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Automatic Mixture Control
Can you Hac it?
By Gerald J. Olenik,
Pres. Green Sky Adventures, Inc.
Around mid September we procured a Kitfox Model II for the purpose of developing our HKS engine conversion package. Consequently there was a need to get the plane from it's home in North East Ohio, to our location in North Central Florida. I was familiar enough with its maintenance history, that flying the Kitfox on a thousand mile trip over a couple hundred miles of mountains seemed a reasonable gamble. It was during this flight, that I was reminded what a pain it can be to fly the 2 strokes without automatic mixture control. Prior to removing the 582, a decision was made to install a High Altitude Compensator (HAC) on this plane and explain the process. The need to adjust fuel/air mixture with changes in air density is nothing new. Air density varies with altitude and with daily or seasonal climate changes causing our engines to run richer or leaner.. An operational symptom of a fuel rich engine is rough running. Conversely, too lean a mixture results in very smooth running, often preceding piston seizure on 2 strokes. So, what's better, too rich, (rough) or too lean (dead)? Neither of these are very attractive, but you must admit, a rough engine is better than none at all.
Why worry about jetting and mixture?
Wait a minute. Why such extremes? Isn't there some compromise or middle ground? Of course there is! Bing 54 carburetor equipped, 2 stroke aircraft engines, allow a relatively broad operating range before a change in jetting is required. If your engine is jetted for the middle of that range, as with standard jets, and you don't fly higher than a few thousand feet or in extreme cold or fast, quite possibly your engine will never be so rich that it runs rough or so lean that it seizes. Two variable components in the mixture equation, other than the carburetor are Load and Air Density. Changing either of these will cause an engine to run richer or leaner. It won't necessarily run badly unless drastic changes are made in one of these components or there are multiple changes.
Variable 1: Correct Load
Let's say we have a 65 HP 2-stroke engine that makes its maximum power at about 6,500 rpm, with WOT, (wide open throttle) in standard atmosphere, (sea level, dry air, 59° F, 29.92" hg pressure). That is an exact condition that compliments standard carburetor jetting. So, "correctly" loaded, at standard atmosphere, most any throttle position will result in a fuel / air ratio that is not to lean, not to rich, ...Somewhere in the middle. As airspeed increases, the load typically decreases allowing higher rpm for any given throttle position. Fast is a relative term, but for this argument, it is any speed that lightens the load on the engine allowing its maximum WOT rpm to substantially exceed 6,500. The extra RPM is not the enemy, its a messenger, telling a pilot the fuel air ratio has drifted from the middle, towards a potential enemy (the lean side). If a pilot responds by reducing the throttle position to reduce rpm, the messenger is silenced, but the engine is still gulping extra air. It's at a different throttle position but still running leaner than it would without the lightened load.
So, in the case of faster aircraft, the carburetors may need changes from standard jetting to richen the mixture. Or, another approach is loading. Instead of a propeller that allows 6500 rpm at WOT static, how about one that allows 6500 rpm at maximum speed in level flight? This should result in a mixture within safe operating range at any throttle position during flight. Either change will make the mixture a little on the rich side for static operation. That shouldn't be a problem as long as there isn't too much variation in atmospheric conditions.
Variable 2: Air Density
Regardless of the choice in tuning method, there remains that major variable, Air Density. We may occasionally operate at standard atmosphere, but not often or continuously. Standard Atmosphere, (could also be called "Standard Day",) is basically a benchmark for reference, that can be described as: sea level elevation, dry air, 59° F, 29.92" hg pressure. It actually calculates to 1 ft density altitude. Density altitude, therefore, is a useful value that quantifies the effects of temperature, pressure, and humidity on our engine and aircraft. It could be a little confusing, because high density altitude = less dense air, and low density altitude = more dense air.
In the example above, where jets were enlarged, or load was increased to achieve proper fueling in flight, it was satisfactory to be a little rich statically. However, if density altitude increased by 4 or 5 thousand feet, the mixture may not work at all. You may occasionally witness this, watching an aborted take off of a Northern flyer, at Sun n Fun's Paradise City when outside air temps get up near the nineties. I can't be the only one who's ever had this happen! The engine makes a lot of intake roar, at WOT, but only about 30% power.
What about the other side of the coin? If we were cautiously tolerating a lean mixture, perhaps on a long cross country where some high altitude flying may be necessary, couldn't the DA decrease by 4 or 5 thousand feet as the flight progresses? That could drive an already lean mixture to the seizure point.
You don't even have to leave home to have such wide variations. During tests of our automatic High Altitude Compensation kit on a Kitfox Model II, we flew to 10,000 feet msl on one particular day. That day, the density altitude was 2,038 ft msl at our departure point. Two days later, the weather was ideal for more testing, because a cold front had passed bringing DA down to 1960 ft below seal level. That's a 4,000 ft change, absolutely enough to drive an already lean mixture to the seizure point. Obviously, monitoring EGT is essential, as it is the best method of knowing what's happening with the fuel air ratio.
Automatic Mixture Control
The High Altitude Compensating system for Bing Carburetors, is nothing new. Design and testing goes back to the late eighties. On an early "proof" flight, the crew from Robertson B1RD company flew a HAC equipped, Rotax 532 powered, B1RD to over 20,000 ft. automatically maintaining proper mixture. Green Sky Adventures, Inc., has tested HAC carburetors on five different Rotax engines for over ten years on the company's Zippy Sport. From speeds of 40 mph to130 mph, from sea level to 15,000 plus feet, correctly installed, the automatic mixture control is flawless.
