If you don't know me, I am an ask-it basket. This is hardly the first time you'll hear from me. So, how do they work? What determines good atomization? BTW, I was wondering if anyone has a carburetor that works like an airbrush... Instead of compressed air blowing by the needle and nozzle, just use the intake airflow to pull the fuel out? hey, the exhaust pressure would help pump out the fuel too.

Most model airplane two stroke and four stroke engines, Except Y-S, suck the fuel into the carb much like a airbrush does. But this fuel is metered through a needle valve.

Most model engines also use muffler back pressure to force fuel from the tank to the engine.

Enjoy,

Jim

The intake stroke of the piston creates a negative pressure under the carb which pulls in air. The air moves past the fuel spraybar, pulling fuel into the carb by the venturi effect. The amount of fuel is regulated by the needle valves. Muffler pressure helps the engine draw fuel at idle, when the negative pressure is the least.

Dr.1

But isn't muffler backpressure highest at full speed and almost non existent at idle? I think that with muffler pressure helping deliver fuel, the main needle needs to be closed a few clicks at full speed. Because the main needle is closed more, there can be more vacuum at idle at the spraybar for a correct fuel/air mix. On an air bleed carb, this means the air bleed can be closed a little more for a better fuel draw at idle.

You're right, the pressure is greatest at full throttle. I was always told the pressure helped the engine when the fuel draw was least, at idle. I've also seen the pressure help at high speed, too. Fuel draw in our little engines is marginal, at best, and they need all the help they can get.

Dr.1

The newer engines have oversize carbs is why they need muffler pressure. The older ones had much smaller carbs therefore much better fuel draw so didn;t need muffler pressure, in fact the motors in the 60s had no mufflers.

The Bernouli effect is at work in the venturi just like on a wing airfoil.

Big words. Now explain them to the guy.

Dr.1

Air go fast. Pressure go low. Big suck.

Works for me!

Dr.1

> Air go fast. Pressure go low. Big suck. <

I think I will hang those words over my workbench!

Without muffler pressure, fuel draw is lowest at full throttle, because of losses from the turbulance of high flow, and pulses, and because the engine friction and volumetric losses do not allow the engine to maintain the same velocity across the carb. It is best at idle, because the engine speed is lagging from the closed throttle, there is always sufficient velocity to maintain fuel draw. Thus muffler pressure is an ideal method to cure this as its pressure is almost opposite of the carb suction.

When liquids and gasses flow, one of the physical laws in effect is that where velocity is highest pressure is lowest. When air flows over the curved upper surface of a flat bottom wing the air in effect flows faster over the top due to it being a greater distance, this creates a low pressure on the upper surface creating lift because the pressure under the wing is greater. This effect is only a small portion of the lift required to fly so angle of attack makes up the rest. This same effect happens in a carb because the air has to "hurry up" to maintain the same volume in the venturi as is flowing above and below the venturi. Semi symmetric and symmetric airfoils have their own characteristics.

Here is one of the finest carbs I've ever had the pleasure to set up, it's on my MVVS .91. It has a simple wedge shaped butterfly that is tapered on the front and flat on the back creating a venturi, it is a metered carb and is as linear as an Irvine Jetstream and that's saying something. Unfortunately MVVS chose to discontinue it probably due to mfg. cost.

Hobbsy,

Both my statement and your statement are true. However the velocity is highest at idle because of the small throttle opening better volumetric efficiency at low spedd and the other factors I stated. Also pressure drops with velocity in any tube or duct. The airfoil causes a differance in velocity between upper and lower. The curved venturi in a carb is primarily to keep flow laminar (compared to a tube of the same diameter of the smallest area of the venturi), not to reduce the pressure. A good venturi will keep a better ratio of fuel air mixture from high to low speed or altitude compared to a straight tube. Fox and others used a simple square tube for control line models, but that would be a poor design for an RC engine.

Close Air go fast. pressure go low. Big PUSH

Atmospheric air is pushed in by the differential between atmosphere & the low pressure in the carb throat.

Sounds like something i'd tell my girl on those nights...

