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07-29-2013, 12:31 PM
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Join Date: Jun 2006
Posts: 316
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Gascolator
This is a letter we wrote to a customer about gascolator use in fuel injected engines.
To fully understand the situation you must first understand why a gascolator was used in the first place. Back when carburetors were the only source of fuel metering on aircraft, water was a detriment to the operation of the engine. Of course water won?t burn but the main reason was that water being heaver than fuel would sink to the bottom of the float bowl. Water also has a higher surface tension than fuel. Since the main jets are located in the bottom of the float bowl and the metering head (the suction created by the venturi to suck the fuel out of the bowl) is low on a carburetor, the water can actually block the flow of fuel through the main jets due to surface tension, thus starving the engine of fuel.
Enter the fuel injection system. There?s no float bowl, the fuel is under pressure (20-30 PSI). So in this case even if there?s some water in the system the fuel control will flow the liquid what ever it is. Granted the engine cannot burn water but there will be no interruption of the delivery of fuel to the engine.
One thing that is in the carburetors favor is that with the float bowl, if any foamy fuel or fuel vapor is in the system, the float bowl will vent off the vapor and the main jets are only exposed to liquid fuel. Correct fuel metering in this case is mostly not effected. Unless the engine driven fuel pump or boost pump is vapor locked the engine will get fuel. In this respect, carburetors are less affected by hot operation. More on that subject later.
Now look at the aircraft fuel system. In an RV aircraft the fuel tanks are the lowest point in the fuel system. Water will be in the sumps of the tanks. If water is found during a sumping operation then further investigation should be made concerning the entire aircraft fuel system condition.
1. Installing a gascolator in any other place other than the wing roots would result in the gascolator being higher than the lowest point in the fuel system. Not the place to catch water.
2. Since the RV aircraft can do some aerobatic maneuvers, rolling the aircraft upside down would dump any water into the fuel system if any were held in the gascolator.
3. Since the fuel injection system does not have a float bowl, fuel vapor is a problem and can cause poor fuel metering. Avgas boils at around 130 degrees F at sea level. The boiling point gets lower as altitude increases and also with a decrease in fuel pressure on the suction side of the fuel pump (flow losses in the fuel system). Since the gascolator is typically installed on the firewall, it is subject to a high heat environment. The volume of fuel in the gascolator does not change very fast at low power therefore the fuel in the gascolator picks up heat, which can lead to fuel vapor issues and possible vapor lock of the engine driven fuel pump.
4. Installation of the gascolator on the fire wall though not recommended on this installation can be accomplished by the following:
a. Make sure the gascolator can withstand 30 PSI, as it will be pressurized when the boost pump is on.
b. Install a blast shield over the gascolator and provide blast air to keep the gascolator cool.
c. Understand that having a volume of fuel on the firewall (gascolator) may result in rough engine operation and poor idle, under hot conditions.
d. With fuel injected installations we want to minimize the volume of fuel in the engine compartment as much as possible. This applies to hose routing and components that increase the volume of fuel that can be heated.
Hope this helps
Don
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07-29-2013, 03:15 PM
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Join Date: Oct 2012
Location: Scottsdale, AZ
Posts: 150
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That does help a lot, thanks! I guess the only question I have then is if I remove the gascolator, do I need to add fuel filter somewhere to catch sediment? I know there is one in the fuel injection servo, is that all I need?
Quote:
Originally Posted by Don at Airflow
This is a letter we wrote to a customer about gascolator use in fuel injected engines.
To fully understand the situation you must first understand why a gascolator was used in the first place. Back when carburetors were the only source of fuel metering on aircraft, water was a detriment to the operation of the engine. Of course water won?t burn but the main reason was that water being heaver than fuel would sink to the bottom of the float bowl. Water also has a higher surface tension than fuel. Since the main jets are located in the bottom of the float bowl and the metering head (the suction created by the venturi to suck the fuel out of the bowl) is low on a carburetor, the water can actually block the flow of fuel through the main jets due to surface tension, thus starving the engine of fuel.
