VansAirForceForums  
Home > VansAirForceForums

- POSTING RULES
- Donate yearly (please).
- Advertise in here!

- Today's Posts | Insert Pics

  #21  
Old 08-17-2018, 04:50 PM
DanH's Avatar
DanH DanH is offline
 
Join Date: Oct 2005
Location: 08A
Posts: 7,796
Default

Quote:
Originally Posted by Tom @ N269CP View Post
The "fail to high pressure" failure mechanism is still a puzzle.
Maybe not. Humor me; check that pulsator chamber and vent passage. If it is as I suspect, I'll explain.

Quote:
For some reason the pump is putting out more flow than required by the carburetor (assuming carb is fine).
Let's not confuse "flow" with "pressure". This kind of pump can generate rated pressure with no flow. Within the normal gph envelope, pressure should drop very little as flow rate increases.

Quote:
As output pressure is controlled by the spring force (F = k * x), it's possible the pump output strokes may be bottoming at a shorter compressed spring length.
The spring is compressed when the pump lever lifts a pull rod during the intake stroke. The compression dimension (x) is fixed by the mechanics of the cam, pushrod, pivot, and lever. The spring is compressed the same amount with every every rotation of the pump drive cam. The physical relationships can't change, so more spring compression than normal is not possible.
__________________
Dan Horton
RV-8 SS
Barrett IO-390
Reply With Quote
  #22  
Old 08-17-2018, 09:26 PM
Tom @ N269CP Tom @ N269CP is online now
 
Join Date: Oct 2017
Location: Green Cove Springs, FL
Posts: 43
Default

Quote:
Originally Posted by DanH View Post
Maybe not. Humor me; check that pulsator chamber and vent passage. If it is as I suspect, I'll explain.



Let's not confuse "flow" with "pressure". This kind of pump can generate rated pressure with no flow. Within the normal gph envelope, pressure should drop very little as flow rate increases.



The spring is compressed when the pump lever lifts a pull rod during the intake stroke. The compression dimension (x) is fixed by the mechanics of the cam, pushrod, pivot, and lever. The spring is compressed the same amount with every every rotation of the pump drive cam. The physical relationships can't change, so more spring compression than normal is not possible.
Understood.

Let's say the pump normally delivers 2 gph at 3 psi outlet pressure at idle....this is where it normally operated . However, now it delivers 2 gph at 9 psi at idle. As discharge pressure is generated purely by spring force, this means the spring is "now" more compressed at the bottom of its downstroke....ie. for some reason its downstroke must now be less than normal to deliver 2 gph at 9 psi. As the carburetor is only taking 2 gph, this suggests the "effective" pump displacement per inch of downstroke is now higher than it should be for some reason. This is consistent with reports by others of flooded carburetors and running rich. I always lean to peak rpm at idle and have a flowmeter.

I'm arranging to have the pump replaced on Tuesday. I'll inspect the internals once it's out and report back. I could also give Lycoming a call on Monday to see what their experience is with failure to high pressure.

FWIW, I'm a process machinery engineer. We'll figure this out.
__________________
Kind regards,

Tom

==================================
RV-8 N269CP
O-360-A1A w/Hartzell CS prop on 100LL
Slick-IC+PMag ignitions
Steam gauges, EI UBG-16 & FP-5, Garmin Aera 660
TruTrak autopilot (GPS coupled)
Reiff engine preheater
AntiSplat oil mist separator
MH O2 system

Location: Durango, CO (KDRO)

Last edited by Tom @ N269CP : 08-17-2018 at 09:52 PM.
Reply With Quote
  #23  
Old 08-17-2018, 10:11 PM
lr172 lr172 is offline
 
Join Date: Oct 2013
Location: Schaumburg, IL
Posts: 3,007
Default

Quote:
Originally Posted by Tom @ N269CP View Post
Understood.

Let's say the pump normally delivers 2 gph at 3 psi outlet pressure at idle....this is where it normally operated . However, now it delivers 2 gph at 9 psi at idle. As discharge pressure is generated purely by spring force, this means the spring is "now" more compressed at the bottom of its downstroke....ie. for some reason its downstroke must now be less than normal to deliver 2 gph at 9 psi. As the carburetor is only taking 2 gph, this suggests the "effective" pump displacement per inch of downstroke is now higher than it should be for some reason. This is consistent with reports by others of flooded carburetors and running rich. I always lean to peak rpm at idle and have a flowmeter.

