Fuel System Design

7/30/2019 Fuel System Design

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Set p Engine-UL System Design Jan 2008

POOR DESIGN AND INSTALLATION OF THE FUEL SYSTEM IS THE

MOST LIKELY REASON IF YOUR FLIGHT ENDS TOO SOON.

This article can help prevent that.

Engine failures are rare. Almost all so called "engine failures" are returned to service without ever

working on the engine itself. Its the systems that support the engine which usually fail ... and fuel

systems fail more often than all other things combined. Most of those failures are easily prevented.

A WORD OF CAUTION

This subject always brings out the internet "experts". If any readers doubt that real people can be as

screwed up as some of the "experts" mentioned in this article they should check the archives of the

engines-ul2 list starting Nov 6, 2007. Just that one discussion included many of the "expert" claims

mentioned in this article.

FUEL

Most American pumps have a label on them which shows the advertised octane was calculated as

RON plus MON divided by two. European countries use the RON method only. If your engine

manual says to use 91 RON then you can use American 87 octane gas since its the same as 91

RON.

Aviation fuel is often called 100LL. Its 100 octane but the low lead part is misleading. It does have

less lead than aviation fuel had years ago but it still has much more lead than automotive gas. That

lead is hard on our type engines because it fouls plugs and causes rings to stick if we use it on a

regular basis. A few gallons of aviation fuel once in a while doesn't matter much but I still avoid it

as much as I can.

Some "experts" claim that the volatiles evaporate out of gas rapidly so fuel goes "stale" fast. There

is a tiny bit of truth in that but it certainly doesn't happen very fast. How long does gas have to sit in

your car or lawn mower before you notice any difference? In my plane 3 or 4 weeks certainly

doesn't cause any problems and I have gone twice that long with only a very minor difference in the

performance ... and I even premix the oil with my fuel. Don't get paranoid about "stale" gas.

Gasoline picks up small flakes of rust and dirt as its pumped hundreds of miles through

underground pipe lines. The large storage tanks at the end of the pipeline add water and more crap.Even the truck that delivers gas to your local gas station may contribute and of course water and

crap in the gas station tank gets stirred up when the tank is filled. Gas companies try to filter the gas

and most even have a filter right at the pump nozzle but some water and crap always gets to our

tanks.

Some pilots filter the gas going into the tanks on the plane with felt or chamois cloth and some use a

Mr. Funnel. It doesn't matter what you use to filter your fuel or how careful you are to avoid

condensation in the tank, sooner or later there will be water and crap in your tank so your fuel

system has to handle some of it.


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ELECTRIC FUEL PUMPS

The Mikuni pumps are designed to suck fuel up to the engine and can easily handle the 3 or 4 feet

of vertical lift needed with our engine and tank locations. Faucet recommends mounting their

electric pumps below the fuel tank because they can NOT produce much suction.

Many people think that an electric fuel pump is a way to provide an extra margin of safety. That's

okay but make sure that you understand what you are doing or you can wind up LESS safe. Dual

pumps can be used safely but you need to do it correctly.

If you use pumps in series each pump must provide enough fuel pressure by itself in case the other

pump fails. With both pumps running there is too much pressure at the carb. To prevent that you

need a pressure regulator but then you depend on ONE regulator to keep working. Wasn't your

orginial concern that you depended on ONE fuel pump to keep working? What have you gained?

There was an article on two pumps in parallel published in an UL magazine which assumed that

Facet pumps had a check valve built in. One friend crashed when he had fuel starvation with thatsetup. I made another friend do some tests before he flew that setup and we found out that he would

have also had fuel starvation problems. That article was published about 2002 but that same bad

assumption is still floating around on some web sites.

We normally use the Facet 40104 or 40105 pump where the fuel tanks are in the wings. Where the

fuel tanks are mounted low in the plane we usually use the Facet 40106 so the carb sees enough

pressure. Check them out at

http://www.facet-purolator.com/solidstate.asp

The labels on top of the columns are screwed up but if you study the info you can figure out that

NONE OF OUR PUMP MODELS have a check valve built in. Hint .. If you shift the label "fitting

size" to your left to be in line with the fitting descriptions of the model numbers then the columns

on the right will match up better and make more sense.

