve or where can I get them?
 
Purge valve spring

Kurt,

You should make these and market them to the VAF group. A lot of guys are using purge valves and like Walt said, Airflow Performance doesn’t offer these for sale on their website. As part of the purge valve assembly it seems this would be a pretty important part to leave to the builder’s discretion.
 
Kurt,

You should make these and market them to the VAF group. A lot of guys are using purge valves and like Walt said, Airflow Performance doesn’t offer these for sale on their website. As part of the purge valve assembly it seems this would be a pretty important part to leave to the builder’s discretion.

Don't get me wrong, I think if the proper cable is installed with due diligence the install is safe without the spring, it's no less safe than your throttle or mixture cable install.

However, I've seen quite a few installs that are less than ideal and use a standard cheap Bowden cable, in those cases a spring would be good insurance, I would install them on all valves if they were readily available.
 
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