VansAirForceForums  
Home > VansAirForceForums

-POSTING RULES
-Advertise in here!
- Today's Posts | Insert Pics

Keep VAF Going
Donate methods

Point your
camera app here
to donate fast.






VAF on Twitter:
@VansAirForceNet

  #11  
Old 11-24-2022, 10:10 PM
cgarts1 cgarts1 is offline
 
Join Date: Mar 2020
Location: Lyons CO
Posts: 76
Default

Quote:
Originally Posted by DanH View Post
This isn't black or white. More a matter of carefully considered compromise.

Let's start by asking "What is it's intended purpose?"

Is there something special about your application which makes it unlike the multitude of Bendix systems operating without a return? An example might be the intent to operate on auto fuel with high vapor pressure.
The original intent was to use an Aeromomentum engine which required a return line. That didn't pan out. Went with the Lycoming. The plumbing was there. Avstar recommended the fitting during a discussion with support. I didn't insist on using the return line but it seemed like the guy I talked to liked it. That is certainly speculation on my part from a conversation 10 months ago.

That said, it seems like a system that automatically bleeds down may be beneficial. Is this really different than the AFP purge valve?
__________________
Chuck Garton
RV-14A flying 140600
Reply With Quote
  #12  
Old 11-25-2022, 08:25 AM
DanH's Avatar
DanH DanH is offline
 
Join Date: Oct 2005
Location: 08A
Posts: 11,242
Default

Quote:
Originally Posted by cgarts1 View Post
The original intent was to use an Aeromomentum engine which required a return line. That didn't pan out.
Ahh.

A classic PWM port injection has a pressure regulator in the return flow path. The regulator holds the injector supply at a specific pressure above atmospheric regardless of engine demand. Think of it as a constantly variable restrictor. An alternative introduced later is a regulated pressure electric supply pump and no return line. Given a known pressure and a known injector flow rate at that pressure, the EFI computer merely calculates required injector open time. Injector flow is start/stop/start/stop.

A Bendix RSA type is a constant flow system. That includes Avstar and Precision (both near clones), and Airflow Performance (not a clone, but similar in how it regulates fuel flow). They meter using a pair of connected diaphragms further connected to a ball valve, which feeds the divider. The air diaphragm has venturi pressure on one side and dynamic pressure on the other. The fuel diaphragm has supply pressure on one side and pressure after a jet drop on the other. The combined pressure deltas move the ball valve. Being based on delta, the system is relatively insensitive to supply pressure, and will operate from roughly 15 psi to the limits of its internal seals, possibly as much as 90 psi.

Here's the thing. A Lycoming's engine driven pump is effectively a pressure regulated supply, typically 25 to 30 psi. It's done mechanically by sizing the pump spring. It's also a variable volume pump. An engine cam raises the pump diaphragm to max travel/max spring compression, then releases. With standard RSA plumbing (no return) the spring only pushes the diaphragm down a small fraction of an inch, in direct proportion to outlet demand. At idle, the movement is tiny. At WOT, it is more. Given a return line with restrictor, and WOT operation, the diaphragm travel would be greater yet. Note that being spring driven, as travel increases, spring force (thus fuel pressure) decreases....not a lot, but it's there.

More critical, note the limited system headroom. Best case, supply is maybe 30 psi, while minimum to run right is roughly 20 psi. If restrictor flow rate plus engine demand approaches pump capacity, supply pressure drops.

All the above is background. Let's return to the previous question.

Quote:
Went with the Lycoming. The plumbing was there. Avstar recommended the fitting during a discussion with support. I didn't insist on using the return line but it seemed like the guy I talked to liked it. That is certainly speculation on my part from a conversation 10 months ago.
If recommended, why?

Quote:
That said, it seems like a system that automatically bleeds down may be beneficial. Is this really different than the AFP purge valve?
Very different. An AFP purge valve is closed during normal operation. There is no return flow with the engine running.
__________________
Dan Horton
RV-8 SS
Barrett IO-390

Last edited by DanH : 11-26-2022 at 06:24 AM.
Reply With Quote
  #13  
Old 11-25-2022, 09:52 AM
Toobuilder's Avatar
Toobuilder Toobuilder is offline
 
Join Date: Jul 2009
Location: Mojave
Posts: 5,101
Default

Do I understand correctly that there is a constant retun bleed (calculated leak) that is installed in the servo to divider valve line?