I'm arranging to have the pump replaced on Tuesday. I'll inspect the internals once it's out and report back. I could also give Lycoming a call on Monday to see what their experience is with failure to high pressure.

FWIW, I'm a process machinery engineer. We'll figure this out.
These pumps have an inverse relationship between flow and pressure potential. At low GPH rates it can produces internal pressures greater than rated pressure and at max flow, they usually produce just under the rated pressure. However, there is an internal mechanism to reduce output pressure to a max of rated pressure, plus or minus a tolerance. I don't know the details of the mechanism used to regulate pressure, but presume it is related to diaphragms and/or springs. Whatever that mechanism is, it is compromised/failed in your pump allowing higher pressures at lower flow rates and why you see the highest pressure as idle. This is the same mechanism that keeps pressures at rated even with 0 flow.

Larry
__________________
N64LR
RV-6A / IO-320, Flying as of 8/2015
RV-10 in progress

Last edited by lr172 : 08-17-2018 at 10:16 PM.
Reply With Quote
  #24  
Old 08-18-2018, 07:22 AM
DanH's Avatar
DanH DanH is offline
 
Join Date: Oct 2005
Location: 08A
Posts: 7,796
Default

Quote:
Originally Posted by lr172 View Post
However, there is an internal mechanism to reduce output pressure to a max of rated pressure, plus or minus a tolerance.
No such mechanism Larry. As Tom noted in engineer-speak, output pressure is controlled by the spring force, (F = k*x).

David Carr posted an excellent pump tour some years ago, and I saved it all as a Word doc. FWIW, I also have a high pressure pump (for constant flow fuel injection) dismantled on my bench right now.

Here is a link to the Word doc:

https://www.danhorton.net/VAF/High&#...ismantled.docx

Illustration below from the Lycoming overhaul manual, 4-cyl pump cam, pushrod, and pump lever. Lycoming calls the cam an "eccentric", which is closer to the truth, as it is round with an off-center axis, rather than the familiar egg-shape lobe with the axis concentric with a base circle. The eccentric turns at one-half engine RPM.

In this illustration, the eccentric is shown rotated with its lowest point at the pushrod, which allows the fuel diaphragm to reach the limit of its output stroke, if there is no output restriction, and enough time to empty the pump chamber. However, when the engine is running, the diaphragm never moves that far.

See the Word doc. The lever lifts the diaphragm pull rod to compress the pump spring, but cannot push the diaphragm. When the eccentric rotates off its high point, removing force from the pushrod and lever, the diaphragm moves downward into the pump chamber by spring pressure alone. The lever, however, does indeed move through the entire range you see in the illustration. There is a separate light spring stacked on top of the pull rod's lift cap which keeps the the slack out of the lever/pushrod/ecccentric contact faces while the end of the diaphragm's pull rod is floating above the end of the lever.

See the return spring under the lever in the illustration? IIRC, that's the old pump style with a laminated arm. It's not present on late model pumps. The newer design incorporated the light spring you'll see in the Word doc.

__________________
Dan Horton
RV-8 SS
Barrett IO-390

Last edited by DanH : 08-18-2018 at 10:57 AM.
Reply With Quote
  #25  
Old 08-18-2018, 11:58 AM
DanH's Avatar
DanH DanH is offline
 
Join Date: Oct 2005
Location: 08A
Posts: 7,796
Default

Previously I wrote:

Quote:
In this illustration, the eccentric is shown rotated with its lowest point at the pushrod, which allows the fuel diaphragm to reach the limit of its output stroke, if there is no output restriction, and enough time to empty the pump chamber. However, when the engine is running, the diaphragm never moves that far.
Let's do the math and see how far it actually moves. Check me.

Assume 15 gph for a 180 hp engine at 2700 RPM, best power mixture. That's 0.25 gallons per minute, or 2 pints. The pump eccentric rotates at half speed, so the linkage pulls the diaphragm upward 1350 times per minute. Thus, the quantity per pump cycle is 2/1350, or 0.0014814 pints per stroke, which converts to 0.042775 cubic inches.

