Fuel System Questions

[Battson]

My plan is to install the transducer just before the fuel distributor which will place it after the return line so all the fuel it is measuring will be fuel used. The tech I talked to at GRT said this won't work. He couldn't tell me why it wouldn't and he didn't know of anyone that had tried it. Because it not working won't be a safety of flight issue I think I'll give it a try.

It works just fine with the right setup.

I have certainly seen it done that way on some certified aircraft, first hand. I am 95% sure that is how Cirrus do it on the IO-550, but I could also be thinking of a Cessna 206. That is the FT-90 Red Cube, or maybe the Gold Cube(?). They put it in the flexible hose connection. Don at airflow performance also recommended installing it that way.

You may want to look into heat shielding the thing, but I can't recall seeing heat shielding on the install I saw. I think engine vibration was the biggest concern for the FT-90 Red Cube, and by installing it in the flexible hose I think they got around that problem too.

[whee]

Just for the sake of clarify, I will restate a few things which are already very obvious to everyone here, in order to build to a logical conclusion:

The fuel system isn't designed to operate with anything more than the very occasional droplet of water in it, during flight. The only *safe* option is to remove the water pre-flight.

Gascolators are good traps for catching heavy particles at any time, and they have a coarse screen inside them to prevent big stuff getting past and blocking downstream filters. But water is not *that* much heavier than fuel, can't be screened out, and the gascolator is really only designed to catch droplets during flight, rather than anything more serious.

So, the gascolator could be installed anywhere in the system at achieve that end. But it is installed at the lowest point to catch water before flight begins, which naturally drains downhill.

That water can come from three main sources:

1. Due to condensation overnight while the plane is parked. That water sinks, flows downhill, and pools at the lowest point - either the tank drain or the gascolator. It can be drained before flight.
2. Leaks in the tank cap or elsewhere in the system, during rain while parked outside. Again - drained as above.
3. Water entering the system during re-fuelling. That needs to be given time to settle, and drained from the tanks (and gascolator if there's a lot) before flight.

If you have no gascolator, or its not at the lowest point - then you have no way of draining any water which gets past the fuel tank's low point. That water then has no choice but to be injected into your engine.

So my point is, there are basically two different functional requirements of that item within the fuel system:

1. In-flight water trap + strainer function
2. Lowest point water drain pre-flight function

In conclusion - even if the current thinking is you don't need to fulfil the first requirement (which is a big call), you still need to install something to meet the second requirement anyway, at least a drain valve - and it might as well be a gascolator. You get the added benefits of the strainer, bigger water trap, and no loss of fuel pressure even if a huge volume of nastiness ends up in your system

I'm not sure that the gascolator will be catching much, if any, water while in flight unless it is fairly large. I agree that a gascolator does a good job catching heavy particles but with a large area fuel filter I'm not sure a gascolator is needed to do that job. I agree with your conclusion that if a drain is going to be installed at the low point it might as well be a gascolator. However, since my fuel valve will be mounted approximately where your gascolator is I don't have a good none obtrusive spot to mount a gascolator. By placing a drain in the line I can place that drain forward of the fuel valve in an accessible location and I won't need a removable panel to allows access to remove the gascolator bowl for maintenance.

The decision to install or not install a gascolator is a serious one and I am not taking it lightly. I think it is worth noting that many RVs with FI do not have gascolators installed because the manufactures of their FI systems recommend not installing one. I am really enjoying appreciate all the points being made. Please don't think I'm dismissing anyone's thoughts because I'm not.

[Battson]

I am also unsure how much water, if any, they can catch in flight. But they are the only line of defence in that regard, something you end up thinking about when flying in the rain.

A final thought - if you don't go for a gascolator, please do change that inline filter after less than 10 hours flying, for the first time.

Mine system was full of unavoidable swarf, dust, fibres, insects, and other debris left over from the building process. I have no idea how so much accumulated in the system. My gascolator had teaspoons full of junk inside. The fuel filter also had some stuff inside, but it was all finer grade and the fuel pressure was only down about 10-20%. The gascolator caught most of the debris.

And before anyone asks, I was meticulously careful about always keeping all open lines taped over, tanks fillers taped over, etc. during the building process.

[marcusofcotton]

I'm not sure that the gascolator will be catching much, if any, water while in flight unless it is fairly large.

Which seems like an especially good reason for it. It doesn't happen often, but it does happen. What if you are remote camping with the family? Sometimes it pays to look at what the certified aircraft are doing, there is a reason for it.

Something else that hasn't been mentioned but perhaps should be in this thread (you may already know this but many don't): Many paper filters (all???, most???) will severely or totally block flow in the case of water in the fuel. As mentioned earlier, a transparent housing is nice and with a fine screen.

fwiw (I agree, good conversation!)