The bottom line is this. When there is more pressure at the outlet port than is being created by the

pump, fuel will flow backwards through the pump. Any time you use more than one pump you

should install a check valve on the outlet of each pump to prevent reverse flow of fuel through the

pumps.

With engines mounted high and fuel pumps mounted low there is one PSI pressure on the pumpoutlet port for every 37 inches in height just from the head pressure of the fuel in the line up to the

carb ... even with the engine off and the pump not running. For discussion, call it 1 to 2 PSI head

pressure at the pump outlet when the pump is off.

Fire up the engine and the Mikuni will add 4 to 7 PSI to the top of that head pressure and give about

5 to 9 PSI total at the electric pump outlet. If the electric pump can only pump 6 PSI, fuel from the

Mikuni goes backwards through the electric pump even when it is running. The

end result is less fuel available for the carb than was available before you installed the electric

pump.

Don't believe it? Go out to the flight line and find any plane thats running an electric pump inparallel with a Mikuni. Pull the suction line off the suction port of the electric pump and plug that
http://www.facet-purolator.com/solidstate.asphttp://www.facet-purolator.com/solidstate.asp


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and the pulse line drains back into the engine. Do NOT bolt the pump solid to the engine. Use some

form of mounting which isolates the pump from vibration and engine heat

Long pulse lines, leaks in the pulse line and thin wall tubing which expands and contracts with the

pulses also weaken the pulses. Weak pulses move the diaphram less so less fuel is pumped. Pulselines must be less than 18 inches long so the pump must be located near the engine pulse

connection. You can swap the engine pulse connection from one cylinder to the other to help keep

the pulse line short.

It's not easy to do all that so even plane manufacturers sometimes take the easy way out. Problems

may develop over time as condensate builds up in the fuel pump or pulse line.

I recommend the round Mikuni pump since it can pump up to 9 gallons (35 liters) an hour ... but

that is only when they are installed correctly. Remember that they will pump less if they have any

installation problems. Both outlets come from the same internal chamber so just plug off one outlet

if you only need one output line.

Its VERY rare to hear of any problem with a Mikuni pump if it is installed correctly. They have

been used on many brands of cycles, ATVs, jet skis and snomobiles for many years. They don't

require electrical power and they are simple devices with few moving parts. Those parts are

replaced during a low cost rebuild which is so easy that anyone can do it.

I think they are more reliable than electric fuel pumps so I use a round Mikuni which I replace

every two years and I don't have any backup pump on my plane. If I was concerned I would install a

second Mikuni pump for a backup. Most engines have pulse connections on each cylinder. There is

no reason why both pulse connections could not be used at the same time to drive two Mikuni

fuel pumps. Do NOT connect two pumps to the same engine impulse connection. Both would

recieve weak pulses.

FUEL TANKS AND THEIR CONNECTIONS

Be a little leary of fiberglass and other composite fuel tanks. Every once in a while we hear about a

tank which partially disolved because of ethanol or some other fuel additive.

I don't like holes anywhere in a fuel tank except at the top. Its not a question of IF they will leak ...

its a question of WHEN (and how much) they will leak. All connections should be at the top to

avoid that problem.

I don't care what you use to filter your fuel or how careful you are, there WILL be crap and water in

your fuel tank. Your first line of defense is to leave a little room at the bottom of the tank where the

crap can settle out before it gets into the fuel lines. Make a tool to clean your tanks and use it often.

Use a piece of 1/4 inch OD stainless or aluminium tubing long enough to reach the bottom of your

tank through the filler opening and still leave enough for you to hold as you guide the bottom end to

the low spots in the tank. Slip a piece of 1/4 inch ID Tygothane tubing over one end of the metal

tube then attach a squeeze bulb to the open end of the Tygothane tubing. By squeezing the bulb you

can suck out trash and water. Just like cleaning your tropical fish tank at home.

