alan0899

G'day All,
Below, is a classic example of the misconception about the use of muffler pressure.
This example, tells me a couple of things, 1) His tank is too low, that's why he needed the pump,
2) His engine could not SUCK enough fuel in, again caused by either bad compression, & consequently bad crankcase suction, or tank position, or both.
If the engine won't run properly on the ground WITHOUT muffler pressure, then you have a problem. & muffler pressure will NOT solve that problem.



I'm running a STG75 with the Pitts and i could not get enough back pressure in the fuel tank to keep it running so i tried plugging one pipe which helped a little but not enough. I could not keep the fuel pressure at idle and it kept stalling. I ended up plugging the pressure tap and putting a perry fuel pump on mine and it runs great now. It idles so slow you can almost count the prop blades going around. Not quite but almost.

Lou Crane

Alan, G'Day 2U2,

Agree, generally, with your points.

I would offer that if all of the excuses you cite are dealt with properly - presuming the engine is suitable for basic set-up corrections - muffler pressure still has a certain usefulness.

Wouldn't it be better if the excuse-makers tried to set up their models properly, first? And kept them in good enough condition that failing compression or case leakages did not raise problems?

I've used crankcase-tap driven Perry pumps, and they do well. If your flying demands make necessary the absolute positive feed they provide, then they are your good solution. For others, the extra machinery and its maintenance and tuning may not be necessary. I prefer things as simple as possible, so long as they do what is needed. In many cases, scrupulous prep goes a long way toward solving most problems. Where it doesn't, further work is needed.

The ground may rise up and smite Thee if Thou celebrateth with excessive zeal, but I hope it seeks only to smite Thy model airplane on most other occasions...

alan0899

G'day Lou,
I agree with your points, muffler pressure does have it's uses, But it's not a cure all.
Lots of people on RCU think that it is muffler pressure that gets fuel into the engine, which is totally WRONG. And this is the point I was trying to convey, it is crankcase suction, in a 2 stroke, that sucks the fuel into the engine. If the engine won't run properly, on the ground, then there is a problem, either the engine is not run in properly, I hate the term "break in", as this suggests that something is broken, or there is a fuel system problem, tank height, feed lines, etc.
Muffler pressure, is used to keep fuel up to the engine during aerobatics, & that is all.

Years ago, 2 stroke engines didn't have mufflers, or they were very poor ones, & so they didn't have muffler pressure, but the engines ran beautifully.
But they also didn't perform the aerobatics that we see today. Hence the advent of muffler pressure.

mike109

G'day Alan I am presently putting an OS Gemini 160 into a Sig Extra. I am doing it so that I can play with the engine. The plane is just there to cart the engine around. But ... I have discovered that the tank is going to be way too high relative to the carby which is, as you probably know under the engine in the "scale" position. I have seen other threads commenting on fuel leaking out of setups like this. OS recommend that the tank be positioned so that the top of the tank is just above the carby but there is no way to do this in this airframe. I am not planning any serious maneuvers in this plane. I am sure it can do them but I can't! And now the question - do you or does any one else have a way of regulating the fuel to the engine in a situation like this? Perry pump? Regulator? If so what sort? The engine has no provision for muffler pressure. It has no mufflers. The carby throat is quite small so it should be a good "sucker" though. Thanks for any advice you may be able to give. Mike in Dubbo.

alan0899

G'day Mike,
How's lovely Dubbo, getting a bit cool, no doubt.
Anyway, the Gemini, would not have a problem, with the tank height, while running, but it may siphon fuel into the carby, & that will just leak out the air intake, while it's sitting on the ground.
I would get the tank as low as possible, remember that, because the carby is below the engine, fuel mist has to be sucked UP, into the engine, when it is running, & that's why I don't believe there will be a running problem with your setup.

Lou Crane

Mike, & Alan, of course...

Care to borrow a thought from the stringy-thingies? In CL, our carrier event (deck takeoff, timed high speed, timed low speed, landing on same take-off deck) has a profile event category. To carry enough fuel for 3-5 minutes of near-hover ( and only 7!!) low speed laps, many use an 'inboard' main tank with an outboard 'local header' tank to establish the relationship between "tank" and "carb(?y?)".

It IS simpler to locate the relative pieces as we have centrifugal force to rely on. In RC (more or less free) flight, gravity is always there - as is Perry-pump-, or muffler-pressure, or carb suction forces... There's a thing called uniflow venting layout VERY useful in CL, PA particularly. It is sort of self limiting in resupply, so a gravity (or relative gravity direction) feed will cut itself off when the vent tube or connector tube is immersed... Awkward to explain briefly, but the effect is that the air vent into the local small chamber "defines" the effective fuel surface in terms of positive or negative gravity head height. For CLPA, we make the separation from the vent open end inside the tank shell to the fuel pickup reference location as small as possible, so that head changes are minimized despite a maneuvering 'g' range from +/- 1.0 to perhaps over +/- 35 'g', with operating directions from ~22° to over 70° off spanwise.