[Bdflies]

On a previous plane build (Continental IO520-D), I used a 'high performance' fuel filter. The photo attached, is representative. I welded a steel AN nipple into the lower portion, so I could drain it, prior to flight. The paper element is quite large, so complete blockage would be hard to imagine. The replaceable paper element is much better at removing particulates than a gascolator screen, which generally excludes small children and most animals.😎 I considered it to be the best of both worlds, in that I had a good filter and I could drain any water from the housing. Worked great for the 12 years I flew it and still flying, as far as I know.
BTW, I'm using Bob's gascolator on the Patrol I'm building.

Bill

[JimParker256]

I, too, am enjoying the discussion and learning something along the way. (That's practically a given for me, as a first-time builder!)

Water contamination from condensation and from parking outside (on trips - I keep it hangared at home) are the main reasons I would use a gascolator, even on an FI engine. It's not intended to trap water during engine operation or in flight, but rather to separate it out and trap it while sitting on the ground. I once pre-flighted a Cessna where I pulled five GATTS jars full of water out of ONE tank before getting a clean sample. The gascolator had another almost-full jar of water. I decided I was not going to fly that airplane until the fuel system was drained and flushed, but the short version is that maintenance found the fuel cap was allowing standing rain water to enter the tank while parked...

The large screen on the gascolator is also a good way to trap any "stuff" that manages to make its way into the fuel system, whatever the source. There's a lot of surface area on that gascolator screen, compared to any of the in-line fuel filters...

Speaking of which, I am still looking for a fuel filter that includes a bypass-when-clogged capability. Aircraft Spruce has this one but it has a metal-housing. I'd really like to have a clear housing so you can see if it's getting clogged. If forced to go with this one, and since there's no visual indication when it is clogged and starting to bypass, I would put a second (clear) filter downstream from it. The instant I saw ANYTHING in that second filter, I'd clean the metal one (and replace the paper element in the 2nd filter as well). The good news is that the metal-bodied Spruce filter has a "washable" metal filter screen, so that should be a relatively easy process.

Back to questions about fuel system design for a fuel-injected engine... My certified Rockwell Commander 114 (with turbo-normalizer) uses the standard Lycoming-supplied system, so there is no fuel returned. As a result, the engine vapor-locks (or at least develops vapor bubbles in the fuel lines) quite readily. It can be a royal pain to start when it is hot - and I don't just mean when the engine has been running, but pretty much any time during our 105+ degree summer days...

I understand that Continentals (and at least some aftermarket FI systems for Lycoming engines) are much better in this regard, because they DO circulate fuel, and you can run the boost pump for a while to circulate cool(er) fuel through the lines to flush out those vapor bubbles in the lines, greatly reducing the potential for vapor-lock problems. But if you want to take advantage of that, I would think that your fuel system design should:

• return the excess fuel to a header tank, or
• return the excess fuel to the wing tank, or
• circulate the excess fuel through sufficient length of fuel lines to radiate the heat away

And if you're relying on the last solution, wouldn't running these "radiator" lines just above the tunnel (where it has to be warmer than the surrounding atmospheric air due to the exhaust flow) mean that you might not be able to achieve the cooling you need? And is it a good idea to have pressurized fuel circulating inside the cabin, right below our feet? Just thinking out loud and wondering...

PS - it was exactly all these questions and issues that led me to decide that I'm going with a carbureted engine in my Patrol:

Just as God (and Bob Barrows) intendedl...

[whee]

Which seems like an especially good reason for it. It doesn't happen often, but it does happen. What if you are remote camping with the family? Sometimes it pays to look at what the certified aircraft are doing, there is a reason for it.

I meant I think it would take a fairly large gascolator to slow the fuel enough that the water and debris would drop out of it during flight. I have been looking closely at certified installations but I think sometimes they just do things the way they have always been done for ease of certification.

Bill, I like what you did with the filter for the 520 but is big, kinda heavy and has a paper element. I'd like to avoid paper if possible and I only have 3" of vertical space to mount a filer or gascolator.

Jim, That Golan filter on ACS is a good find. I just talked to Golan and at 10 micron it is rated at 15GPM (yes, gallons per minute). It has a prescreen and a 10micron filter element (60 and 40 micron elements are available). You can mount it in reverse so if the element gets plugged it will compress a spring so fuel can get by and the screen will catch any large debris. They claim no pressure loss and say it will work great on the suction side of the pump. I'm sure there will be some loss but it sounds like good way to go. ACS has it cheaper that they can sell it for directly.

Another thing to think about. When parked I always turn my fuel valve off. Because of this any water in the tanks that might make its way to the gascolator can't get there. On the Luscombe I turned the fuel valve on as soon as I arrived at the plane in hopes any water would drain to the gascolator during preflight. Draining the gascolator was the last thing I did before getting in the plane but I don't know if 15min is enough time for the water to get to the drain. There are no tank sumps on a Luscombe and 100% of the fuel is usable. If there is any water in the tanks it will makes its way to the gascolator and probably through the engine. As I note earlier never had a fuel related issue with the Luscombe. If you always turn your fuel valve off when parked then is the gascolator really doing anything in regard to catching water when parked? How long do you have to leave the fuel valve on for the water to drain into the gascolator? Will the water ever actually reach the gascolator because of the design of some fuel valves the outlet is on the forward side of the valve requiring a short uphill run before the 180* turn back to the gascolator (see the picture of Battson's fuel system).