Enter the fuel injection system. There?s no float bowl, the fuel is under pressure (20-30 PSI). So in this case even if there?s some water in the system the fuel control will flow the liquid what ever it is. Granted the engine cannot burn water but there will be no interruption of the delivery of fuel to the engine.
One thing that is in the carburetors favor is that with the float bowl, if any foamy fuel or fuel vapor is in the system, the float bowl will vent off the vapor and the main jets are only exposed to liquid fuel. Correct fuel metering in this case is mostly not effected. Unless the engine driven fuel pump or boost pump is vapor locked the engine will get fuel. In this respect, carburetors are less affected by hot operation. More on that subject later.
Now look at the aircraft fuel system. In an RV aircraft the fuel tanks are the lowest point in the fuel system. Water will be in the sumps of the tanks. If water is found during a sumping operation then further investigation should be made concerning the entire aircraft fuel system condition.
1. Installing a gascolator in any other place other than the wing roots would result in the gascolator being higher than the lowest point in the fuel system. Not the place to catch water.
2. Since the RV aircraft can do some aerobatic maneuvers, rolling the aircraft upside down would dump any water into the fuel system if any were held in the gascolator.
3. Since the fuel injection system does not have a float bowl, fuel vapor is a problem and can cause poor fuel metering. Avgas boils at around 130 degrees F at sea level. The boiling point gets lower as altitude increases and also with a decrease in fuel pressure on the suction side of the fuel pump (flow losses in the fuel system). Since the gascolator is typically installed on the firewall, it is subject to a high heat environment. The volume of fuel in the gascolator does not change very fast at low power therefore the fuel in the gascolator picks up heat, which can lead to fuel vapor issues and possible vapor lock of the engine driven fuel pump.
4. Installation of the gascolator on the fire wall though not recommended on this installation can be accomplished by the following:
a. Make sure the gascolator can withstand 30 PSI, as it will be pressurized when the boost pump is on.
b. Install a blast shield over the gascolator and provide blast air to keep the gascolator cool.
c. Understand that having a volume of fuel on the firewall (gascolator) may result in rough engine operation and poor idle, under hot conditions.
d. With fuel injected installations we want to minimize the volume of fuel in the engine compartment as much as possible. This applies to hose routing and components that increase the volume of fuel that can be heated.
Hope this helps
Don
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07-29-2013, 03:16 PM
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Join Date: Oct 2012
Location: Scottsdale, AZ
Posts: 150
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For now I've added a blast tube to the gascolator. If I remove it entirely I will re-purpose that to the fuel pump.
Quote:
Originally Posted by Don at Airflow
This is a letter we wrote to a customer about gascolator use in fuel injected engines.
To fully understand the situation you must first understand why a gascolator was used in the first place. Back when carburetors were the only source of fuel metering on aircraft, water was a detriment to the operation of the engine. Of course water won?t burn but the main reason was that water being heaver than fuel would sink to the bottom of the float bowl. Water also has a higher surface tension than fuel. Since the main jets are located in the bottom of the float bowl and the metering head (the suction created by the venturi to suck the fuel out of the bowl) is low on a carburetor, the water can actually block the flow of fuel through the main jets due to surface tension, thus starving the engine of fuel.
Enter the fuel injection system. There?s no float bowl, the fuel is under pressure (20-30 PSI). So in this case even if there?s some water in the system the fuel control will flow the liquid what ever it is. Granted the engine cannot burn water but there will be no interruption of the delivery of fuel to the engine.
One thing that is in the carburetors favor is that with the float bowl, if any foamy fuel or fuel vapor is in the system, the float bowl will vent off the vapor and the main jets are only exposed to liquid fuel. Correct fuel metering in this case is mostly not effected. Unless the engine driven fuel pump or boost pump is vapor locked the engine will get fuel. In this respect, carburetors are less affected by hot operation. More on that subject later.
