How many fuel filters do you have/need?

[sirlegin]

We have decided that removing the gascolator from the engine side of the firewall may stop our engine surging in high temps at low rpm's due to the fuel boiling/pre heating here. That leaves a fuel filter at the boost pump and a fuel filter at the spider valve.
This is a 9A with a Mattituck 0-320 with dual lightspeed ignition, fuel injection (Formerly FADEC) and a Catto 3 blade prop.
How many fuel filters are required?
As of right now we get no junk from the wing sumps or water. Hangared.

[terrye]

You should consult the fuel injection manufacturers and boost pump manufacturers for recommendations on fuel filtration.

The Precision/Bendix injection system manuals recommend 32 micron filtration. The finger screen in the servo is 64 micron I believe and is a "last chance" filter, not intended as a main filter.

My boost pump manufacturer says upstream filtration is required, I have 70 micron filters.

If you're really sure the location of the firewall mounted gascolator is causing the engine surging problem, have you considered running a blast tube to and surrounding it with a cover similar to the one Van's sells for the engine mounted fuel pump? Also have you tried running a blast tube to the engine mounted fuel pump?

[Mel]

I have a filter on each tank just ahead of the selector valve.
In the unlikely event that a filter becomes restricted, I can switch tanks and avoid an engine stoppage.

[N427EF]

One is all you need, and it should have the mesh size required by the fuel system designer.
Good decision on removing your gascolator, should help smooth out your
idle operations.
You have probably seen this in another thread.
FWIW I also have 2 filters one for each tank

Old 07-29-2013, 01:31 PM
Don at Airflow Don at Airflow is offline

Join Date: Jun 2006
Posts: 216
Default 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

[Bevan]

One Andair gascolator/filter for each tank (in the wing root), and one 40 micron after the engine driven fuel pump immediately prior to the fuel servo as directed by Prescision (Silver Hawk).

Bevan
injected 0-360
not flying yet.

[BillL]

Originally Posted by N427EF

One is all you need, and it should have the mesh size required by the fuel system designer.

Probably too much information, but be careful with this statement. Each filter protects something. As previously mentioned, the 70 micron filter prior to the boost pump protects the pumps moving parts. The filter (32 micro in this example) ahead of the servo protects the fuel system parts and orificies. There are probably edge filters for errant particles in the system too.

A finer filter typically means higher pressure drop, so they belong after the pumps and only required (minimal) filtration prior to the pumps. This yields a system with the highest flow and temperature capability.

Thanks for the requote from Don. Excellent, specific information.

[grayforge]

I asked this question of both EFII and Precision and got the same answer: The pre-fuel pump filter is the only one needed in this scenario: EFII Boost pump, included pre-pump filter and Precision Silverhawk injection.

Maybe this is because injectors in Mechanical FI systems have much larger nozzle holes than with electronic FI.

[Bubblehead]

I had been battling intermitten clogged injectors and followed Danny King's advice and put an Edelbrock 40 u filter in each wing root. I also rerouted the fuel line on the left wing a bit to better accomodate the Andair fuel selector - which I LOVE.
Since adding the filters in the wing roots about 50 hours ago I have not had a clogged injector. I wish I had followed Danny's advice when he first offered it about 3 years ago. I might have fewer gray hairs!
I opened them up during the oil change a month ago and found a little fuzz in one filter and several black flakes in the other. Either one could have caused a clog.
I have a lot of faith in Don and his product but it would seem that with their 125 u filter protecting the pump there still needs to be a filter to protect the injectors. I know there is a filter in the servo but I have never found anything in that filter.
Time will tell if the filters will take care of the clogged injector problem but right now it looks good.

[Taildrgr]

Originally Posted by Bubblehead

I had been battling intermitten clogged injectors and followed Danny King's advice and put an Edelbrock 40 u filter in each wing root.

Does the Edelbrock filter have 37 degree fittings or are the fittings replacable? I could not tell from their web page. I would like to put these in my wing roots also.

[Bob Axsom]

One for each tank prior to selector valve. No problem with fuel flow 1000+ hours.

Bob Axsom