Interesting. That is contrary to my understanding of the operation of a Bendix style system. Since you really want as much pressure as you can get to the fixed orifice nozzles (using common manifold hydraulic theory to keep regulation consistent), it seems that "stealing" pressure via a fixed bleed will only make the problematic low flow, LOP condition worse.

Will be following this thread in the hopes that I learn something new.
__________________
WARNING! Incorrect design and/or fabrication of aircraft and/or components may result in injury or death. Information presented in this post is based on my own experience - Reader has sole responsibility for determining accuracy or suitability for use.

Michael Robinson
______________
Harmon Rocket II -SDS EFI
1940 Taylorcraft BL-65
1984 L39C - SOLD
RV-8 - SDS CPI - SOLD
Reply With Quote
  #14  
Old 11-25-2022, 10:16 AM
DanH's Avatar
DanH DanH is offline
 
Join Date: Oct 2005
Location: 08A
Posts: 11,242
Default

Quote:
Originally Posted by Toobuilder View Post
Do I understand correctly that there is a constant retun bleed (calculated leak) that is installed in the servo to divider valve line?
No. The subject is a return connected near the servo inlet.

Quote:
Since you really want as much pressure as you can get to the fixed orifice nozzles ...
Strictly an aside, but there are only two practical ways an operator can raise nozzle pressure...advance the throttle, or change the nozzle inserts. IIRC, 0.028" to 0.022" is pretty common.
.
Attached Thumbnails
Click image for larger version

Name:	Scan_0001.jpg
Views:	25
Size:	371.9 KB
ID:	34244  
__________________
Dan Horton
RV-8 SS
Barrett IO-390

Last edited by DanH : 11-25-2022 at 10:19 AM.
Reply With Quote
  #15  
Old 11-25-2022, 10:34 AM
Toobuilder's Avatar
Toobuilder Toobuilder is offline
 
Join Date: Jul 2009
Location: Mojave
Posts: 5,101
Default

Quote:
Originally Posted by DanH View Post
No. The subject is a return connected near the servo inlet.



Strictly an aside, but there are only two practical ways an operator can raise nozzle pressure...advance the throttle, or change the nozzle inserts. IIRC, 0.028" to 0.022" is pretty common.
.
Copy all Dan, thanks.

That said, and not to be pedantic, but high altitude, cruise flight for many of us in the West is accomplished with the throttle against the stop from takeoff to the TOD. But I understand your point that the servo schedules pressure to the nozzles via throttle position (among other inputs). Unfortunately, with the open ended Bendix style system that uses the nozzle sizing as a metering parameter, there is a conflict between a small enough orifice to meter effectively at low flow conditions (LOP and high altitude) AND the demands of substantially higher flow rates at TO power. This is tough to do in a system that tops out at 30 PSI.

So the scheme in this thread effectively pulls a bit of pressure off the total available for the competing demands of small nozzles for effective metering and high pressure required to overcome the small nozzles at high power. If it works now, great, but seems the scheme is adding extra plumbing and reducing performance margin for no real benefit.
__________________
WARNING! Incorrect design and/or fabrication of aircraft and/or components may result in injury or death. Information presented in this post is based on my own experience - Reader has sole responsibility for determining accuracy or suitability for use.

Michael Robinson
______________
Harmon Rocket II -SDS EFI
1940 Taylorcraft BL-65
1984 L39C - SOLD
RV-8 - SDS CPI - SOLD
Reply With Quote
  #16  
Old 11-25-2022, 08:53 PM
cgarts1 cgarts1 is offline
 
Join Date: Mar 2020
Location: Lyons CO
Posts: 76
Default

Quote:
Originally Posted by Toobuilder View Post
If it works now, great, but seems the scheme is adding extra plumbing and reducing performance margin for no real benefit.
It certainly does. I'm convinced. It's getting plugged.
__________________
Chuck Garton
RV-14A flying 140600
Reply With Quote
  #17  
Old 11-26-2022, 06:51 AM
DanH's Avatar
DanH DanH is offline
 
Join Date: Oct 2005
Location: 08A
Posts: 11,242
Default

Quote:
Originally Posted by cgarts1 View Post
It certainly does. I'm convinced. It's getting plugged.
Before we go, return to the photo from post #4

Quote:
Originally Posted by cgarts1 View Post
I just got done rebuilding another engine and it came with a Bendix servo. (see attached). I circled the fitting. It comes out of the same chamber as the inlet.
Is the circled fitting an elbow, or a tee?