I'll drop it here for y'all (regional dialect in SE USA) to ponder. There are many references available, for those who can tolerate an actual physical connection with their models in flight, instead of a hypothetical, vision-delayed (with the question, " Why'd it do THAT?...oh, er, ur, OK, let's try it this way..."), surrogate connection with their models... [>:]

I've learned enough so far, with more coming, from the other disciplines in model aviation, that I'm glad to share anything I use (or might use), with those heretics who haven't found the one true... er, ah, discipline...

NM2K

My Feedback: (14)

Much of the confusion that the latter portion of this thread is attempting to deal with is caused the ignorance of newbies, although some very old timers barely have a grip on how our engines work, it appears.

Remember Duke Fox's side panel advertisements where he would talk to the masses about engine theory upon occasion? Those were some of my favorite reading materials in the model mags. The master spoke to us!

Perhaps someone with energy can reconstruct a web/blog site to reeducate folks regarding engine theory and operation? Duke used to talk about the seal of the piston in the lower crankcase being just as important as the seal of the piston/cylinder in the top of the crankcase of two-stroke engines. He mentioned how an engine can have great upper cylinder compression, but how if the lower piston/cylinder fit was less than optimum, the engine would be unable to draw fuel properly. Things like that.


Ed Cregger

Hobbsy

My Feedback: (102)

The real fuel draw is caused by the Bernouli effect of the air flow velocity increasing as the intake air makes its way past the spraybar. Where velocity is highest pressure is lowest, just like on a wing. It stands to reason that a new engine, or worn out one, with less than stellar compression holding ability is going to have lousy fuel draw because the leakage past the piston (on the compression stroke), reduces the volume drawn through the carb, in turn reducing the airflow velocity and the fuel draw. Most of our carbs are variable venturi carbs reducing the throttle opening also reduces the venturi area so the fuel draw remains fairly constant, in fact in most cases it is greater at part throttle. All of us have rescued a quitting or stumbling engine by simply reducing the throttle opening, if done before the plug goes out.

NM2K

My Feedback: (14)

Where many folks fall short in understanding is in realizing that if the seal formed around the crankshaft and crankcase, base of the carb or backplate leaks just a tiny bit, it can have devastating negative effects upon the Bernoulli effect that you have described.

Many of us are aware that it is important to have a great seal at the top of the piston in the cylinder, but how many consider the seal between the piston/liner-sleeve/lower crankcase? While our engines may appear simple due to a minimum of parts, there are many things occurring simultaneously/near simultaneously, that it can be difficult to wrap one's mind around everything and the implications of variables while at running speed.

For technical discussions, lose the word suction (it doesn't exist in technical parlance) and use the terms negative pressure (not you, Hobbsy).


Ed Cregger

Hobbsy

My Feedback: (102)

Very true, the carb creates the low pressure area and atmospheric pressure tries to fill the void, (balance things out) its that simple, muffler pressure just gives the atmospheric pressure a little assist.

Jim Thomerson

This has been discussed a number of times on other forums. My take is that muffler pressure supplies about 1/3 the pressure differential necessary to get fuel into the engine. About 1/3 because mufflers, carbs, phase of the moon , etc. mean that your results will vary.

MJD

My Feedback: (1)

Exactly - muffler pressure increases the pressure differential between the fuel tank and the venturi, resulting in greater fuel draw. If muffler pressure did nothing at low rpm, then my engines wouldn't go slightly rich when I re-attach the muffler line when they're idling. Muffler pressure accounts for some part of the fuel draw picture everywhere in the rpm range AFAIK - how does a muffler pressure system identify when the airplane is doing aerobatics for example?

I believe the addition of muffler pressure reduces the net effect of variations in head pressure caused by attitude changes and G forces - since those pressure changes are now a lower percentage of the now higher total that results from addition of pressure. Whatever - it works for sure.

MJD

DarZeelon

It seems too late to bother writing this, after the requiem for the glow engine has already been written...

Most newbies will more likely ask if the placement of the Li-Po pack will affect they function of their brushless motor.


