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Oct 07, 2008, 06:24 PM
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Carburetor / Fuel Tank Height Relationship


Hi All,

Many moons ago, before engine mufflers (and crankcases) were tapped for providing positive pressure to fuel tanks, fuel tank positioning was a very critical issue. Both fuel tank distance from the engine and the height of the fuel tank in relation to the engine was pretty important. Then, with positive tank pressure, our engines ran more reliably, tuning was easier and less finicky, and fuel tank foaming was not an issue.

I another thread, the topic of fuel tank height in relation to carburetor position came up. The operating manual for a particular 2-cycle engine advises that the fuel tank center line be inline with the needle valve of the engines carburetor exactly.

I cruised a few 2-cycle engine manufacturers operator’s manuals online and this requirement seems to be hit and miss. I reviewed several of the aircraft plans that I have and some aircraft depict the fuel tank inline with the engines carb, but some do not. As a matter of fact, one aircraft has two inverted mounted 2-cycle engines and the fuel tanks are much higher than the carb’s needle valves. In the case of my Top Flite DC-3, the tanks are mounted on the wing’s mid-section with fuel lines running about a foot out to the engines.

I have 2 J-3 Cub kits: Goldberg and Great Planes. Identical in size and power, two different fuel tank positions in relation to their engines. Neither fuel tank center line inline with the engine's carb needle. Both showing 4-cycle engines on the plan.

Now, there are those that will say to follow the engine’s manufacturer’s recommendations or else, but in the case of some scale aircraft, this just isn’t possible. You’re pretty much at the mercy of the designer to come close to carb/tank positioning.

So, how critical is crab/tank positioning considering a muffler or crankcase pressurized fuel tank?

Thanks!

EJWash
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Oct 07, 2008, 07:49 PM
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The diameter of the carb venturi probably has more effect on fuel suction than exact positioning of the tank. When carbs were opened up to get more power, Bernoulli suction effect was reduced.
A tank considerably below the carb centerline should still work reasonably well as the engine would be tuned to create slightly more suction with the tank in the lower position. I think you would have more problems with the tank too high where it would be more prone to drain into the carb when the engine was stopped.
Oct 07, 2008, 09:16 PM
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Back in the earlier days before mufflers the only way to get fuel into the engine was either by using a small enough venturi for good "suction" at any attitude or by using crankcase pressure. Crankcase pressure can only be used for engines that run full throttle all the time though but don't need a venturi. With just a venturi drawing fuel, tank position was fairly critical for anything other than straight and level flight.

Then along came mufflers and it didn't take long for someone to realise that muffler pressure could be used to aid in getting fuel to the carb which meant the carb bore could be made bigger which meant more airflow which meant more power which meant the advertising guys could make big claims. But with muffler pressure taking over some of the carbs ability to draw fuel it was back to square one as far as tank height being fairly critical.

It's generally accepted that the centre line of the tank (not the outlet from the tank) should be within about 3/8" of being level with the centre of the carb barrel to get a fairly consistent fuel draw from full to empty and with the model in any attitude (upright or inverted).

In the case of a model where the fuel tank is mounted appreciably higher (or lower) than the ideal, it'll hold it's tune quite well in normal level flight but do a turn or go inverted and the G forces will exaggerate the height difference. A 3G turn will make the tank appear to be 3 times higher (or lower) which obviously will cause a big difference in tune. If it's impossible to get a correct tank height then it's time to start thinking about a pressure regulated fuel delivery system.
Oct 07, 2008, 10:22 PM
Zor
Zor
Suspended Account
Quote:
Originally Posted by EJWash1
Hi All,

Many moons ago, before engine mufflers (and crankcases) were tapped for providing positive pressure to fuel tanks, fuel tank positioning was a very critical issue.

Please read post #1

So, how critical is crab/tank positioning considering a muffler or crankcase pressurized fuel tank?

Thanks!

EJWash
EJWash,

Thanks for initiating what can become a very fascinating subject and possibly very controversial as reporters and disagreeableness can flourish as we saw in a previous thread.