If an elbow, and connected to a return line, all the return flow would be passing through the Bendix inlet screen. Trash tends to be cumulative based on volume, so the inlet screen would collect garbage at a higher rate than one which only screens the fuel burned. There are owners out there who fail to clean inlet screens, and...

If a tee, it would be connected to both the supply line from the pump and to the return. If so, it would supply fuel to the wrong side of the screen.
__________________
Dan Horton
RV-8 SS
Barrett IO-390
Reply With Quote
  #18  
Old 11-26-2022, 10:24 AM
Lenny Iszak's Avatar
Lenny Iszak Lenny Iszak is offline
 
Join Date: Sep 2008
Location: Palm City, FL
Posts: 423
Default

One additional thing to consider. The Avstar servos have a real tight idle cutoff. After shutting down the engine the fuel pressure starts climbing quickly. All efis manufacturers issued a service bulletin for the Kavliko pressure sensors that were popping under that overpressure.
My buddy has an Avstar on his -7 and fuel pressure hits 50psi+ after shutdown, then it goes to -- (as it hits the set sensor max). No idea how far up it goes but it popped his original sensor a couple of years ago. Resulted in fuel dripping out of the sensor, running down the wiring harness. Yikes.
__________________
Lenny Iszak
Palm City, FL
2014 RV-10, N311LZ - 850 hrs
OnSpeed development team
Reply With Quote
  #19  
Old 11-26-2022, 11:11 AM
BillL's Avatar
BillL BillL is offline
 
Join Date: Sep 2007
Location: Central IL
Posts: 6,916
Default

Quote:
Originally Posted by Lenny Iszak View Post
One additional thing to consider. The Avstar servos have a real tight idle cutoff. After shutting down the engine the fuel pressure starts climbing quickly. All efis manufacturers issued a service bulletin for the Kavliko pressure sensors that were popping under that overpressure.
My buddy has an Avstar on his -7 and fuel pressure hits 50psi+ after shutdown, then it goes to -- (as it hits the set sensor max). No idea how far up it goes but it popped his original sensor a couple of years ago. Resulted in fuel dripping out of the sensor, running down the wiring harness. Yikes.
BTW, the fuel hoses FWF have a significant effect on this. The 5000psi hoses are very stiff and allow little "spring" for solid expansion of the fuel. I tested the Vans standard hoses on my 7 (Avstar) and the pressure rise was within the sensor range.

I had a real world product issue due to this very same thing (solid ethanol expansion in a sealed volume), it took our team a while to trace it to the hose as it exhibited the behavior after upgrading the field test to production hardware. I actually approved the hose change - - learned a lesson there.
__________________
Bill
RV-7

Last edited by BillL : 11-26-2022 at 11:15 AM.
Reply With Quote
  #20  
Old 11-26-2022, 12:05 PM
Lenny Iszak's Avatar
Lenny Iszak Lenny Iszak is offline
 
Join Date: Sep 2008
Location: Palm City, FL
Posts: 423
Default

Quote:
Originally Posted by BillL View Post
BTW, the fuel hoses FWF have a significant effect on this. The 5000psi hoses are very stiff and allow little "spring" for solid expansion of the fuel. I tested the Vans standard hoses on my 7 (Avstar) and the pressure rise was within the sensor range.

I had a real world product issue due to this very same thing (solid ethanol expansion in a sealed volume), it took our team a while to trace it to the hose as it exhibited the behavior after upgrading the field test to production hardware. I actually approved the hose change - - learned a lesson there.
Woah! Great point Bill! Thanks! I'll have him swap back to a rubber hose.

Sorry about the detour guys, thought that might be why Avstar likes the idea of the restricted return line.
__________________
Lenny Iszak
Palm City, FL
2014 RV-10, N311LZ - 850 hrs
OnSpeed development team
Reply With Quote
Reply


Thread Tools Search this Thread
Search this Thread:

Advanced Search
Display Modes

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

vB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Forum Jump


All times are GMT -6. The time now is 06:14 AM.


The VAFForums come to you courtesy Delta Romeo, LLC. By viewing and participating in them you agree to build your plane using standardized methods and practices and to fly it safely and in accordance with the laws governing the country you are located in.