VansAirForceForums  
Home > VansAirForceForums

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

- Today's Posts | Insert Pics

  #31  
Old 12-08-2015, 08:02 AM
DavidBurton DavidBurton is offline
 
Join Date: Jan 2005
Location: Near Seattle , WA
Posts: 79
Default

You may want to experiment with some mogas from a different source just to eliminate a fuel problem as a potential issue.

I got a tank of gas in 2013 that caused what felt exactly like vapor lock symptoms. After draining the fuel and running the engine long enough to clear the bad gas out of the system the engine performed normally, and has for years. I've never had a repeat of this problem.

I filled my riding lawn mower with some of the fuel I had drained out and mowed about 100' before the engine quit like I had turned the ignition switch off. I assume it took that long for the existing gas in the lines to be burned up and the drained fuel to reach the engine.

As a test I put some of this fuel in my truck and the engine ran OK, but the truck shook like a dog at idle. I added as much high octane fuel as I could to the tank and the truck idled normally after that as soon as the mixture sloshed around a bit.

I spoke with a testing lab about having the fuel I drained out tested, but $1000 was more than I wanted to spend to satisfy my curiosity.

This was summer blend and tests for water in the fuel were negative.
Reply With Quote
  #32  
Old 12-08-2015, 09:11 AM
Danny King's Avatar
Danny King Danny King is offline
 
Join Date: Dec 2004
Location: Southlake, Texas
Posts: 626
Default Gascolators

Good discussion here. I enjoyed the thoughtful post. I have the Doll's G3X redline warning for fuel pressure set a 18 psi. For me, that is an absolute minimum. So that needs to be addressed.

The grandfather worship for the use of gascolators in RV's has always baffled me. They are great for gravity feed systems like C-150's and Cubs, but gascolators are to be placed at the low point in the system so any water can be stopped there. The low point in RVs are the tank drains. That is especially true for taildragers. When mounted on the firewall, gascolators serve as a poor filter and a great heat sink, and IMHO, are just inviting trouble.

Certified Grumman low wing singles do not have gascolators because, like RVs, the low point in the system are the fuel tank drains. If I were you, I'd remove the gascolator and have just a fire shielded fuel line between the firewall fitting and the engine driven fuel pump.
__________________
Danny King
Beautiful Doll 80434 TT 1675 hours
I0360 A1B6 200 HP
Christen Inverted Oil
First Flight 12 July 2000
VAF Dues current for 2020

Last edited by Danny King : 12-08-2015 at 09:26 AM.
Reply With Quote
  #33  
Old 12-08-2015, 09:18 AM
dweyant dweyant is offline
 
Join Date: Sep 2012
Location: 07TS
Posts: 472
Default

Quote:
Originally Posted by 1001001 View Post
To answer this specific question--

As others have mentioned, it is a function of pressure drops in the system and the vapor pressure of the fluid being moved. The bubbles in the system caused by this mechanism are not air, but fuel being vaporized due to the low pressure at the discharge of the fitting.

Cavitation can be a serious problem in any kind of fluid system--in my professional experience it has been seen mostly in water pump systems with disastrous results for pump impellers and other components. Here's an example of what cavitation can do:
This is a result of tiny bubbles forming and collapsing at the impeller surface due to the local static pressure being below the fluid's vapor pressure at the operating temperature. When the microbubbles collapse they can create very powerful shock waves that can damage metal surfaces. The picture is of the rotor of a water turbine on the discharge (low pressure) side, but the same type of damage happens to pumps on the suction (low pressure) side.

Here's a link to a brief description of cavitation.
http://www.engineeringtoolbox.com/cavitation-d_407.html

And another (the source of the above linked image):
http://authors.library.caltech.edu/25019/1/chap6.htm

Cavitation in our fuel lines isn't likely to result in terrible damage like this (unless it happens in the fuel pump itself, perhaps), but the same principle is behind vapor bubbles forming in fuel lines near restrictions that induce pressure drops.
Thanks, that is a great explanation!