If you feel you already know all about this subject I suggest you stop reading right HERE. This posting is aimed at the beginners or uninitiated.

I imagine the thread host welcomes any contribution.

The relative position of the fuel tank and the carburetor can vary considerably and still have reasonable performance of the engine.

Based on ignition requirements in the firing chamber (not the subject of this thread) we have to consider first that the mixture in the firing chamber is not directly related to the tank location.
It is related to the proper fuel-oxygen ratio, temperature and compression. The temperaure is the key factor in initiating combustion and compression is a contributing factor to reach the ignition temperature.

The mixture does not come from the fuel tank. In two stroke engines it comes from the carter (the word carter means the space inside the crankcase below the piston(s). In four stroke engines it comes from the intake manifold

Both are downstream of the carburetor spraybar (fuel orifice in the venturi). We can refer to the fuel-air ratio instead of the fuel-oxygen ratio as both are easier to meaure or monitor in an engine design. It is alredy known that the oxygen content of the air is fixed very close to 20 percent.

The proper fuel-air ratio is is measured by weight which explains the occasional requirement for re-adjustmen of the mixture controls due to the air density and humidity variations.

What we have to consider is "what happens at the spraybar orifice" due to tank location variation.

As far as the air is involved a pulse of air ( a tiny volume) moves pass the spray bar in a restricted area of the carbureor (the venturi) and its instantaneous pressure has a specific variation vs time. The shape (graphical representation of this pressure variation for every suction of the piston can be seen in a development laboratory but is not of importance in this thread.

NOTE: I personnaly have seen these graphs on an oscilloscope as my friend Devlin was a development engineer at a factory manufacturing small engines (some for racing snow machines) and some for drones (fighter aicraft firing practice).

The amount of fuel picked up at each suction will depend on the actual pressure of the fuel at the spraybar orifice surface.

This is where the tank location enters into play. Fortnately we have some leeway in the fuel-air ratio that still satisfy firing conditions; We also have some leeway with the timing of the firing vs the crankshaft rotation (and its conrod and piston).

Some other factors are also involved and of some degrees of importance, namely the methanol- lubricant ratio (%) which affects how mush combustible methanot is admitted at each suction pulse. The ratio of nitromethane (%) which also affects the amount of oxygen introduced.

Most engine manufacturers recomment that the fuel tank be installed such that the half-full top level of the fuel be horizintally level with the spray bar or down to 3/8 inch (9.5 mm)

Note: often expressed simply as the carb or the fuel connection to the carburetor. The real consideraion however is the spray bar orifice. If the orifice of he spray bar has a certain length and the spray bar is vertical (engine installed sideways) then the center of the length of the orifice become the point of reference. Another acceptable reference is the center of the air intake orifice.

We have an uncontrolable factor in the fuel level since the tank has a certain height. Explanation . . . if the tank is two inches (50.8 mm) high inside and the mid level is at 1/4 inch (6.35 mm) below he spray bar reference then the fuel level with the airplane level is from 3/4" (19 mm) above to 1 1/4" (31.75 mm) below the spray bar.

The result is a 2" of fuel pressure difference at the spray bar orifice. A reality of little consequence. That difference still exist even when the tank is pressurized from the muffler line.

Now what if the tank is installed higher like 1/2" to 3" (12 to 75 mm) above the spray bar (in airplane level attitude) ?

We would expect a higher fuel pressure at the spraybar and we do have in static condition (engine not running) depending on the routing of the fuel line(s).

However in dynamic conditions (engine running) that pressure will become proper for the proper mixture ratio due to the pressure drop aross the needle valves.

What if the tank is below the spraybar?

The design of the engine is providing enough suction to draw the fuel up (against gravity). The needle valves would be opened wider than in the precedent case.

You may have often seen fliers pointing the nose of their airplane straight up. That has the effect of leaning the mixture so the needle(s) adjustment should be such as to keep the mixture still on the rich side or at least not on the lean side considering that such airplane attitude in flight does not exist for any long time periods.

There is an implication here that the tank position is not terribly
critical. The length of the fuel supply line is also not terribly critical. The lenth creates a pressure drop due to the fuel flow rate. It wil be unknowingly compensated when adjusting the needle valves.

