A carburetor is a mechanical device that mixes air and gasoline into a combustible mixture. For perhaps a century carburetion was standard in just about anything with pistons—automotive, aviation, agricultural, marine, motorcycle, and industrial engines—but carburetion went functionally extinct in the automotive world in the late 1980s and is ancient stuff pretty much everywhere today—except in third-world motorcycles and first-world light aircraft where they’re still prevalent. Thus, while graybeard hot rodders natively understand carburetors and the intricacies of operating car, bike, and airplane engines with them, today’s new pilots need to familiarize themselves with this time-tested technology.
Carburetors can be very complex, but the typical light airplane “carb” is about as advanced as a carefully arranged box of rocks. Such simplicity helps make them reliable and resilient to dirt and other abuse, plus they are low-cost (or should be: see lawyers). Another plus is there are no electronics in play with carburetors, just fundamental physics.
A primary carburetor function is controlling engine power output. This is done with a round flap in the carburetor’s main air passage. Closing this flap “throttles” the engine in the Victorian sense of the word, by restricting airflow through the carburetor’s throat. And so the pilot has a throttle control knob or lever in the cockpit to set the power output via controlling how much air the engine can breathe.
Controlling fuel delivery in a carburetor is done by differentials in air pressure. Such differentials both meter the correct amount of fuel and physically power fuel delivery. The carburetor’s main air passage (its throat) has a venturi shape. That is, the passage gradually narrows, then reopens in a fairly gentle taper. Air passing through the carburetor speeds up to get through the restricted area, causing the air pressure to drop at that point. This is the motive force in a carburetor.
Off to the side of the venturi is a small chamber with gasoline in it: the float bowl. Small passages connect the float bowl to the venturi, and as the air pressure is slightly lower in the venturi than in the float bowl (which is vented to the atmosphere), air pressure pushes fuel out of the float bowl and into the airstream passing through the venturi. This is the carburetor’s main fuel circuit used during cruise flight.
A separate idle circuit for very low power while idling or taxiing is also provided. A third circuit gives a momentary extra shot of gasoline when the throttle is rapidly advanced; this is because lightweight air responds immediately when opening the throttle but the far heavier fuel lags before increasing its flow rate. This is the accelerator circuit and is one place to investigate if the engine stumbles when rapidly opening the throttle.
Because a cold engine—especially in cold weather—requires extra fuel to start and run smoothly until it warms up, there can be a choke circuit. On carbureted automobiles this is normally a flap over the carb’s air inlet; on Rotax aircraft engines it is mechanical linkage that increases fuel flow, and on Lycoming/Continental engines the choking is handled by the pilot pumping the throttle before cranking the engine. That operates the accelerator pump, squirting fuel into the intake manifold. Furthermore, the mixture control runs quite rich at its full rich setting and this is often enough fuel enrichment for cold starting in cool weather.
For truly cold starts carbureted engines may have a primer. A primer is completely separate from the carburetor. It’s essentially a gasoline syringe that shoots a healthy stream of gasoline into the intake manifold.
Finally, there is the very authoritative mixture control in the cockpit. The mixture modifies the carburetor’s fuel delivery from zero to full flow. Zero flow (idle cut-off) is used to shut off the engine. Full rich is used for starting plus high power used at takeoff and climbing. Because air density falls with altitude, the mixture control is necessary to restrict fuel flow as the airplane gains altitude to avoid running overly rich.
Young student pilots today have never had to think about these circuits or mixture controls because all this is done automatically by electronic fuel injection on auto engines. Therefore, the flight instructor needs to teach carburetor operation and the student must understand why it’s necessary.
As you have so correctly said the instructor needs to explain this operation to the student. It then takes a lot of hours and varying weather conditions until the student begins to understand how and why this all happens, and finally to appreciate the reliability and foibles of gravity, water, atmosphere and carburetor mechanics. It’s not an overnight thing. Really good article Tom.
Assumes that the instructor is not on the same knowledge level as the students that he is charged with instructing
“…on Lycoming/Continental engines the choking is handled by the pilot pumping the throttle before cranking the engine”.
That comment made me cringe a little. While some aircraft (Robinson helicopters in particular) do actually list pumping the throttle before engine start in the POH, many do not. I have also been taught from numerous engine courses that if one does pump the throttle in this manner, it should only be done while the starter is engaged.
