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-   -   Stank Idle (https://vansairforce.net/community/showthread.php?t=184493)

lr172 07-23-2020 08:26 AM

Quote:

Originally Posted by biscuit112 (Post 1448549)
The spring effectively raises the boiling point of the fuel in the stainless lines to the injectors. .

Actually, it raises the pressure, and therefore increases the boiling point, in the line between the servo and the spider. I does not increase pressure in the SS lines after the spider (injectors are too big to create backpressure at idle level fuel flows, which is the reason for the plumbing inside the spider). That -4 hose absorbs more heat than most think it does.

Larry

rv8ch 07-23-2020 11:51 AM

Quote:

Originally Posted by lr172 (Post 1448629)
Actually, it raises the pressure, and therefore increases the boiling point, in the line between the servo and the spider. I does not increase pressure in the SS lines after the spider (injectors are too big to create backpressure at idle level fuel flows, which is the reason for the plumbing inside the spider). That -4 hose absorbs more heat than most think it does.

Larry

Thanks Larry for your diagnosis. I had no idea this would even be something to consider - the back pressure spring.

I watched a Mike Busch webinar yesterday where someone recommended creating an AI to diagnose engine problems - I think he needs you on his team!

biscuit112 07-23-2020 04:38 PM

Quote:

Originally Posted by lr172 (Post 1448629)
Actually, it raises the pressure, and therefore increases the boiling point, in the line between the servo and the spider. I does not increase pressure in the SS lines after the spider (injectors are too big to create backpressure at idle level fuel flows, which is the reason for the plumbing inside the spider). That -4 hose absorbs more heat than most think it does.

Larry

What he said

ltn6a 10-04-2020 02:49 AM

Rough idle
 
Hi All

Just came across this , I have the almost same issue. Io-320-b1a
The engine ideas fine when cold but once hot the rpm hunts with the occasional misfire.
I have had the mags tested and they tested fine , the injector servo ,distributor valve and lines all bench tested where they did replace an injector nozzle but it made no change..
All lines to and from the servo up to the distributor are fire sleeved .
I will look at the spring pressure in the distributor valve and a look at a non metallic spacer. .
Before coming across this thread the only thing left was an induction leak but we have new seals at the heads the sleeves to the sum all look good .
I have also seen mention of the Savvy induction test but Iím not aware of what this is ?
Any further help would be appreciated

Mark

DanH 10-04-2020 07:01 AM

1 Attachment(s)
Replacing a 2 lb divider spring with a 4 lb spring only raises the boiling point by four, maybe five degrees F. It is a relatively ineffective crutch.

The fundamental problem is fuel heating. It would be more effective to slow the rate of heat transfer.

The OP has a rear inlet sump and a top-mounted divider, so (1) the servo is probably exposed to radiant heating from the exhaust, and (2) the long line from servo to divider offers plenty of transfer time at low fuel flow (idle).

The typical aluminum or stainless "heat shield" clamped to a hot exhaust is really a very poor performer. Radiant heat energy strikes the side facing the hot exhaust. Some is reflected and some is absorbed. The absorbed energy spreads through the shield material by conduction. It then leaves the heat shield material by convection (via all surfaces) and re-radiation (surface opposite from the heated side). The re-radiated energy heats the object we wished keep keep cool. Not good.

So, the goal is to slow the rate of re-radiation. The easy way is to slow conduction from the hot side. Simply add an insulator layer to the shield, which can be any material with a low rate of conduction. Again, in our world, good 'ole fiberfrax felt works fine. Cut a patch the size of the shield. Hold in place and tape over it with metallic aluminum duct tape. Now energy transfer from the hot side to the cool side is much slower.

Note the principle, a reflector (here the hot side of the shield) backed by an insulator. The same principle can be applied to the fuel line running from the servo to the divider. Silicone faced firesleeve already provides an insulator, the fiberglass liner. Simply add a reflector to the outside of the sleeve.
For a short length, wrap it with aluminum tape. For a longer length, slip it inside a reflector sleeve, available from lots of sources. Here's an example: http://www.americanfiresleeve.com/hi...rotection.html

Even with a 4 psi spring, I'd still take steps reduce heat transfer. It can only increase the margins.
.

lr172 10-04-2020 07:18 AM

Quote:

Originally Posted by DanH (Post 1467010)
Even with a 4 psi spring, I'd still take steps reduce heat transfer. It can only increase the margins.
.

+1

For my 320, in addition to the spring, I did all of the things mentioned by Dan above to get the hot idle issue under control, except insulating the heat shield, which is a great idea that I will implement next time the cowl is off. Reflective material on the -4 line made a big difference, as did the reflector shields on the exhaust. The vetterman exh on a 320 gets pretty close to the servo and the fuel feed line. I cannot fully address my issue, as my SS injector lines run through the baffling to the hot side (using primer ports)

Oddly, I made no remediation steps on the 10 and see no hot idle issues. I think a lot has to do with the exh so close to the servo and fuel feed line on the 320.

Larry

airguy 10-04-2020 09:05 AM

Quote:

Originally Posted by DanH (Post 1467010)
Replacing a 2 lb divider spring with a 4 lb spring only raises the boiling point by four, maybe five degrees F. It is a relatively ineffective crutch.

Before I installed the SDS system in my airplane, I did LOTS of things to reduce the vapor issue while using 91E10 fuel. I tried the 4# spring in the divider and discovered exactly what Dan says - it only delayed the onset of trouble, and did nothing to really stop it. I ended up later removing it and reinstalling the original stock spring. Don Rivera at Airflow Performance told me I was chasing a losing proposition by doing that, but I had to try it myself to make sure.

What did work, and worked well, was to replace the OEM injector orifices with smaller ones - that presents more backpressure between the injector and the divider in the spider lines, and upstream of there in the divider all the way back to the servo. I got down to a .022 orifice from the original .028, and this effectively cured my loping hot idle on 91E10. The drawback to this plan is also the attractiveness of it - higher pressure drop across the smaller orifice requires a much higher fuel pressure available at the servo to be able to move enough fuel for full takeoff power. I had already installed dual electric pumps and had an adjustable fuel pressure regulator on them, so that was easy to do - I raised my fuel pressure up to 40psig and then it was happy. I could move enough fuel for full power, and still had enough back pressure to avoid boiling the fuel in the lines at hot idle.

I also double-firesleeved all my FWF fuel lines for insulation, and had removed my engine-driven fuel pump (primary fuel heater) when I installed the dual electrics.

I flew that setup for about 550 hours with excellent results - and then I upgraded to the SDS which is even better!


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