Now look at the aircraft fuel system. In an RV aircraft the fuel tanks are the lowest point in the fuel system. Water will be in the sumps of the tanks. If water is found during a sumping operation then further investigation should be made concerning the entire aircraft fuel system condition.
1. Installing a gascolator in any other place other than the wing roots would result in the gascolator being higher than the lowest point in the fuel system. Not the place to catch water.
2. Since the RV aircraft can do some aerobatic maneuvers, rolling the aircraft upside down would dump any water into the fuel system if any were held in the gascolator.
3. Since the fuel injection system does not have a float bowl, fuel vapor is a problem and can cause poor fuel metering. Avgas boils at around 130 degrees F at sea level. The boiling point gets lower as altitude increases and also with a decrease in fuel pressure on the suction side of the fuel pump (flow losses in the fuel system). Since the gascolator is typically installed on the firewall, it is subject to a high heat environment. The volume of fuel in the gascolator does not change very fast at low power therefore the fuel in the gascolator picks up heat, which can lead to fuel vapor issues and possible vapor lock of the engine driven fuel pump.
4. Installation of the gascolator on the fire wall though not recommended on this installation can be accomplished by the following:
a. Make sure the gascolator can withstand 30 PSI, as it will be pressurized when the boost pump is on.
b. Install a blast shield over the gascolator and provide blast air to keep the gascolator cool.
c. Understand that having a volume of fuel on the firewall (gascolator) may result in rough engine operation and poor idle, under hot conditions.
d. With fuel injected installations we want to minimize the volume of fuel in the engine compartment as much as possible. This applies to hose routing and components that increase the volume of fuel that can be heated.
Hope this helps
Don
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07-29-2013, 05:54 PM
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Join Date: Jun 2006
Posts: 316
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Installation for fuel injected engines
We recommend that the boost pump be installed on the cold side of the firewall also. There should be some type of fuel filter before the boost pump to protect the boost pump.
Don
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07-29-2013, 06:07 PM
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Join Date: Oct 2012
Location: Scottsdale, AZ
Posts: 150
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My boost pump is installed directly in front of the wing spar between the seats. I'm not the builder, but I looked at your website and I think I may have your boost pump and fuel filter. My plan is to fly with the modifications I've already made for now and remove the gascolator and add a cooling shroud and blast tube for the fuel pump at the next annual. Because of the way it's installed, I can't bypass it without removing it from the firewall and I don't have time to do that right now. Thanks for all the help!
Quote:
Originally Posted by Don at Airflow
We recommend that the boost pump be installed on the cold side of the firewall also. There should be some type of fuel filter before the boost pump to protect the boost pump.
Don
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07-29-2013, 11:15 PM
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Join Date: Feb 2006
Location: Goodhue, MN
Posts: 160
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Slightly off topic, a question for the folks who see this behavior with their engines -- what sort of OAT's do you see it at? I have heard much discussion of this happening on "hot" days, but I have encountered it on ground and in flight at anything over ~55F, which is a stretch to call "hot." The stutter is much as described in the OP. Disappears with higher fuel flows above 1200RPM, but the engine feels like it may quit at any moment on the ground (and has, three times now...I tend to taxi with more throttle than I'd like as a result.)
This is really the only problem I've had with my 9A...
IO-320, ECI from Aerosport.
AFP Injection system.
Catto 3 blade prop.
Vetterman muffler exhaust.
Andair pump and filter, which has been serviced along with the servo inlet filter.
Boost pump on/off makes no difference.
Red cube moved to cabin between pump and FW.
FWF fuel lines shortened as much as possible.
Engine fuel pump has been replaced, and blast tube w/shroud installed.
Servo to spider line covered with heat rejection sleeve.
Never had cooling problems on engine...oil temps work hard to get much above 180, and typical cruise CHTs average 300 in current weather.