The main fuel supply should have a metal dip tube which goes through the top of the tank and ends

about 1/2 inch above the bottom of the tank to keep it above any water and grit thats in the tank. DoNOT let dip tubes lay against the bottom or side of the tank. That can seal off the open end or let


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vibration rub a hole in the tank. Don't use copper tubing with fuel because it corrodes in gas

quickly. I know that brass fittings do NOT corrode but brass is NOT the same as copper tubing

which DOES corrode. Aluminium or stainless tubing will work fine. Very large engines might need

as much as 3/8 OD X .035 wall tubing but 1/4 OD X .032 wall will easily handle 5 or 6 gallons per

hour. Do NOT use screens or filters on the bottom of the tube. Any screen or filter will stop upeasier than the tube will, you will have a hard time cleaning or replaceing it and you already need a

filter right at the pump inlet anyway.

If the tank doesn't have any connections on top use a stainless steel PANEL MOUNT COUPLING

through the top of the tank. McMaster-Carr #5182K375 fits 1/4 OD tubing.

This coupling is like a normal tubing coupling except it has a section in the middle which is

threaded so that it can be inserted through a hole and mounted solidly in place by a nut. If you

install a 7/16 viton, nylon, or PTFE washer as a gasket outside the tank it will seal in fumes.

McMaster-Carr #90295A186 is nylon. To protect it use a #92141 stainless washer under the

mounting nut.

The tubing nut and ferrule inside the tank are not used so the dip tube can be removed later. The

tubing nut and ferrule outside the tank will hold the dip tube in place, seal around it and allow

removal of the dip tube.

There is a small shoulder inside the coupling that needs to be drilled out with a 1/4 inch bit before

the tubing will slide all the way through the coupling.

Most tanks have a small filler opening so here are some tricks to help install connections. Large

drill bits make a sloppy hole in soft plastic so drill a small hole then use a tapered hand reamer to

enlarge the hole to the correct size. Don't forget to deburr the hole.

Drop a string through the hole then fish the string out through the filler opening. Remove both

tubing nuts and the mounting nut from the coupling then push the string through the coupling. Tie a

nut on the end so it can't pull back through the coupling then drop the coupling in the tank. The

string will help guide the coupling to the hole as you pull it out. Hold the coupling in place as you

cut the string. Install a viton or nylon gasket then a stainless flat washer to protect the gasket. Use

Locktite and install the mounting nut. Put both pieces of the ferrule and the tubing nut loosely on

the top of the coupling and slide the dip tube through the coupling to the bottom of the tank. Pull it

back up at least inch before you tighten the tubing nut. Some tanks with flat tops may dish down

in the middle over time. Pull the dip tube further up to allow for that.

Only planes capable of inverted flight need flexible lines inside the fuel tank. Those flexible lines

WILL be a real pain in the butt later. Do NOT use them.

I use a seperate 1/4 inch dip tube for my hand pump primer because defective primers can leak air

into the main fuel suction line. Some people do tee into that line to supply the primer. If you must

do that, use a metal or nylon tee fitting. Plain plastic fittings may break easily and are not rated for

use with gasoline.

Of course vent lines must come out of the top of the tank. A cheap fitting to use for vents is a metal

valve stem for tubeless truck tires from an automotive store. They have a nut for solid mounting

through the tank but the rubber seal furnished with them may not be suitable for use with gasoline.Throw it away and use viton or nylon washers as gaskets as discussed above for dip tubes. Remove


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and discard the valve core before you slide Tygothane tubing over the fitting. If you run the vent

tubing down past the bottom of the tank, fuel will not leak out if you flip the plane over. I stick a

fuel filter on the open end. It keeps bugs out of the fuel tank and serves as a spare fuel filter on cross

country flights. If you extend the vent line out the bottom of the plane it will keep fumes and any

slosh overflow out of the cockpit. Any vent built into the filler cap can be sealed to prevent sloshleakage and fumes when the tank is vented this way.

New tanks and lines often have construction crap left in them which causes problems on the first

flight. Clean and flush them out before you trust them.