The recommendation of installing the fuel tank as close to the firewall as possible is aimed at reducing the mixture ratio difference between level and vertical flight. It is an advantage mainly for aerobatic and pattern airplanes.

Long fuel lines are often supported by fuel pumps and some engines are equipped with fuel pumps that help stabilize the pressure at the needle valves.

Zor
Oct 08, 2008, 12:05 AM
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EJWash1's Avatar
Thread OP
Thanks All,

Good start.

I am of the opinion that the bottom line is positive fuel pressure at the needle valve. After all, isn’t that what we tune the engine to? In a non-pressurized fuel system it makes sense that the tank be positioned mid-needle valve because the first part of fuel consumption is gravity feed on the needle valve (positive pressure) and the remaining half is suction feed. As a matter of fact, in writing this post, I remember that fuel tank size had something to do with the engine fuel draw. In other words, the smaller the tank, the less effort for second-half tank suction feed, but the shorter the available flight time. The larger the tank, the more suction pressure needed in the second-half of tank suction feed.

The challenge is in the method of keeping positive pressure to the needle valve throughout all phases and attitudes of flight. Has muffler (and/or) crankcase pressure tapping provided this or not? Obviously, there is a degradation with fuel tank distance from the engine and fuel tank height in relation from the carb needle valve.

In the case of large-scale model aircraft, I am surprised that I have not seen a landing gear air tank used as a pressure accumulator to aid in positive pressure on fuel systems.

EJWash
Oct 08, 2008, 01:06 AM
Zor
Zor
Suspended Account
RJWash,

I have to insert red inserts to assure the comments apply to your part of the writing.

Quote:
Originally Posted by EJWash1
Thanks All,

Good start.

I am of the opinion that the bottom line is positive fuel pressure at the needle valve.
- No . . . not necessarily. the engine is a suction device
After all, isn’t that what we tune the engine to?
- No . . . we do not tune the engine to get positive fuel pressure at the needle valve(s). We tune the needle valves to control the rate of flow at all rpm. The needle valves are obstructions in the fuel flow.
In a non-pressurized fuel system it makes sense that the tank be positioned mid-needle valve because the first part of fuel consumption is gravity feed on the needle valve (positive pressure)
- That gavity feed is a very small pressure. The engine suction is responsible for most of the draw and continues to be such to the bottom of the tank.
and the remaining half is suction feed. As a matter of fact, in writing this post, I remember that fuel tank size had something to do with the engine fuel draw. In other words, the smaller the tank, the less effort for second-half tank suction feed, but the shorter the available flight time. The larger the tank, the more suction pressure needed in the second-half of tank suction feed.
- No . . . you cannot write "The larger the tank, the more suction pressure needed in the second-half of tank".
As the engine draws the fuel the difference in level in the tank plays a very small role. The engine suction is by far the significant factor to etablish the fuel flow. The needles are an obstruction to the flow and determine the pressure at the spray bar opening
The challenge is in the method of keeping positive pressure to the needle valve throughout all phases and attitudes of flight.
- If you had positive pressure at the needle valves there would be constant leakage at the spray bar when the engine is not sucking and the mixture would be much too rich. The engine sucks only about 25% of the time.
Has muffler (and/or) crankcase pressure tapping provided this or not?
- Provided what ? positive pressure at the needle valves?
Obviously, there is a degradation with fuel tank distance from the engine and fuel tank height in relation from the carb needle valve.
- A longer line acts as if it was contributing to the needle valves opening. A short line, valve more closed, a longer line, valves more open.
In the case of large-scale model aircraft, I am surprised that I have not seen a landing gear air tank used as a pressure accumulator to aid in positive pressure on fuel systems.
- What are you referring to? A tank on the landing gear to provide ram air pressure?

EJWash
Awaiting you next post.
Best regards _ _ _

Zor
Last edited by Zor; Oct 08, 2008 at 07:13 AM. Reason: Rechecking spelling next day
Oct 08, 2008, 02:01 AM
AMA 910957
EJWash1's Avatar
Thread OP
Hi Zor,

I have to learn how to the “quote” feature works here, so I hang with me.