My dad and I have been talking and if we can fit a gascolator in a good spot we will install one because having one won't hurt anything. We wanted to uses a Steve's Aircraft gascolator but I don't think it will fit in the available space so we will likely use a BH unit if we find room for one.

[Battson]

Just to offer the counter view...

I have never found even a drop of water in the Bearhawk's fuel system.

I refuel in the rain when I have to, fly in rain, park outdoors overnight in the rain, park outdoors on hot days + cold nights with very empty tanks - I've tried everything. Never found water by draining.

I've done nothing special to water-proof the system. My tank filler necks stand proud of the wing skin, per the instructions. I always keep the caps down snug and done up firmly.

[JimParker256]

Battson, that's good to know, and further proof that Bob knew what he was doing with the design of the Bearhawk fuel system. I'm going to built it as he designed it. I'll sleep better, if nothing else!

[patrickh99]

Good topic, I've been kicking around different ideas along the lines previously discussed, with plans to use after-market fuel injection. I plan to have two fuel pumps, with separate filters for each, possibly feeding one from each tank, or maybe using a header tank.

Depending on your gascolator, some do have screens that are fine enough to filter out water. If the screen is similar in texture to the cloth you cover the fuselage with, water will not pass.

If you ever fly in very cold weather, including higher altitudes, you need to be mindful of ice crystals as well. This is particularly troublesome for mogas with alcohol, since it retains any water within the alcohol until the temperature drops far enough. Then the water comes out of suspension and freezes into solid (lump) or crystal form, hence the widespread admonishments against gasohol usage. Straight avgas can also get ice crystals from humidity in the air as tanks 'breathe' from fuel usage and temperature changes.

Carburated would be much simpler, but the efficiency (LOP) and/or extra horsepower available with EFI make a compelling case.

[JimParker256]

It is fairly well known that carbureted engines are more difficult to operate LOP, but there are some tricks that can help. The issue is uneven distribution of the fuel-air mixture to the cylinders, and uneven atomization of the fuel. Anything that causes the mixture to "swirl" is helpful in that regard. C-182 pilots often find that they can apply just a bit of carb heat and the engine will run LOP much more smoothly. The guys who teach the Advanced Pilots Course (www.advancedpilot.com) teach that technique for those who want to run LOP with carbureted engines. It seems that introducing just a little bit of air through the carb-heat circuit causes the fuel-air mixture to even out quite a bit. I tried this with my high-compression carbureted O-320 engine in a Grumman Traveler (using about 1/4 carb heat) and it smoothed out enough to run about 60-75 degrees LOP.

One of the great things about being in the experimental category is the ability to tweak things. My hangar mate has a Jabiru 3300-powered CH-750, and they have an X-shaped insert (90-degree intersection) that is installed in-line in the air intake just ahead of the carburetor. By rotating that X-shaped insert about 30 degrees at a time, he managed to get his engine to run much smoother and run cooler. The engine distributor tells us that this X-shaped insert helps to even out the fuel-air distribution between the cylinders, and you rotate it between test flights to find the optimal position. That particular engine uses an altitude-compensating Bing carburetor (so no mixture control, and therefore no LOP operations), but I plan to see if this same X-shaped carburetor inlet vane will help even out the fuel-air distribution and enable smooth LOP operations. But that's a ways off for me...

[Zzz]

Let's not get into carb vs FI... That's a tired discussion all its own.

I'm starting to think the best case for using a gascolator is so you don't have to admit to anyone that you're not using a gascolator.

[Blackrock]

The O-540 in 28M runs lean of peak fairly well using the partial carb heat technique Jim describes. However, the sweet spot where it runs well LOP is a narrow range and it isn't always easy to find.

Zzz wrote: "I'm starting to think the best case for using a gascolator is so you don't have to admit to anyone that you're not using a gascolator."

How true. LOL It really is just a type of filter (the ones with screens). Sometimes a name (label) is all about perception, not function.

[Blackrock]

A good discussion on the Van's forum with regards to gascolators and fuel injected engines:

"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"

[Battson]

Very interesting about firewall installation of gascolators - I bet that explains why some FI planes are a bitch to hot-start, while others are a dream.
My gascolator is under the belly, and ours hot starts on the 5th blade every time.

[marcusofcotton]

Thanks for posting this Blackrock.

[Blackrock]

Some more good discussion including a formula for sizing the bowl:

[whee]

I'm getting ready to install my fuel lines and am trying to get it right the first time. I drew up a sketch of what I'm currently thinking and am curious what you guys think. I wouldn't say this is my final revision, I have too many unanswered questions, but this where I'm currently at.

Untitled by Jon Whee, on Flickr

Remember: I'll have Continental fuel injection that will see high flow rates, a duplex fuel valve with no "Both" position, a engine driven fuel pump and a electric boost pump from EFII.