KBVI? I've flown my Cardinal into KBVI a few times. Really nice airport. I have family in East Palestine. Coming up for Christmas (flying united this time).

-Dan
__________________
Dan Weyant
RV-9A N96KD
Done and Flying 4/30/2015
Reply With Quote
  #34  
Old 12-08-2015, 09:30 AM
Elwell81 Elwell81 is offline
 
Join Date: Sep 2012
Location: Western Airpark (06WN) Yelm, WA
Posts: 41
Default I appreciate all of the input

The only reason I installed the gascolator is I like the serviceable aspect of it. With the Andair gascolator, I can remove/clean/replace the filter element without undoing any fuel fittings. It gives me the opportunity to know what I have as far as contamination, and a simple inline replaceable filter doesn't do that.

As I said in previous post, I installed the gascolator in the cabin, under the fuel selector valve to prevent a heating issue.

Autogas is on the back burner now (no pun intended). I need to get my fuel pressure sorted out. I think trying the autogas just magnified an issue that I was already experiencing.

I ordered a new mechanical fuel pump to try out. Best case I get my fuel pressure back, worst case is I have a spare fuel pump. I'm finding it hard to believe there is a restriction problem upstream of the mechanical fuel pump causing this, but stranger things have happened. I'll update when I get the new pump installed.
Reply With Quote
  #35  
Old 12-08-2015, 10:12 AM
rvbuilder2002's Avatar
rvbuilder2002 rvbuilder2002 is offline
 
Join Date: Jul 2005
Location: Hubbard Oregon
Posts: 9,035
Default

Quote:
Originally Posted by Danny King View Post
Certified Grumman low wing singles do not have gascolators because, like RVs, the low point in the system are the fuel tank drains. If I were you, I'd remove the gascolator and have just a fire shielded fuel line between the firewall fitting and the engine driven fuel pump.
Just another data point.......

Certified Pipers (the low wing kind) with Carb'ed engines do have gascolators, even though the apparent lowest point on those is also the sump drains at the tanks.

I don't disagree that the gascolator can be a source of fuel heating, but thousands of RV-'s flying successfully with them proves it is not a bad design (I flew one in the heat of Phx AZ for 5 years).

In this case the OP says the gascolator is aft of the firewall, so it is not likely contributing to the problem.
__________________
Opinions, information and comments are my own unless stated otherwise. They do not necessarily represent the direction/opinions of my employer.

Scott McDaniels
Van's Aircraft Engineering Prototype Shop Manager
Hubbard, Oregon
RV-6A (aka "Junkyard Special ")
Reply With Quote
  #36  
Old 12-08-2015, 10:30 AM
az_gila's Avatar
az_gila az_gila is offline
 
Join Date: Jan 2005
Location: 57AZ - NW Tucson area
Posts: 10,011
Smile

Quote:
Originally Posted by Danny King View Post
.....
Certified Grumman low wing singles do not have gascolators because, like RVs, the low point in the system are the fuel tank drains. If I were you, I'd remove the gascolator and have just a fire shielded fuel line between the firewall fitting and the engine driven fuel pump.
They sort of have a gascolator...

There is a similar sized filter than needs service every inspection integral to the firewall mounted electric fuel boost pump. Facet rates it as a 74 micron filter.



As you mention, it is not the lowest point in the fuel system, but it is the equivalent of a gascolator filter after the fuel pump.

I'm assuming that the "gascolator" functions both as a water collection point and as a filter.
__________________
Gil Alexander
EAA Technical Counselor, Airframe Mechanic
Half completed RV-10 QB purchased
RV-6A N61GX - finally flying
Grumman Tiger N12GA - flying
La Cholla Airpark (57AZ) Tucson AZ
Reply With Quote
  #37  
Old 12-08-2015, 12:06 PM
rv7charlie rv7charlie is offline
 
Join Date: May 2006
Location: Pocahontas MS
Posts: 3,884
Default

The ethanol test can be done in less than 2 minutes, and more reliably, if you have a capped container. I use an olive jar, ~1.5" dia ~6" tall. ~1" of water, up to my permanent mark on the jar. Fill with gas. Shake vigorously; let it sit for ~30 seconds & watch the water settle out. E-gas will show ~1/8-1/4" rise in the 'water' level at the bottom, depending on how much ethanol is in the gas.

