Carburetor icing, or carb icing, is an icing condition which can affect any

carburetor A carburetor (also spelled carburettor) is a device used by an internal combustion engine to control and mix air and fuel entering the engine. The primary method of adding fuel to the intake air is through the venturi tube in the main meteri ...

under certain atmospheric conditions. The problem is most notable in certain realms of aviation. Carburetor icing is caused by the temperature drop in the carburetor, as an effect of fuel vaporization, and the temperature drop associated with the pressure drop in the venturi. If the temperature drops below freezing,

water vapor (99.9839 °C) , - , Boiling point , , - , specific gas constant , 461.5 J/( kg·K) , - , Heat of vaporization , 2.27 MJ/kg , - , Heat capacity , 1.864 kJ/(kg·K) Water vapor, water vapour or aqueous vapor is the gaseous pha ...

will freeze onto the throttle valve, and other internal surfaces of the carburetor. The Venturi effect can drop the ambient air temperature by 70 absolute degrees Fahrenheit (F) (40 degrees

Celsius The degree Celsius is the unit of temperature on the Celsius scale (originally known as the centigrade scale outside Sweden), one of two temperature scales used in the International System of Units (SI), the other being the Kelvin scale. The ...

). In other words, air at an outside temperature of 100 degree F, can drop to 30 degrees F in the carburetor. Carburetor icing most often occurs when the outside air temperature is below 70 degrees F (21 degrees C) and the

relative humidity Humidity is the concentration of water vapor present in the air. Water vapor, the gaseous state of water, is generally invisible to the human eye. Humidity indicates the likelihood for precipitation, dew, or fog to be present. Humidity depe ...

is above 80 percent. Unfortunately, the warm air temperature often causes pilots of aircraft to overlook the possibility of carb icing. The

ice Ice is water frozen into a solid state, typically forming at or below temperatures of 0 degrees Celsius or Depending on the presence of impurities such as particles of soil or bubbles of air, it can appear transparent or a more or less opaq ...

will form on the surfaces of the carburetor throat, further restricting it. This may increase the Venturi effect initially, but eventually restricts airflow, perhaps even causing a complete blockage of air to the carburetor. The engine begins to run more rich as ice formation increases. Without intervention (carb heat or leaning) this can only continue until the mixture is outside of the "chemically correct" range for combustion. Icing may also cause jamming of the mechanical parts of the carburetor, such as the

throttle A throttle is the mechanism by which fluid flow is managed by constriction or obstruction. An engine's power can be increased or decreased by the restriction of inlet gases (by the use of a throttle), but usually decreased. The term ''throttle'' ...

, typically a butterfly valve. While it applies to all carburetors, carburetor icing is of particular concern in association with piston-powered

aircraft An aircraft is a vehicle that is able to fly by gaining support from the air. It counters the force of gravity by using either static lift or by using the dynamic lift of an airfoil, or in a few cases the downward thrust from jet engines ...

, especially small, single-engine, light aircraft. Aircraft powered by carbureted

engine An engine or motor is a machine designed to convert one or more forms of energy into mechanical energy. Available energy sources include potential energy (e.g. energy of the Earth's gravitational field as exploited in hydroelectric power gen ...

s are equipped with carburetor heat systems to overcome the icing problem. In cars, carburetor icing can occasionally be a nuisance. The

inlet manifold In automotive engineering, an inlet manifold or intake manifold (in American English) is the part of an engine that supplies the fuel/air mixture to the cylinders. The word '' manifold'' comes from the Old English word ''manigfeald'' (from th ...

and parts of the carburetor often have warm water from the cooling system or exhaust gas circulating through them to combat this problem.

Motorcycle A motorcycle (motorbike, bike, or trike (if three-wheeled)) is a two or three-wheeled motor vehicle steered by a handlebar. Motorcycle design varies greatly to suit a range of different purposes: long-distance travel, commuting, cruising ...

s can also suffer from carburetor icing, although some engine designs are more susceptible to it than others.

Air-cooled engine Air-cooled engines rely on the circulation of air directly over heat dissipation fins or hot areas of the engine to cool them in order to keep the engine within operating temperatures. In all combustion engines, a great percentage of the heat ge ...

s may be more prone to icing, due to the absence of warm coolant circulating through the engine.

References

Weather hazards to aircraft Engine problems Carburettors {{Aviation-stub

See also

References