Thanks for your post. I learned a lot and you cleared some assumptions on my part.

I did not consider the engine as being a “suction device”. Good analogy. Is this the case with both 2 and 4-cycle engines? If this is the case, you shoot down my offerings of providing a constant positive flow to the needle valve. I assumed that if there was always positive pressure at the needle valves, there would never be starvation unless there simply was no fuel to pass along. But, you offer that since the engine is constantly drawing fuel that the needle valve is in fact a constrictor of fuel flow, and will not need pressure behind it. That the pressure is greater on the fuel tank side of the needle valve as opposed to the needle valve spray bar side of the carburetor.

What I was introducing by offering an air tank was to put positive pressure at the needle valve, but I guess this is unnecessary, given your “suction device” theory. I guess that would be like pressurizing a baby bottle: the baby WILL draw the milk, so what’s the point of pressurizing the bottle? But, is this the case in all flight modes and attitudes?

As to there being constant positive pressure at the needle valve and leakage , that positive pressure would have to only be present when the engine is running, no?

“If you had positive pressure at the needle valves there would be constant leakage at the spray bar when the engine is not sucking and the mixture would be much too rich. The engine sucks only about 25% of the time.”

So, are you saying that these engines only work well when they suck??? You are saying that the needle valve(s) are fuel regulator(s). You are saying that there is no need for positive pressure before the needle valve because the engine is a suction device and the needle valve will regulate how much fuel flows by it predicated by this suction prior to it. I am saying that the positive pressure before the needle valve would create a more uninterrupted and constant fuel flow before the needle valve sends the fuel downline after it. Maybe we are saying the same thing?

EJWash
Oct 08, 2008, 02:20 AM
Registered User
turk1's Avatar
Quote:
Originally Posted by downunder

In the case of a model where the fuel tank is mounted appreciably higher (or lower) than the ideal, it'll hold it's tune quite well in normal level flight but do a turn or go inverted and the G forces will exaggerate the height difference. A 3G turn will make the tank appear to be 3 times higher (or lower) which obviously will cause a big difference in tune. If it's impossible to get a correct tank height then it's time to start thinking about a pressure regulated fuel delivery system.
In that anology can we say, problems of running during low angle take off but was great on ground testing,comes from that extra g force, pulling fuel back into tank rear?
Oct 08, 2008, 09:30 AM
Registered User
downunder's Avatar
Quote:
Originally Posted by EJWash1
I am of the opinion that the bottom line is positive fuel pressure at the needle valve.
Only if you consider the difference in pressure at the inlet to the needle valve and the exit through the spray bar. The exit pressure is whatever pressure drop the venturi gives below atmospheric pressure. Typically a good venturi will be around 0.5psi below atmospheric so with a normally vented tank where the breather is open to the outside air you have a difference of 0.5 psi pressure forcing fuel past the needle valve.

RC carbs generally don't have good venturis because they're too large so their pressure drop might only be maybe 0.2psi but muffler pressure could be about 0.3psi above atmospheric so this can be added to the 0.2 at the venturi to come back to the 0.5psi of a good venturi.

Now look again at that earlier good venturi above but this time use muffler pressure. You now have the 0.5psi from the venturi plus the 0.3 from the muffler so there's now 0.8psi to push fuel through the needle valve orifice. Obviously this would mean the needle valve doesn't need to be opened as much as before to get the correct mixture.

With a tank that, when full, has the fuel level 1 inch above the centre of the carb this equates to an added pressure (fuel head) of near enough 0.04psi so call it 0.54psi felt at the needle valve. When near empty and 1 inch below the carb centre there's 0.46psi at the needle valve. This just means that if you tune the engine with a full tank it'll be slightly leaner when near empty. And VERY lean when empty .

All these figures are only examples but reasonably close to the real world.