Because it is a diaphragm with a bellows at its perimeter, its motion can be described as cylindrical displacement. The effective diameter of the fuel diaphragm is about 1.75", so radius is 0.875". The volume of a cylinder is

volume = radius squared * pi * length

Here we want to determine the length (i.e. diaphragm stroke) needed to displace 0.042775 cubic inches, so the equation becomes

length = volume / (radius squared * pi), or

length = 0.042775 / (0.875^2 * 3.14), or

length = diaphragm stroke = 0.01779

The diaphragm moves about 0.018" per stroke...less than half a millimeter, at best power and 2700. (Same pump on a hot rod 540 at 30 gph would be twice the stroke...a whopping 0.032")

Consider the implications for spring force, F = k * x. I'll get a measurement to be exact, but for now, assume the compressed spring has a length of 1.75". During the pumping stroke, it relaxes by 0.018", a very small percentage of its length, so F varies by only a tiny bit. That's why the pump is almost constant pressure, regardless of flow, idle to redline.
__________________
Dan Horton
RV-8 SS
Barrett IO-390
Reply With Quote
  #26  
Old 08-21-2018, 09:42 PM
Tom @ N269CP Tom @ N269CP is online now
 
Join Date: Oct 2017
Location: Green Cove Springs, FL
Posts: 43
Default

We replaced the mechanical fuel pump this afternoon. During the subsequent test run, indicated fuel pressure at 1070 rpm was 4 psi. With the boost pump running in tandem, it was 6 psi. At 1700 rpm, the fuel pressure was also well within range (did not make note of the pressure...it was getting late). The high pressure (>8 psi) anomaly appears to have been resolved. I would also note that the pressures were rock steady. Whereas before pump replacement, the fuel pressure tended to be somewhat erratic. I did not replace the fuel pressure gauge today, and will keep the new gauge as a spare.

I partially disassembled the old mechanical pump. I cannot fully disassemble it as I lack the tools necessary to detach the plunger rod & upper/lower diaphragms and spacer ring from the actuator lever & spring housing. But I did manage to remove the lower cover, pulsator diaphragm and valve housing.

There were puddles of fuel under the pulsator diaphragm in the recesses on both the suction and discharge side of the lower cover. The pulsator diaphragm appeared to be in reasonably good condition, no tears, but the inlet side of the diaphragm was somewhat stiffer than the discharge side. No sign of fouling in the small vent hole in the lower cover. The throughbolts fastening the pulsator cover/diaphragm to the valve body were barely more than finger tight, which seems abnormal.

From what I could see of the upper/lower pump diaphragms, they appeared to be in reasonably good condition as well...better than yours, Dan. No outward signs of wear...further disassembly is necessary for a complete disposition. But there were a couple of strands (~1" long) of soft fibrous material that fell out of the space between the two pump diaphragms. Some oil also appeared to spill out of this space, but I cannot rule out overspill from the upper actuator housing.

Not sure if/when I'll be able to completely disassemble the plunger/diaphragm/spring assembly as I only have limited hand tools.
__________________
Kind regards,

Tom

==================================
RV-8 N269CP
O-360-A1A w/Hartzell CS prop on 100LL
Slick-IC+PMag ignitions
Steam gauges, EI UBG-16 & FP-5, Garmin Aera 660
TruTrak autopilot (GPS coupled)
Reiff engine preheater
AntiSplat oil mist separator
MH O2 system

Location: Durango, CO (KDRO)
Reply With Quote
  #27  
Old 08-22-2018, 05:36 AM
DanH's Avatar
DanH DanH is offline
 
Join Date: Oct 2005
Location: 08A
Posts: 7,796
Default

Quote:
Originally Posted by Tom @ N269CP View Post
I would also note that the pressures were rock steady. Whereas before pump replacement, the fuel pressure tended to be somewhat erratic.
Ok, I'm a believer. There really is something going on here. As before, the fun part is determining what and why.

You mentioned a 3-6 psi range in your first post, but now rock steady. That's interesting. Can you expand on "erratic"?

Quote:
I partially disassembled the old mechanical pump. I cannot fully disassemble it as I lack the tools necessary to detach the plunger rod & upper/lower diaphragms and spacer ring from the actuator lever & spring housing.
It's actually not hard to do. File off one end of the pump lever pivot pin, then drive it out of the case with a punch, after which the lever and bushing will slide out. With the lever's fork no longer hooked under the pull rod cap, the diaphragms, pull rod, and springs will all pop out of the case with a good tug. There is nothing holding them but a seal pressed into the case.

