bird

bird

Well Known Member
Hi everyone,

I am hoping for some input and opinions on my fuel system. Planning on an 0360 a1a, with a carburetor. My question is about filtering. Vans says to put a filter before boost pump, good idea to prevent clogging the aux pump and I plan on doing that. I want to keep my fuel system as simple and reliable as possible. So, if I put a good filter there do I still need to put a gascolator on the firewall? My thinking is that there is only a pump in between the two. I understand the need for a place to tap for fuel press transducer, a fitting maybe at the firewall. My friend has a gascolator on his 8 and I noticed he can't sump it unless the aux pump is on, it is higher than tanks. I am open to any ideas and recommendations. As for the filter before the aux pump( best type, name brands etc). Oh and if it makes any difference, I am planning on a red cube after the engine pump and before the carb. Thanks to all in advance.


Bird
 
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As posted earlier:
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

In other words, put the gascolator below the level of the wing tanks if you can.
 
Gascolator

You're going to get a lot of opinions!

If it were my airplane I would use a single gascolator mounted on the firewall and use the Van's optional finger screens (not the sawcut tubes shown in the plans) on the fuel pickup tubes in the tanks.
1. The sumps on the tanks do a good job of draining any water during preflight as they are the lowest point on the fuel system.
2. The Van's optional finger screens should be adequate to filter fuel for the carb boost pump.
3. Gascolators in the wingroots are a pain to mount and plumb and are twice as expensive as a single gascolator on the firewall. I know, I'm doing this on my RV-9A, but it's injected and has a boost pump less tolerant of contamination.
4. Although the gascolator mounted on the firewall is higher than the fuel tanks, the bowl will allow any dissolved water to drop to the bottom. Of course this is drained off during preflight.
5. The cheapest, simplest and easiest gascolator to mount to the firewall is the Van's one. I think its filtration rating is 100 microns. If you want an upgrade, consider the Andair GAS375. Its filtration rating is 70 microns, but it's not so easy to mount. It has some nice features such as the safety wire drilled bowl ring.
6. Regarding heating of the fuel in a firewall mounted gascolator, consider that virtually every Cessna has their gascolator mounted on the firewall. If it proves to be a problem it's easy to run a blast tube to cool it, similar to the engine driven fuel pump.
 
Thanks for the info this is helpful to me, my tanks are already done, I don't really want to open them, i did the saw cut per instr., but I am debating that verses adding The vans recommended filter before the aux pump. I am leaning towards the screens in each tank, that would eliminate a single point for clogging and give me the abilty to swap tanks in the event of a total screen clog in either tank. For the firewall i have an andair 375 and getting or building a bracket is not a problem for me. So it looks like its going to be a gascolator and either screens in tanks or a filter before the aux pump. Thanks again and any other thoughts are welcomed.

Bird
 
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Not advice, just my thoughts.

Lowest point is the drain in the tanks - I occasionally see some small droplets here on the first drain. 2nd drain always clear.

I now use the Andair "Gascolator" as a filter, mounted under the LH Mid Cabin Cover. Takes about 2 minutes at servicing time to undo wirelock, unscrew bowl and remove bowl / filter and clean etc. Never seen any water in the bowl, despite the drain being plugged and sealed (since it is in the cockpit).

Talking to people far more knowledgeable than me, and also the regulations, the "requirement" for a Gascolator seems "varied". If I find someone who says I "must have one or will die" I describe the Andair part as a Gascolator. To others I describe it as a filter. If you wanted a genuine drainable Gascolator, you could fit the Andair in the wing root, or the cockpit floor area with a proper drain hole / access.
 
My system

RV-6A, O-360 A1A, 1 filter for each tank (4) no gascolator. 1,000+ hrs~9 years operation. No problems.

Bob Axsom
 
I am using FI so I am not using a Gascolator but.
A screen in the tank is great but not serviceable. I would want a serviceable filter before the aux. pump.
A Gascolator is more that a filter, it is a water separator. Even water partially mixed in the fuel will be separated. A carburated engine will not run if water gets in the main jet. I would avoid this at all costs. I would install a Gascolator that I can sump on every fuel fill-up.
The C172 I fly has fuel injection AND 13 sump drains.
 
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