IMO, the whole vapor pressure issue is a *hardware system* issue; not a fuel issue, as was hinted at in a recent post about planes like the RV-12, which were designed for auto e-gas. Since the advent of fuel injection in cars, is there anyone reading this who can say that they've ever had vapor lock (edit: in their fuel injected car)? I'm old enough to remember carb'd cars having the problem fairly often, and guess how a typical Lycoming engine delivers its fuel. (I'll make it easy: it uses an ancient auto style diaphragm pump mounted on the engine (where it's nice & HOT) to *suck* fuel *up* a long skinny line from the fuel source.) Supply fuel under at least minimal positive head to the pump, make sure that all fuel forward of the firewall is under pressure, and no fuel injected engine will ever experience vapor lock as long as it isn't on fire. The above would *not* apply to carb'd engines; you can't pressurize the float bowl.

Is there anyone who's running one of the auto style injection systems (using only redundant electric fuel pumps mounted in the cabin) who's ever experienced vapor lock? I consider this a valid point, because even 100LL can, and does, vapor lock in some situations (see above).

Charlie

Last edited by rv7charlie : 12-08-2015 at 02:12 PM.
Reply With Quote
  #38  
Old 12-08-2015, 02:02 PM
krw5927 krw5927 is offline
 
Join Date: Aug 2010
Location: Wichita, KS
Posts: 1,958
Default

Quote:
Originally Posted by Pittsartist View Post
5) Back to Vapour Pressure. If you dilute 100LL with mogas by even 5% the vapour pressure of the mixture becomes virtually the same as pure mogas.
Do you have a source for this? Sounds like an interesting read.
__________________
Kurt W.
RV9A
FLYING!!!
Reply With Quote
  #39  
Old 12-08-2015, 02:08 PM
ChiefPilot's Avatar
ChiefPilot ChiefPilot is offline
 
Join Date: Jan 2005
Location: Twin Cities, MN
Posts: 1,565
Default

Quote:
Originally Posted by krw5927 View Post
Do you have a source for this? Sounds like an interesting read.
I don't think that's true. I had heard this before and set about to test various ratios. The full thread is here. My tests would conclude that the above statement is not correct.

Summary: Adding a little 100LL to mogas disproportionally raises the vapor pressure. Pure mogas for this test tested at 16", 100LL tested at 21", and a ratio of 50/50 tested at about 20.2".



There's more to it, so look at the linked thread if you're so inclined.
__________________
Brad Benson, Maplewood MN.
RV-6A N164BL, Flying since Nov 2012!
If you're not making mistakes, you're probably not making anything
Reply With Quote
  #40  
Old 12-08-2015, 02:33 PM
krw5927 krw5927 is offline
 
Join Date: Aug 2010
Location: Wichita, KS
Posts: 1,958
Default

Quote:
Originally Posted by ChiefPilot View Post
I don't think that's true. I had heard this before and set about to test various ratios. The full thread is here. My tests would conclude that the above statement is not correct.
That was my understanding too, Brad. I was hoping to read the other side of the coin if one exists.

Edit: Found some factual references to compare RVP between mogas and 100LL.

Shell (here) lists RVP for 100LL at 38-49 kpa (5.5-7.1 psi) at 38C (100F).

The ASTM spec for auto fuel costs $$ to purchase, but in this paper by CARB, the ASTM values for certain areas of California are published. (The numbers are also in psi at 100F.) You can see how they vary by month and area, and how in general are higher than 100LL (especially in winter).
__________________
Kurt W.
RV9A
FLYING!!!

Last edited by krw5927 : 12-08-2015 at 02:50 PM.
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 01:57 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.