Quote:
But I did manage to remove the lower cover, pulsator diaphragm and valve housing. There were puddles of fuel under the pulsator diaphragm in the recesses on both the suction and discharge side of the lower cover. The pulsator diaphragm appeared to be in reasonably good condition, no tears, but the inlet side of the diaphragm was somewhat stiffer than the discharge side. No sign of fouling in the small vent hole in the lower cover. The throughbolts fastening the pulsator cover/diaphragm to the valve body were barely more than finger tight, which seems abnormal.
You think the pulsator chambers were full before disassembly, or did you manage to keep the pump upright when removing the bottom bolts?

Quote:
From what I could see of the upper/lower pump diaphragms, they appeared to be in reasonably good condition as well...better than yours, Dan. No outward signs of wear...further disassembly is necessary for a complete disposition. But there were a couple of strands (~1" long) of soft fibrous material that fell out of the space between the two pump diaphragms. Some oil also appeared to spill out of this space, but I cannot rule out overspill from the upper actuator housing.

Not sure if/when I'll be able to completely disassemble the plunger/diaphragm/spring assembly as I only have limited hand tools.
Fibrous material is also interesting. The vent to atmosphere is really tiny.

If you like, send it to me for disassembly.
__________________
Dan Horton
RV-8 SS
Barrett IO-390
Reply With Quote
  #28  
Old 08-22-2018, 09:21 AM
Tom @ N269CP Tom @ N269CP is online now
 
Join Date: Oct 2017
Location: Green Cove Springs, FL
Posts: 43
Default

Quote:
Originally Posted by DanH View Post
Ok, I'm a believer. There really is something going on here. As before, the fun part is determining what and why.

You mentioned a 3-6 psi range in your first post, but now rock steady. That's interesting. Can you expand on "erratic"?

You think the pulsator chambers were full before disassembly, or did you manage to keep the pump upright when removing the bottom bolts?

Fibrous material is also interesting. The vent to atmosphere is really tiny.

If you like, send it to me for disassembly.
By "erratic", I never knew where the fuel pressure was going to settle after startup. Anecdotally, it could shoot to a higher level then come down, flicker up and down by a couple of psi over a period of several seconds between first start and takeoff. It wasn't steady or predictable. But with the boost pump-only it was steady. It was like that since I bought the plane last August. I didn't pay much attention to it as fuel pressure was always within Lycoming's wide allowable pressure range. But when it went and stayed above that range, that caught my attention.

When I removed the pulsator cover the diaphragm was stuck to it around the entire circumference. The fuel puddles were evident when I peeled the pulsator diaphragm off the cover. The pockets were roughly 30-50% full.

I always try to be careful when disassembling failed machinery as solids and other debris can be strong evidence pointing to root cause. Many years ago when I was with Shell, we had a Kobe pitot pump that was failing main shaft bearings repeatedly after it was shutdown and restarted. The mechanics would disassemble the pump and clean it up before the reliability engineers would examine it. The pump was located downstream of a sand filter in a tertiary oil recovery process. I got involved and instructed the mechanics not to clean up anything after the next failure. After the next failure we found a considerable amount of sand inside the internal rotating casing. The pitot pump was basically an excellent centrifuge which removed carried over sand distributing it evenly around the ID of the rotating casing. But when shutdown the sand would slough off to the bottom of the casing creating a massive rotor unbalance which overloaded the bearings. So, I was careful when disassembling this pump.

It's probably best that I ship the fuel pump to you. All I have is a small collection of hand tools now. I sold off my well-equipped workshop with Shizuoka CNC knee mill a few years ago (sniff, sniff). If you can PM me your address, I'll get it on its way ASAP.
__________________
Kind regards,

Tom

==================================
RV-8 N269CP
O-360-A1A w/Hartzell CS prop on 100LL
Slick-IC+PMag ignitions
Steam gauges, EI UBG-16 & FP-5, Garmin Aera 660
TruTrak autopilot (GPS coupled)
Reiff engine preheater
AntiSplat oil mist separator
MH O2 system

Location: Durango, CO (KDRO)
Reply With Quote
  #29  
Old 09-01-2018, 09:56 PM
DanH's Avatar
DanH DanH is offline
 
Join Date: Oct 2005
Location: 08A
Posts: 7,796
Default

Tom, with the pump diaphragm assembly out of the pump body, this one seems obvious.