Most newbies that don't already know all this, probably never will...[sm=sad_smile.gif]

Sport_Pilot

Ed and Hobbsy has already said that the engine doesn't suck in the fuel but that the carb venturi creates a vacuum based on pressure differences from the Bernoulli effect. I would like to point out that many modern high performance engines have very large carb venturi and have less vacuum even though they draw more air. If one of these engines if propped for lower speed they may not be able to run without muffler pressure, or with low muffler pressure with those large pits mufflers.

alan0899

G'day All,
If the carb venturi creates a vacuum based on pressure differences from the Bernoulli effect, Where does the vacuum come from, to produce this pressure difference?
It comes from the negative pressure produced by the piston/liner seal, as the piston goes up.
Hence what I previously called crankcase suction, which describes the effect perfectly, even if it is not technically correct.

Hobbsy

My Feedback: (102)

The partial vacuum, (low pressure area), is created by the increased airflow velocity caused by the air flowing through the narrower passage created by the spraybar, in a car carb it is caused by the venturi being smaller in diameter than the carb neck above and below the venturi. The carb has no idea whats moving the air it only reacts according to the laws of physics to the movement.

alan0899

G'day Hobbsy,
I agree totally, but my point is, that the air flow through the venturi is created by the piston movement.
Something has to cause the air to flow!
True or not.

jeffie8696

I have to think the tank pressure is much like the fuel pump on a carbuereted engine. Simple venturi effect fuel draw can't pull fuel all the way from the back of your car to the top of the engine so it needs a little assist. I have run countless engines without tank pressure but they are very sensitive to tank height. Take a cox engine and attach a small 4 oz tank to it then start it and run it . While it is running raise and lower the height of the tank and see the effect on the engine. Do the same thing with a pressure tap system and you will see very little if any effect. Yes? Something I ran into recently was a deficiency in my test stand fuel system. As it turns out I was not getting enough fuel volume to the engine with the setup and I could turn the high speed needle all the way out several turns without a change in the engine despite the tank being perfectly level with the carb(running fullout high rpms with a small prop). I modified the system taking out a restrictive line and installing a one way pressure valve to the tank and Boom. Problem went away. It created another problem with residual tank pressure pumping fuel after the engine has stopped and wanting to flood the crankcase. Not a problem if it was inverted but.................

Lou Crane

In reply to Jeffie and several preceding posts...

Jeffie's signature tagline deserves another look.

When Castor was the only practical oil to use in glow fuels, some of its characteristics helped in less obvious ways. E.g., it tends to thicken at higher temps - higher than we usually, or should run our engines. Still the tendency is there. Engines like the eternal Fox 35 lasted, and last, so long due to the tendency for hot castor to penetrate the porous Meehanite-type cast-iron piston wearing surface with a varnish-like coating. Maintains and replaces wear!

Castor's viscosity holds up well compared to other oils at practical operating temps and RPM; the varnishing tendency is a disadvantage for the carefully precise fits in newer ABC/ABN/AAC engines. ANY obstacle to reaching and using design-temperature running fits is unwelcome for these engines... However, Castor's viscosity does help slack fitting parts keep useful base (crankcase) compression. Viscosity of liquids is often measured by time to flow through a standard orifice. The 'time' available for the oil spray coating the cylinder wall below the piston on the downstroke to 'leak' is brief. That time is also all that is available for the pretty continuous wet coating along a plain crankshaft bearing. Newer metals in engines, and the synthetic oils that keep the metals clean, generally thin more at raised temperatures, and may leak more in a 'traditional fit' older design iron/steel engine.

So, Castor may help "old fashioned" engines seal better for crankcase guage-low and guage-high pressure conditions (guage pressure is ambient atmospheric, so we could say that pressure changes go guage-low or guage-high, instead of using the word suction?) Still, decent base compression seal helps Castor do its job. Don't know about Duke Fox's comment that base compression seal is "just as" important as combustion chamber seal... Combustion compression is several atmospheres above guage, while the irregularly shaped and larger case cavity volume, plus the limited shaft degrees timing for the entry passage to be fully open to allow flow into it, limit the average guage negative pressure during the intake period... Estimates of pressure drop, from Bernoulli's theorem, using average and peak flow estimates for cylinder volume and choke dimensions and a generous efficiency SWAG for "% fill" suggest average pressure drop of less than 1/3 atmosphere... (fixed choke and spraybar dimensions, full operating RPM.)

So, a meaningful point about muffler pressure: I've heard that it may average out about 1 or 2 psi positive, which is less than the 3 to 4+ psi negative we may find at the fuel jet in the carb/venturii throat at WOT. Still, there are two advantages: 5 or 6 psi is better than 3 to 4 as a pressure difference feeding fuel. Also, as a 'closed vent' system, the contribution from average pressure in the muffler is not subject to ram air pressure effects on an open vent tube end. (I fly CL, and open vent tube ends can easily be observed to tend to richen as the model comes around to face the wind. Inertia - kinetic energy - maintains the model's V a bit higher than it would be in windless conditions, for a brief time.) So, just as you'd like the engine to lean a bit from its rich cruise setting to meet maneuvering loads on the downwind side of the lap, it sounds richer... Muffler pressure averages out better in the air space in the tank, in these conditions, aside from the slight improvement in the fuel supply pressure conditions.