For turk1, an accelerating model will lean out slightly. If it accelerated at 1G then this would be the same as ground running but with the nose held vertically and would get worse as the fuel level dropped because then it'd be like holding the nose up with a near empty tank. Any model capable of hovering can accelerate with at least 1G at takeoff.
Oct 08, 2008, 09:36 AM
Zor
Zor
Suspended Account
Hi again EJWash1
Again please read the red inserts for the same reason as previous.

Quote:
Originally Posted by EJWash1
Hi Zor,

I have to learn how to the “quote” feature works here, so I hang with me.

- I am "hanging" with you. The quotes " " are used to emphasize a word or a group of words or to associate words together.
I just love postings without any formatting. Like a whole page or more all in one huge long paragraph.

- Each item or subject should have its own paragraph to make the reading easier and avoid non related things.

Thanks for your post. I learned a lot and you cleared some assumptions on my part.
- We often have to make assumptions due to lack of data (information) or due to misinformation (bad info) or ambiguous writings. Not everyone has mastered the art of written communications.
I did not consider the engine as being a “suction device”. Good analogy.
- It is not an "analogy", it is a reality. An IC (Internal Combustion) engine is like a compressor sucking one side and pushing the other side. It sucks the mixture on the intake and pushes the exhaust out. The fact that the intake stuff can explode and create pressure inside the engine gives us our power. A piston engine (reciprocating engine) on a full size airplane operates with typically an intake manifold of 10 to 30 inches of mercury of absolute pressure. That is the same as saying 0 (zero) to 20 inches of vacuum. Some engines are equipped with a blower in the intake or similar device to increase the manifold pressure above the ambient atmospheric pressure. These engines are "supercharged engines". See the use of quotes to help associate the word supercharged with the word engines.
Is this the case with both 2 and 4-cycle engines?
- Yes.
If this is the case, you shoot down my offerings of providing a constant positive flow to the needle valve.
- Sorry, in a way, about shooting you down. If my "enemy friends" think I am wrong, they will quickly post to contradict me.
I assumed that if there was always positive pressure at the needle valves, there would never be starvation unless there simply was no fuel to pass along.
- Of course if there is no fuel availabe (empty tank) the engine is sucking only air which does not fire very well but if you would put a reservoir on the exhaust and turn the crankshaft by some mean you would have a compressor.
But, you offer that since the engine is constantly drawing fuel that the needle valve is in fact a constrictor of fuel flow, and will not need pressure behind it.
- That is correct. The needle valves are an obstruction in the fuel flow allowing us to control the AVERAGE flow rate. Similarly the carburetor is an obstruction in the air flow allowing the rate of air flow allowed in thus controlling the power being developped by the engine. The ignition in the firing chamber (dylinder head) is a matter of temperaure. It is not a matter of how much mixture is in there. The ratio of air and fuel (mixture) has to be within a specific range for the fire (combustion) to initiates, to start burning.
That the pressure is greater on the fuel tank side of the needle valve as opposed to the needle valve spray bar side of the carburetor.
That, again, is correct. Any fluid flowing throuh a restricted area has a higher pressure on the upstream side (where it comes from) than on the downstream side (wherever it is going).
What I was introducing by offering an air tank was to put positive pressure at the needle valve, but I guess this is unnecessary, given your “suction device” theory.
- It is not just a theory (an hypothesis), it is a proven knowledge understood since the IC engines were developed in the 18th century.
I guess that would be like pressurizing a baby bottle: the baby WILL draw the milk, so what’s the point of pressurizing the bottle?
- Well _ _ _ you could always drown the baby. So could you drown (flood) the engine.
But, is this the case in all flight modes and attitudes?
- It is the same in all airplane attitudes accompanied with slight variations in the mixtue ratio as it is affected by the gravity acting on the fuel in the tank. The engine suction to create a fuel flow can be helped by the weight of the fuel if the level is above or can be hampered if the level is below.
The effect of centrifugal forces (G forces) has been covered by "downunder" in his posting.