Both high and low pressure pumps have a seal on the pull rod shaft. Those seals fail by tearing out the center portion. This is your low pressure pump:



Slightly different view. Between the two, you can see how the center rips out:



Here, when the spring is compressed to a degree similar to an almost fully raised position, the inner core of the damaged seal catches on the edge of the remainder of the seal still in the upper spring seat. When it does, any additional raising of the pull rod must compress both the pump spring and the seal parts. There is a distinct momentary increase in force required to compress the pump spring, until the seal parts pop past each other...the extra spring force we were looking for.



Can I prove it? Nope, not without a spare accessory case, pushrod, and pump eccentric. However, I can send it back to you. You play with it and see if you're not left with the same conclusion.

The fuel and oil diaphragms were intact in all respects. All the vent passages were open, including the tiny restrictor in the main vent to atmosphere. There was minor evidence of oil between the diaphragms, "minor" described as much of the surfaces being oil-free. Given an intact oil diaphragm, the little oil present had to have gotten there via bolt holes due to low clamp force. I suspect the fuel in the pulsator cover chambers got there via the vent passage down the side of the case, sneaking between the underside of the fuel diaphragm and the pump chamber flange, again due to low clamp force, the bottom bolts being not much more than finger tight in your description.
__________________
Dan Horton
RV-8 SS
Barrett IO-390

Last edited by DanH : 09-02-2018 at 06:06 AM.
Reply With Quote
  #30  
Old 09-08-2018, 04:09 PM
Tom @ N269CP Tom @ N269CP is online now
 
Join Date: Oct 2017
Location: Green Cove Springs, FL
Posts: 43
Default

Quote:
Originally Posted by DanH View Post
Tom, with the pump diaphragm assembly out of the pump body, this one seems obvious.

Both high and low pressure pumps have a seal on the pull rod shaft. Those seals fail by tearing out the center portion. This is your low pressure pump:



Slightly different view. Between the two, you can see how the center rips out:



Here, when the spring is compressed to a degree similar to an almost fully raised position, the inner core of the damaged seal catches on the edge of the remainder of the seal still in the upper spring seat. When it does, any additional raising of the pull rod must compress both the pump spring and the seal parts. There is a distinct momentary increase in force required to compress the pump spring, until the seal parts pop past each other...the extra spring force we were looking for.



Can I prove it? Nope, not without a spare accessory case, pushrod, and pump eccentric. However, I can send it back to you. You play with it and see if you're not left with the same conclusion.

The fuel and oil diaphragms were intact in all respects. All the vent passages were open, including the tiny restrictor in the main vent to atmosphere. There was minor evidence of oil between the diaphragms, "minor" described as much of the surfaces being oil-free. Given an intact oil diaphragm, the little oil present had to have gotten there via bolt holes due to low clamp force. I suspect the fuel in the pulsator cover chambers got there via the vent passage down the side of the case, sneaking between the underside of the fuel diaphragm and the pump chamber flange, again due to low clamp force, the bottom bolts being not much more than finger tight in your description.
Hi Dan,

Thank you so much for taking a more detailed look for the root cause of failure. That failed seal is certainly abnormal and your hypothesis regarding the resulting higher spring force sounds plausible. Installing a new pump certainly seems to have resolved the high FP issue.

Sorry for the late reply but have been out flying in this beautiful late summer weather, and now dealing with yet a new "squawk" to fix.

I don't need the old pump back, so feel free to trash it or use as a paperweight.

Thank you again, Dan, for all your help.
__________________
Kind regards,

Tom

==================================
RV-8 N269CP
O-360-A1A w/Hartzell CS prop on 100LL
Slick-IC+PMag ignitions
Steam gauges, EI UBG-16 & FP-5, Garmin Aera 660
TruTrak autopilot (GPS coupled)
Reiff engine preheater
AntiSplat oil mist separator
MH O2 system

Location: Durango, CO (KDRO)
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 05:11 PM.


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.