FUEL, VENT AND PULSE LINES

ALL these lines should be Tygothane only since gasoline fumes attack lines just like gasoline.

Tygon is NOT the same as Tygothane. Be very careful. Some tubing is rated "for fuel use" and that

is NOT the same as "for gasoline use". If tubing is not rated for gasoline it is not safe for our use

and very little of the tubing sold anywhere is rated for gasoline. Color doesn't tell you what the

tubing is rated for and makes it harder to see air bubbles inside the line. Some plastics becomesoft or disolve over time when used with gasoline. Other plastics become brittle after a year or so

then crack or break under vibration. Rubber automotive fuel line can shed rubber particles from

inside and may get soft with our oil/fuel mixtures plus you can't see air bubbles inside the line.

McMaster-Carr sells clear Tygothane tubing which IS designed for gasoline use and it stays flexible

for years. Part #5549K33 is 1/4 ID X 3/8 OD and #5549K34 is 1/4 ID X 1/2 OD.

The heavy wall type must be used for the impulse line and on the suction side of the fuel pump or

vaccuum may cause those lines to collapse. The lines on the outlet side of the pump and the vent

lines can be either wall thickness.

Automotive type worm gear hose clamps often let air leak into suction line connections because the

small size we need doesn't form a circle very well. McMaster-Carr sells nylon double snap grip

clamps which work well. Part #9579K63 is typical but get the right size. They also sell spring steel

wire formed into a circle as "constant tension spring" clamps. Part #5324K61 is typical but get the

right size. They also sell a band tightened by a nut and bolt. Part #5412K48 is typical but get the

right size.

If all else fails wrap two turns of safety wire around the connection and twist it tight but don't cut

into the tubing.

FUEL FILTERSThere is a lot of discussion about fuel filters. Some Cessna pilot "experts" claim that any small stuff

which goes through the screens they use is so small that it goes through their fuel pumps, carbs and

engines with no problem. Obviously that has worked for them for a long time but you need to

remember the differences in planes and engines.

Their engines are much larger so they need larger jets in their carbs. Our smaller jets are easier to

stop up.

The main problem is that our fuel pumps depend on two internal flapper valves being able to seal

properly up to 100 times a second at 6000 RPM. Grit that gets through screens can keep those

flapper valves from sealing shut properly. When that happens our pumps may not pump enough fuel

to keep us in the air. Most of us install a fuel filter just before the fuel goes into the fuel pump toprotect the pump from grit.


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Purolator (and others) make a filter with a glass bowl to trap sediment. You can take them apart to

replace the filter and clean out the bowl. It works fine on the pump output and there is no problem

with them there. Unfortunately we need a filter on the pump intake and there is suction there. You

may have already noticed that suction is harder to seal than pressure if you had a hard time keepingair bubbles from leaking into your suction line connections. The seal at the filter bowl was NOT

designed to handle suction and if you try to use it on the suction side of fuel pumps it WILL suck

air into your fuel system. Many UL pilots have known this for years and avoid them ... but every

once in a while a newbie will use one then post that he has a problem with bubbles in the suction

line.

Some internet "experts" claim that filters should not be used on the suction side of pumps and use

those posts as "proof". You should remember that many of the "experts" use wing tanks to gravity

feed fuel to a lower engine. Their fuel doesn't need to be sucked up to the engine so they are not

familar with our fuel pumps or the suction needed. They claim that the bubbles are "vaporizeing"

out of the fuel because a high vacuum is needed to suck fuel through a filter. Thats not true.

It is true that a high vacuum in fuel lines and/or high temperatures can cause "vapor lock" when fuel

vaporizes but that is NOT what is happening here. The vacuum used normally isn't much more than

we generate when we suck on a hose to siphon gas and those bubbles are present even when the

temperature is below freezeing. Its real easy to prove that those bubbles are simply air leaking into

the suction line ... just replace the glass bowl filter with a different type of filter and there are no

more air bubbles in your suction line.

These same "experts" have claimed that our other filters leak air into suction lines. Thats not true.

The ONLY reports of air leaks are those glass bowl filters and tubing connections which use the

wrong clamps.