Dave,


Cost may have been one reason for discontinuing production of this carburettor, however, the newer #3216/3219/3215, with all their various models and variations are probably as linear as the #3222 that your MVVS .91 sports.

The older versions of the same carburettors, had the blunt-end idle needle, that caused the mid-range to be awfully rich, with perfect top and bottom ends.

For a time, an upgrade has been offered (#3216-0253), using the new, tapered needle and a replacement nozzle, but the behavior of the new carburettor is incomparably better than the old, even upgraded version.


The basis of operation of an R/C carburettor is the fuel metering, done by several means.
The basic means is the tapered low-speed needle that moves further into a nozzle, as the throttle is closed (the barrel moves sideways along a guide), making the fuel orifice smaller and allowing the engine to suck less fuel, as it inhales less air, thus achieving a close to ideal mixture, at all throttle openings.

Some carburettors meter the fuel differently, like, for example, the Perry carburettor, the aforementioned #3222 MVVS, the Webra Dynamix and a couple of other designs. Even the so called low-speed needle, works in a different manner in different two-needle carburettor types.

Here is a better look at how MVVS gets the venturi effect in this carb.

For Colin Stevens, Colin I dismanteled the MVVS carb to make sure of the direction for leaning and richening the midrange. Rotating the adjustment CCW==richer midrange, CW==leaner midrange. The carbs in picture #5 all have a common trait, small adjustment equals big change, minute adjustments are the rule of the day.
Pic. #1 full lean
Pic. #2 full rich
Pic. #3 where this one runs best
Pic. #4 full rich.

Hey Ask-it-Basket - you got flames going everywhere. Now is a good time to sit back with a bowl of popcorn and a soda jug - The show has just started

I wish I had asked this question.[:@]

---------------


I began using R/C throttled engines long before "muffler pressure" was invented. The only reason it is "needed" today is because the manufacturers have increased the carb's flow rate to the point that they need help from somewhere. Muffler pressure is used as that somewhere, but not always successfully.

Back before muffler pressure, air bleed carbs worked much better than today. Even very high quality and expensive engines utilized air bleed carbs successfully, for the most part.

Today I frown when I see someone adding muffler pressure to a K&B Sportster engine. They are asking for trouble. We modelers love to enhance our engines using our own ingenuity whenever we can. Especially if it is as easy as drilling a hole, tapping said hole and then screwing in a pressure nipple. Unfortunately, what most folks do not realize is that in spite of its looks, the Sportster series is a rather high tech product. The mufflers are actually tuned to provide a boost over open stack running power figures and they are also one of the quietest factory stock mufflers that you will find. The Sportsters carbs are metered and throated to provide the perfect amount of suction for good fuel delivery. Adding muffler pressure throws the Sportster carb out of calibration. Depending upon manufacturing variations, you may, or may not, be able to tune the carb satisfactorily when using muffler pressure. I remember when these features were new and "leading edge". Today, newbies come along and assume that these engines are low tech garbage and modify them willy-nilly. Then use their unsatisfactory results to reaffirm their original suspicions of low quality/low tech. Sometimes it is difficult not to say something. After all, I no longer have access to the magazine articles that would back up what I would tell them. They would just see some old timer that they think is trying to defend an old product that they happen to like. I used to be arrogant like that too...<G>


Ed Cregger

There's a web site, www.howstuffworks.com that may help answer your questions. It is a very useful source of information

I know I'm agreeing with someone hear......I have had many motors idle well with the muffler pressure line off, advancing the throttle open is where the problem comes in. My instructor 25 years ago said if a muffler came off in flight don't adjust the throttle......just set up for landing and bring her in and most times she won't deadstick at a constant throttle setting......He was right I have landed more than a few times with out a muffler(no pressure) and the engines stayed lit all the way to landing/Idle, but don't miss the landing cause throttle up (quick pressure change)go arounds are probably not going to happen.

The funny point is that he has never shown up again. I was going to ask him if he had ever heard of the old fashioned method of learning, instead of being an as-kit basket, it's called reading.
John

EDIT-> All said in good fun.