Came here to object to this wording as well. Pumping prior to starting the engine only fills your airbox with fuel and risks an airbox fire. Pumping *while cranking the engine* will make use of the suction in the manifold to draw that fuel into the cylinders.
“A third circuit gives a momentary extra shot of gasoline when the throttle is rapidly advanced”.
I have found that “rapidly” doesn’t actually have to be all that rapid at all, as I discovered one day while playing with a carburetor that had been uninstalled from an engine and still had fuel in it. The rapidness of throttle advancement only seemed to control the forcefulness of the extra fuel jet.
Wait, wait . . . Must be some marvel aficionado. Kind’a left my older-n-dirt, bone simple, Stromberg out in da cold.
“Third circuit” for when the throttle is rapidly advanced?
We don’t need no stinking “Third circuit”! Real Pilots don’t “rapidly advance” the throttle. Er . . . well we learn not to. (grin)
Tom, good description. I see an issue with the first picture that gives me some heartburn. There is a fuel input line with an aluminum adapter fitting to a t-fitting with what looks like a pressure gauge. I am uncomfortable with that much weight hanging free in a high vibration environment. This could lead to failure (cracking) of the carburetor inlet fitting. May be just my paranoia.
Tom,
Why does a carb need a mixture control for altitude? At a higher altitude, there is lower pressure on the float bowl so less fuel being pushed through the main circuit. Also lower density, so won’t that create a lower pressure drop across the venturi?
The amount of fuel delivered from the jet depends on the pressure drop (relative vacuum) in the venturi. But there are 2 different reasons pressure drops: (A) fast moving air, and (B) thinner air at altitude. If (A), then more fuel is the right thing to do. If (B) then more fuel is the wrong thing to do. The simple carburetors used in most GA airplanes can’t tell (A) from (B) – either way it’s just a pressure drop.
Altitude compensating carburetors have been made, but they are more complex and not used in most GA aircraft.
I can’t believe we are still expect kids to learn to operate antique technology like this. No wonder GA is so messed up.
Antique technology? Paddles and bailers are required in liferafts and oars (idiot sticks) are still required in lifeboats. If kids can’t put down their ipads or phones to learn some basics they don’t deserve to be in charge of a three dimensional high speed machines.
I learned to fly in the early 1980’s and there was a lot of stuff to learn back then to get a PPL. Much more has been added to the basic curriculum since. My expertise is aerospace composite materials, but I fly an “antique classic” Beech Bonanza with a pressure carburetor. Even as a 5000 hour pilot and A&P, all I really care about in the cockpit is the throttle, mixture and fuel selector. An A&P is not even supposed to disassemble a carburetor any more. You overwhelm new students with too many details, when they should be learning stick and rudder skills!
Understanding carburetor basics is understanding why the engine still runs. Understanding why tricycle geared aircraft tend to be more controllable on the ground versus tailwheel aircraft with their c.g. behind the mains falls into the same category of “gotta know this” it seems to me.
Lots of tailwheel time Tom. We never discussed theory. We went out and developed “muscle memory”… lol!!!
I agree with the “stick and rudder skills,” but current flight instruction is missing elements of operational understanding for the airplane and the engine being flown. Both engine and airplane have specific characteristics that influence decision making, and while the lowly carburetor may seem least of the worries, understanding its operation is imperative in knowing when to ignore, and when to not.
Yes, doubly so when it burps with a slug of water or becomes a frozen relic at the crash site.
<< this is because lightweight air responds immediately when opening the throttle but the far heavier fuel lags before increasing its flow rate >>
The fundamental reason for the transient lean condition when the throttle is opened too quickly is not because fuel is heavier than air and slower to respond, it’s because the engine itself takes time to spin up and keep the intake air moving.
In more detail:
For a given rate of airflow (mass per second), the speed at which air travels through the bore depends on the area of the bore. Bigger bore = slower air and vice versa. And pressure depends on airspeed. Faster air = lower pressure and vice versa.
When the throttle is suddenly opened, the throttle bore’s effective area suddenly increases, which slows down the air passing through it. Slowing air increases relative pressure, which draws less fuel through the jet, causing a lean condition. This is only temporary; as soon as the engine responds to the increased airflow, it speeds up, “sucking harder” which draws more air and re-establishes the flow of air and fuel through the carb.
When you open the throttle slowly, this lean condition doesn’t happen because the engine has time to speed up as airflow increases, keeping the airflow moving.