Some fuel filters use a sintered bronze element. That type element doesn't have much surface area

so it won't take long for them to get stopped up. I don't recommend them.

It's hard to beat the plain old pleated paper type fuel filters which have been used in cars, boats

cycles and many other applications for years. I highly recommend them. Some "experts" claim that

small amounts of water in fuel will stop them up. Thats not true. A simple experiment with a filter

and a squeeze bulb will easily suck water through the filter. You should trap water in the fuel tank

before it can get to the filter anyway. The Cessna "experts" should think of the tank as a big

gascolator. We just use a filter outside instead of a screen inside.

Some "experts" claim that its better to use a gascolator because filters will stop up. The reason

filters stop up is because they are doing their job of keeping stuff out of our pumps. Simply replace

filters twice a year or any time the engine will not reach full RPM or if you notice higher than

normal EGT readings at full throttle and there is no problem.

SQUEEZE BULBS, CHOKES AND PRIMERS

A squeeze bulb will only pump fuel to refill the carb bowls and lines which have drained down

while the plane was sitting. It will NOT squirt fuel into the intake manifold to help the engine start

like a hand pump primer. It will NOT enrichen the mixture to help cold engines run smoother like a

choke. Some people have had so much trouble with squeeze bulbs that they refuse to use them.Squeeze bulbs should be replaced once a year to minimize problems like splitting, leaking, shedding


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rubber particles into the fuel or failure of their built in check valves. I do NOT recommend them.

Our "choke" is actually just a seperate fuel path inside the carb which allows fuel to flow through a

separate jet. That enrichens the mixture to help cold engines run smoother but it doesn't "prime" the

engine or fill the carb bowls and fuel lines. I don't use mine at all.

A hand primer pump squirts a little fuel into the intake manifold every time you pump it. Proper use

eliminates any need for a squeeze bulb and the choke. It does take a little experience to learn how

many times to pump it before starting the engine and when to pump it to keep the engine running

while the carb bowls and fuel lines refill and the engine warms up a little. I recommend them

highly.

FUEL PRESSURE GAUGE OR LIGHT

I'm a firm believer in KISS (Keep It Simple Stupid) so I won't bother putting one on my plane.

Some people do like to have a fuel pressure gauge or a light that comes on when fuel pressure drops

below 2 PSI. Thats okay as long as you understand what you are really seeing. You need to see thepressure right at the carb inlet. If you just tee into the fuel line there and run a line down to a gauge

mounted on the panel you will see the head pressure of the fuel in that line added to the actual

pressure at the carb. You need a 10 PSI gauge but normal cheap gauges are NOT very accurate near

the ends of their scales and can not survive vibration very long. You have also added the potential

problem of fuel leaks and fire in the cockpit. Electronic sensors for an EIS or a pressure switch for

an indicator light installed near the carb solve those problems but are they really useful?

Expect a wide variation in readings during the normal operation of the plane and make sure that the

pilot understands what those variations mean. Most fuel problems are a very slow reduction in the

amount of fuel delivered as a pump wears out or a filter gets stopped up. A fuel pressure gauge

might let me see that problem develop over a long time but I don't need to include that gauge in my

normal scan of panel instruments.

The EGT gauge IS included in my normal scan of panel instruments and it will give me early

warning of fuel starvation problems by showing higher than normal EGT temperatures any time a

weak fuel system can not provide enough fuel at full throttle on climbout. Even if a filter did

suddenly get stopped up enough to give high EGT while I'm at cruise speed, I know that if the EGT

will go back down when I reduce throttle I can limp a long way to a safe landing at reduced throttle.

Neither the fuel pressure gauge or the EGT will give any warning of a sudden failure so fly like the

prop could stop turning at any time and practice emergency landings.

THE BOTTOM LINE

You should NEVER attempt the first takeoff until you are 100% positive that your new fuel system

will provide enough fuel under all conditions. Tie the plane down and run it at full throttle to be

certain that it won't starve for gas during takeoff and check closely for leaks then be prepared for

the engine to quit

Fuel System Design

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