When you apply carburetor heat to melt ice that has formed in the throat, or venturi, of the carburetor, you may notice that the engine begins to run even rougher. This happens because the fuel mixture, already enriched because the ice is choking off some of the induction air flow, is suddenly made even richer by the addition of hot air.
This triple whammy can make the mixture so fuel-rich it will not ignite in the cylinders. The solution is to lean the mixture (and sometimes it takes some pretty radical leaning) and get a burnable mixture going to the cylinders.
Let's review some carburetor basics. Airflow through the carburetor venturi results in a pressure drop that draws fuel from the float chamber. The mixture control can vary the amount of fuel supplied for a given amount of air. Opening or closing the throttle actually changes the amount of air flow, and the carburetor automatically supplies (more or less) the correct amount of fuel to mix with that amount of air.
Carb ice forms because the pressure drop in the venturi causes the air to "cool," and draw heat away from the surrounding metal of the carburetor venturi. Ice then can begin collecting on the cooled carburetor throat. This is the same principle that makes your refrigerator or air conditioner work.
Meanwhile, fuel being drawn through the fuel discharge nozzle into the airflow atomizes into very fine droplets that evaporate easily. When the fuel changes from a finely atomized liquid to a vapor it, too, cools—stripping more heat from the surrounding metal.
The result is that the carburetor's internal temperature may drop below freezing, even on a warm day. If the ambient air contains sufficient moisture (which can be the case even in seemingly dry air), frost (carburetor ice) can form on the inside of the carburetor.
It's important to understand that carburetor ice results not from a decrease in airflow through the carburetor, but the change in pressure caused by the restriction in the venturi.
The carburetor operates according to Bernoulli's principle. This principle states, in essence, that the static pressure of a non-compressible gas varies inversely with the velocity of the gas as it flows through a tube of varying cross-section. (Due to the laws of the conservation of energy, total pressure remains constant, and because total pressure is equal to static pressure plus dynamic pressure, then dynamic pressure must increase.)
Static pressure decreases as a result of the increase of the velocity of the air flow, not as a result of the change in the mass of air flowing through the tube.
Each time a normally aspirated, four-cycle engine (which describes the engines in most trainers and simple four-place aircraft) completes two crankshaft revolutions, it draws a volume of air equal to the engine's displacement (less small losses because of throttle position and system friction) through the carburetor. Given a constant throttle position, this volume essentially remains the same whether the carburetor is wide open or clogged with ice.If the carburetor venturi is constricted because of ice, the velocity of the flow must increase because the amount of air flowing to the cylinders is constant. This increase in velocity is much more significant than the small decrease in mass flow caused by the restriction in the venturi because of ice.
An increase in velocity, Bernoulli says, will cause a further decrease in static pressure within the venturi, which means the ambient static pressure acting on the fuel in the float bowl will push more fuel through the metering jet, resulting in a richer mixture.
In most cases, pilots can get rid of accumulations of carburetor ice by using carb heat. Nothing more is necessary. This proves that the system works as designed—warming the carburetor venturi and body—especially if we are conscientious in applying carb heat before reducing power.
Also, many of today's training airplanes use Lycoming engines, which mount the carburetor on the oil sump. This gives the carburetor another source of heat. Because of this, Lycoming engines seem to be less susceptible to carb ice.
Rarely do engines quit when you apply carburetor heat, so pilots have trouble accepting that it can happen. I was an unbelieving pilot until the engines in two different airplanes stopped on me in the same week. I was able to get the engines running again because I remembered to pull the mixture almost to idle cut-off in both cases. The engines generated enough heat to melt the ice.
Having adequate heat to melt ice becomes a real problem during prolonged low-power operations because the engine just isn't generating enough heat in the system. There are several partial solutions to this problem.
First, apply carb heat well before you reduce power. This preheats the carburetor and keeps ice from forming in the first place. If you do this when descending from altitude and in the landing pattern, you can push carb heat off on short final, so you won't have to worry about it in the event of a go-around.
Second, if you need to make a prolonged, low-power descent, "clear" the engine periodically by applying power, heating up the carb heat system, and burning out any ice that may have accumulated.
Finally, if applying carb heat results in loss of power, or even in significant "roughening" of the engine, you must immediately open the throttle and pull the mixture control out far enough to smooth out the engine. As the ice melts, restore the mixture gradually to the original position.