Until recently, HACs were available only as a complete package which not only included the HAC chamber, lines, and fittings, but also came with two (one if single carb configuration) complete Bing 54 carburetors, and two application specific Rotax/K&N filters, all at a price of around $700. Green Sky Adventures, Inc. now offers HAC technology in a much more affordable package which, in most cases, eliminates the need to replace existing carburetors or airfilters. Prices start as low as $189 for internet orders and installation time is typically one hour.
The basic kit includes: HAC chamber, Fittings, Lines, Jets, and come calibrated, ready to install.
Late model carburetors (right) are equipped with an extra passage to the venturi, which is connected as a source of low pressure, through a flexible hose, to the variable side of the HAC. Earlier production castings have the undrilled boss (above) Still earlier castings have neither.
Here's how it works
Did you ever wonder what makes fuel flow up the jets, from the carburetor float bowl, enroute to the engine? Is it vacuum at the venturi? That's not quite descriptive enough. It is better defined as a differential pressure between the float bowl and the venturi. What's the pressure in the float bowl? Here is an important hint, the bowl is vented to outside atmosphere. So, it should be the same atmosphere, or ambient pressure, that's feeding the airfilter. If it is not, the differential between the venturi and float bowl is going to change and so will the mixture...Maybe richer, maybe leaner. Just think of that for a second. If fuel air mixture can be thrown off, accidentally by improperly venting the the float bowls, why not control the venting to effectively control mixture? That's exactly what your Bing 54 can do, automatically, when a High Altitude Compensator is added. If pressure in the float bowl is reduced, relative to the venturi, less fuel makes its way up the jets, and consequently, mixture is leaner.
Changing to larger main jets, in most cases, is the only required jetting change.
Vent lines are cut and tees installed to pick up float bowl pressure from the HAC The airfilter is drilled to accept a hose coupling for the static line
The HAC unit has two chambers separated by a diaphragm. One chamber is completely sealed (except during calibration) and air density within remains constant. The other has airfilter, float bowl, and venturi connections, so air density on this side is variable. It is in this chamber, that ambient pressure can be reduced by feeding it to the venturi via a connection on late model carburetors. The amount of ambient reduction is controlled by a tapered needle which changes position with deflection of the diaphragm. Think of the diaphragm as a flexible wall between two chambers that allows the sealed side to expand or contract as changes in ambient pressure occur on the other side. The reduced (from ambient) pressure gets routed to the float bowl(s) via their vent lines with a resulting leaning affect..
If that supply of low pressure is shut off, float bowl pressure returns to ambient, and the mixture goes as rich as the jets will allow. With HAC installations, standard jetting starts out several steps richer. This is why we say the default, or failure mode is typically toward the rich side.
Are there potential risks? The down side
Green Sky Adventures, Inc. currently calibrates HAC units to conform to a service information document once known as 2S 89-E. Mixture control range is from approximately -4,000' msl to +18,000' msl.
Diaphragm failure defaults to a 0 feet msl mixture. In that very unlikely event, a HAC equipped engine operating at 18,000 feet would most likely quit. At half that altitude, it would run rough and loose a couple hundred rpm. If the vent or venturi lines fail, the mixture defaults to full rich.
The HAC unit will not react instantly to extreme atmospheric change. So if it lives in a heated hangar, it will take a couple minutes to correct itself when it's rolled outside on a cold winters day.
The HAC alters the mixture of both Carburetors simultaneously. In the event of a differential pressure between the two air filters, a corresponding differential mixture will result. If a low pressure exists at the air filter where static pressure for the HAC is sensed, BOTH carburetors will be leaned. For that reason, on dual carb HAC installations, use of the single oval filter with provision for both carbs is preferred, or a balance tube may be installed between separate airfilters.
What does the HAC do for me? The up side
I have a preference for airplanes that can be flown at all normal attitudes, throughout their speed range, at any and all throttle positions, without quitting (exceeding EGT limits, upper or lower) In pursuit of that preference, there is a tendency to overload, and/or over fuel.
I also have another preference. Even though the Ultralight Anthem from Airventure 2003 still rings in my memory, "....Low and Slow, Way to go..." Occasionally it isn't the way to go. There are those flights that take us over hazardous terrain. My second preference is to reduce exposure time to that hazardous terrain, just in case I don't get my first preference. Sometimes flying high, it's like pushing the fast forward button.
With a 65 knot cruises speed, if a 12 knot headwind can be converted to a 15 knot tailwind ground speed increases by 50 percent. Even if it takes 9,500 ft to do it...why not?
In the Kitfox Ohio/Florida trip mentioned above, it was not a hypothetical. One leg between Parkersburg WV, and North Wilkesboro NC was definitely over hostile territory. The engine was tuned perfectly on the cool September morning for preference number 1, at the departure point up by Lake Erie. By mid day over the mountains of West Virginia, the engine was beginning to exhibit rich symptoms at 4,500 feet and there was a quartering headwind. The wind at 9,500 would have been almost 20 knots on the tail, but lack of mixture control sentenced the flight to lump along, low and slow.
This allowed plenty of time to reflect on how badly I wished this plane had our HAC kit.
Now it does.
Float bowl pressure is bleed off past a tapered needle in the HAC chamber and routed to the venturi connection on one carburetor.
A static line between the airfilter and HAC provides ambient pressure
Mounting the HAC does not have to be elaborate. Though this temporary method for our test makes use of a coolant line for support and vibration isolation, heat from the coolant line could effect calibration.
The Panel at 10K