I've just accepted it as normal so far, but it's bad enough that I would hesitate to take it to OSH as the long taxi would be tough to manage wrt throttle and brakes. I imagined that "hot" would be OAT 85+...
__________________
Joshua Wyatt | Goodhue, Minn.
RV-9A N627DW @ KRGK - Flying since 2012
AFS 4500s | Stratux | SDS EM-5
rv9a.pacificrimsound.com
Last edited by jwyatt : 07-29-2013 at 11:19 PM.
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07-30-2013, 05:53 AM
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Join Date: Oct 2012
Location: Scottsdale, AZ
Posts: 150
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You don't give me much hope for fixing it on mine, but I am starting to wonder if we see this problem more often with wood props because a heavy metal prop might just spin right through it without much of a hiccup. I have the Catto 3 blade and one of the A&Ps across from my hangar mentioned that the Long EZ next door also does it a little bit and he has a Catto 3 blade too.
Quote:
Originally Posted by jwyatt
Slightly off topic, a question for the folks who see this behavior with their engines -- what sort of OAT's do you see it at? I have heard much discussion of this happening on "hot" days, but I have encountered it on ground and in flight at anything over ~55F, which is a stretch to call "hot." The stutter is much as described in the OP. Disappears with higher fuel flows above 1200RPM, but the engine feels like it may quit at any moment on the ground (and has, three times now...I tend to taxi with more throttle than I'd like as a result.)
This is really the only problem I've had with my 9A...
IO-320, ECI from Aerosport.
AFP Injection system.
Catto 3 blade prop.
Vetterman muffler exhaust.
Andair pump and filter, which has been serviced along with the servo inlet filter.
Boost pump on/off makes no difference.
Red cube moved to cabin between pump and FW.
FWF fuel lines shortened as much as possible.
Engine fuel pump has been replaced, and blast tube w/shroud installed.
Servo to spider line covered with heat rejection sleeve.
Never had cooling problems on engine...oil temps work hard to get much above 180, and typical cruise CHTs average 300 in current weather.
I've just accepted it as normal so far, but it's bad enough that I would hesitate to take it to OSH as the long taxi would be tough to manage wrt throttle and brakes. I imagined that "hot" would be OAT 85+...
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07-30-2013, 10:27 AM
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Join Date: Oct 2012
Location: Scottsdale, AZ
Posts: 150
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no luck
Unfortunately improving the baffling and adding the blast tube really didn't help at all. My CHTs are maybe down 5 or 10 degrees on average, but it's hard to say for sure.
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07-30-2013, 02:22 PM
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Join Date: Jun 2006
Posts: 316
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Hot idle problems.
Yes, of course no weight on the crank doesn't help the hot idle problem either. But you have some installation issues to clean up yet. Remove the gascolator from the firewall, remove any pressure drops from the suction side side of the pumps. The flow meter should be in the metered fuel line.
If all that does not help then the last resort is to install smaller nozzle restrictors to raise the nozzle back pressure. This will not completely cure the problem but usually helps. We need to know the take off fuel flow and inlet fuel pressure to calculate the nozzle size that can be installed.
Don
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07-30-2013, 03:05 PM
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Join Date: Nov 2012
Location: Cincinnati,Ohio KHAO
Posts: 128
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Spark Plugs.....Hmmm
Bj, Just couldnt help but notice this comment on the Champion fine wire plugs.
I never liked them. They have a screw in the center of the plug where the connection (electrical) is made. Below it is a spring and a formed resistor. The resistance from the electrode end and that screw should be 1K ohms to maybe
2K Ohms, that s it. I have found a number of plugs that have 20K and greater resistance which gives you a "miss", but probably not like what you are seeing. What if many of these plugs have this high resistance, Hmmmm... High temperature, greater resistance. I only use Tempest now on my 180Hp Lyc and REM40E only. These have a sealed resistor which reads 1K no matter how many hours I put on them. Take a few plugs out and check, can't hurt.
__________________
John R Prince
RV-7 want-a-b
Dues Paid for 20
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