Untimed crankcase pressure taps provide more pressure than muffler taps, and timed case pressure - e.g., exposed to case positive pressure , by tapping to meet the shaft port during the piston downstroke only - gives several psi more. The only higher pressure available is external to the engine, as in a stretched balloon bladder for FF or CL Combat. Perry pumps, if I recall, give slightly less than untimed case tapped pressure, but have the advantages of being adjustable, and NOT being subject to the guage-negative upstroke pressure conditions.

Sport_Pilot

I believe this to be optimistic for any modern two stroke engine. Four strokes and older two stroke engines perhaps.

downunder

I think it's rather optimistic too. I've never tried it but others who have report that muffler pressure is easily determined by running the engine without muffler pressure (easy to do on the test stand just by opening the needle to compensate) and dropping a long length of tubing attached to the muffler nipple into a jar of water. At first it bubbles furiously but as you lower the tubing the bubbles slow down and the point where they stop gives the water column pressure. By all accounts this is somewhere around 6" below the surface which works out to be something like .25psi.

Older RC engines though had a venturi sized more like what's used in a CL stunt engine which need a very good "suction" to keep the fuel flow as constant as possible. The actual pressure drop is a bit hard to measure but I've heard it's around the 20" water column mark. But that's with a small venturi giving high air velocity. RC carbs tend to be about twice the area of a CL venturi or an old RC carb which means the air velocity is about half. But pressure drop through a venturi varies by the square of velocity so a large RC carb will only have 1/4 the pressure drop of an old carb. This small pressure drop ("suction") is where muffler pressure becomes necessary to add to the difference in pressure between the tank and venturi.

scratchonly

I have the same situation in a zero (tank 2 1/2 inches too high) with a 90 4S inverted. A Cline regulator was used and works great; trouble is you need some muffler pressure. Can you tap your pipe for a fitting? A nipple is supplied with the unit. I have read somewhere that you can get pressure from a T on the vent line but I can't remember where I saw it. Good luck.

Lou Crane

Downunder and Sport Pilot, thanks for the comments...

I just gave the numbers range for what I calculated for dimensions and RPM observed. The assumptions about flow velocity profile, smoothness during the port-open period, and percentage of crankcase fill vs cylinder displacement volume were all highly favorable. The actual situation is more likely that obstructions to flow, non-smooth flow due to shaft valve opening and closing, etc., make the actual pressure drop different from the simplified calculations, and that individual setups may vary in differing ways.

In turn, I'm surprised to hear that muffler tap pressure looks to average out ~1/4 psi! There may be an effect of "pipe resistance" (not tuned pipe, just the length of tubing involved) and the 'softer' cushion of airspace in the tank... The peak pressure during the variations through each exhaust cycle passing through the muffler may affect the airspace, then bleed back down, but too slowly to return to atmospheric or lower before the next higher pressure pulse arrives. So, a cumulative average pressure in the tank airspace may be a bit more than would show in the vertical depth water-immersion check mentioned.

It will also depend a lot on the muffler... In benching a .40 4C, with a small 'collector' "muffler", tapped for pressurizing the fuel, I observed VERY strong and definite signs of considerable pressure: the fuel surface (bottle tank) directly facing the muffler tap inlet flow had a 'bowl' form from the "air" jet hitting it, and the engine starved off instantly if the tap pressure line were pinched or pulled off. I haven't seen indicators that definite with any 2C engine muffler pressure flow...

At any rate, a forward facing open vent line for a tank feeding by venturii "suction" alone, can be subject to ram pressure variations as the model changes V or direction. Whether muffler tap pressure is significantly high or not, we gain a fuel supply venting system closed against external velocity-related ram pressure variations.

Hobbsy

My Feedback: (102)

Lou, you can take any four stroke and put a check valve in the exhaust pressure tap to provide pressure for a Cline or IronBay regulator and the tank will be as hard as a rock with a bulging sides. A site to see and feel.

longdan

Does the size of the tank have anything to do with it? The 0.25 psi acting on the top surface of the fuel in the tank creates a force which pushes fuel towards the engine, kind of like pushing down the plunger in a syringe. Since the force = pressure x area, an engine fed from a larger tank (with more area)would benefit a lot more from mufler pressure.