NEW sniffle valve question

scsmith · #1 07-29-2008, 01:28 AM

Hi,
there is another thread going on about sniffle valve utility, but I would like to focus on what i think is a design problem.

The sniffle valve for horizontal induction engines acts as a drain when the engine is not running. It allows gravity to open a check valve when the manifold pressure is the same as the exterior pressure (or higher).

The valve closes only when there is some suction from the manifold pressure being LOWER than the sniffile outlet pressure.

So, here's the question: with a ram-air intake at wide-open-throttle, and the sniffle valve drain vented to the belly region with just a tube end, it is VERY LIKELY that the manifold pressure is higher than the outlet pressure, and the valve will open.

How many airplanes are flying around with the manifold pressure being vented through the sniffle valve? There is no fuel lost in this case, just a bit of manifold pressure. Maybe its insignificant?

But it seems to me the thing to do is bend the drain tube into the flow so it acts like a little pitot tube, and then will have pressure slightly higher than the ram-intake pressure to keep the sniffle valve closed.

Thoughts?

Steve
RV-8QB FWF

DanH · #2 07-29-2008, 08:08 AM

<<Thoughts?>>

I think you raise an interesting point.

airguy · #3 07-29-2008, 08:19 AM

Would it be possible or practical to put an orifice in there as well? This would still allow a slow(er) dripout of excess fuel, but would prevent large manifold pressure losses. Something on the order .025" or thereabouts?

Bob Brown · #4 07-29-2008, 09:05 AM

I soldered a smooth radiused copper tube in my sniffle valve and attached a rubber hose on the end of it, which is safety wired to the lower cowl attach bracket on my 7A. I was pretty proud of myself because it avoids the chance of having fuel on or around the exhaust system...now I'm wondering whether that was wise or not, there's probably negative pressure at the cowl outlet for sure...

Aden Rich · #5 07-29-2008, 12:36 PM

I think we are beating a dead horse here. I don't want to sound too snobby but there are literally hundred of valves out there with thousands of hours on it and they are working. If the valve can blow open-which I really don't think it does, it's not enough to worry about. My RV-6 has over 400 hours on it without so much as a hic-up when it comes to running smoothly. The only problem is hot starts and this was because the valve was not located at the back of the sump in the taildragger mode. Once that was fixed, it never had issues again. As I posted in another thread, I have an extra valve-new-free for someone that needs one.

scsmith · #6 07-29-2008, 05:02 PM

The check valve in the sniffle valve does have a restriction, so, yes, the flow leakage would be fairly slow.

Regarding Aden's comments, there would be no way to know if those hundreds of valves out there flying around are loosing some manifold pressure (and thus performance). Most bug smashers have a lot of loss in the intake system, and even wide open throttle have manifold pressure below free stream. it is only when you use a ram-air inlet, and bypass the air filter, that you are likely to get high enough manifold pressure to make the valve open and thus leak a little.

I think the answer is to put a small forward bend in the drain tube where it sticks out the bottom (at the cowl mounting lip) so it acts like a pitot tube.

Steve
RV-8QB FWF