As to there being constant positive pressure at the needle valve and leakage , that positive pressure would have to only be present when the engine is running, no?
- No, as you just said, is correct (it is NO). Whatever the pressure is upstream of the needle valves, the variable pressure drop across the needle valves as you adjust them will compensate for the upstream pressure so that the needed pressure at the spray bar orifice is proper for any setting of the carb opening.
“If you had positive pressure at the needle valves there would be constant leakage at the spray bar when the engine is not sucking and the mixture would be much too rich. The engine sucks only about 25% of the time.”
- Yes that is what I said.
So, are you saying that these engines only work well when they suck???
- Sucking is what they do and I think that is what I said.
You are saying that the needle valve(s) are fuel regulator(s).
The needle valves are NOT "fuel regulator(s). They are better understood as pressure control devices downstream. Some may argue that is the same thing. Each brain function a little differently.
You are saying that there is no need for positive pressure before the needle valve because the engine is a suction device and the needle valve will regulate how much fuel flows by it predicated by this suction prior to it.
- I am saying that the pressure upstream (fuel tank side) of the valve is the ambient atmospheric pressure plus the pressurization of the muffler line (if used) plus or minus the weight colum of the fuel level location (above or below the valves) less the pressure drop in the fuel line from tank to valves, less any pessure drop due to fuel filter (if used), less any drop across a filler valve (if used).
Of course the effect of a fuel pump has to be considered if used.
I am saying that the positive pressure before the needle valve would create a more uninterrupted and constant fuel flow before the needle valve sends the fuel downline after it.
- You can say whatever you wish to say. I will not argue wih you.
Personally I say what I understand is taking place and my mind is satisfied of my understanding.
There is NO "uninterrupted and constant fuel flow before the needle valve". The engine suction, as seen by the needle valves, takes place when the crankshaft aperture opens to the carb in a two stroke engine and when the intake valve opens in a four stroke.
Maybe we are saying the same thing?
- I do not know why you say "Maybe". It does not appear to me that we have the same understanding of what is taking place in the fuel supply to the carb and engine.
EJWash
Sorry, pehaps, for the long posting. I would not know how to shorten this without letting the general reader all confused.

Regards to you and to all de

Zor
Oct 08, 2008, 11:11 PM
Registered User
Hi Zor and Ejwash,

Thanks for starting a separate thread on this topic that I started

Was going through your posts and must say that it is getting a bit technical and confusing here.

I think what most beginners / intermediate flyers would like is a yes or no answer?

To sum up the question?

"Is tank height to relative to carb critical? Yes or No?"


Thanks
Last edited by gpang788; Oct 08, 2008 at 11:33 PM.
Oct 09, 2008, 12:09 AM
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EJWash1's Avatar
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EJWash
Last edited by EJWash1; Oct 09, 2008 at 12:42 AM. Reason: post removed by EJWash
Oct 09, 2008, 02:36 AM
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turk1's Avatar
Quote:
Originally Posted by downunder

For turk1, an accelerating model will lean out slightly. If it accelerated at 1G then this would be the same as ground running but with the nose held vertically and would get worse as the fuel level dropped because then it'd be like holding the nose up with a near empty tank. Any model capable of hovering can accelerate with at least 1G at takeoff.
I have seen many times,an engine( not necessary be new) runs and adjusted perfect on ground,passed all kind of tests including vertical upnose and even shaken on that position to up and down to listen any rpm changes during such +,- acceleration, begins to take off run and stops gradually during accelerating on the ground or just after low angle take off.
I cant comment on such a case because no reason for that.Do you have any idea?
Oct 09, 2008, 06:58 AM
Zor
Zor
Suspended Account

Guessing and assuming


Quote:
Originally Posted by EJWash1
EJWash
Hello EJWash,

I have to guess and assume you posted in response to post #11 and then decided the no response was the thing to do.

I have not responded to post #11 as yet.
I am trying to decide on that.

I am anxious to see hundreds of responses with only one word.

A YES or a NO.
Do we have to know what the YES or the NO is responding to ?

Zor
Oct 09, 2008, 07:29 AM
Zor
Zor
Suspended Account
Quote:
Originally Posted by turk1
I have seen many times,an engine( not necessary be new) runs and adjusted perfect on ground,
Etc _ _ _ _
Do you have any idea?
I think you are asking downunder so I will refrain from replying but the answer is pretty evident.

Zor


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