Carburetor heat
Encyclopedia
Carburetor
, carburettor, carburator, carburettet heat (usually abbreviated to 'carb heat') is a system used in automobile
and piston-powered light aircraft
engines to prevent or clear carburetor icing
. It consists of a moveable flap which draws hot air into the engine intake. The air is drawn from the heat stove
, a metal plate around the (very hot) exhaust manifold
.
contains significant levels of water vapor
; as the air is drawn into the engine through a carburetor it is cooled sufficiently to cause that vapor to condense out and freeze on the butterfly valve
. This cooling has two causes: the air pressure drops due to the expansion caused by engine suction and the increase in speed through the butterfly valve (in part-throttle operation) which drops the air temperature; and the liquid gasoline being introduced into the airstream must evaporate
, and the heat of evaporation is extracted from the airstream, cooling it. The temperature in the carburetor can drop below the freezing point
of water, and if the air is humid
, ice
can form inside the carburetor. As the ice builds up, less air can pass through the carburetor, causing a drop in power and, in severe cases, the engine will eventually stall.
Temperature drops of 20°C (36°F) or more are often encountered within the carburetor, so ice can build up even when the outside air temperature is well above freezing. Perhaps paradoxically, cold winter weather is less prone to icing, since cold air contains much less moisture. A warm day with high humidity is considered the most likely scenario for carburetor icing. Since there is less air acceleration through the carburetor at full-throttle
operation, icing is usually not a problem then.
Carburetor heat uses hot air drawn from the heat exchanger or heat stove (a metal plate around the exhaust manifold
) to raise the temperature in the venturi section high enough to prevent or remove any ice buildup. Because hot air is less dense than cold air, engine power will drop when carburetor heat is used.
Engines equipped with fuel injection
do not require carb heat as they are not as prone to icing - the gasoline is injected as a steady stream just upstream of the intake valve, so evaporation occurs as the fuel/air mixture is being drawn into the cylinder, where metal temperatures are higher. The exception is monopoint or TBI injection systems which spray fuel onto the throttle plate.
Some multipoint injection engines route engine coolant through the throttle body to prevent ice buildup during prolonged idling. This prevents ice from forming around the throttle plate but does not draw large amounts of hot air into the engine as carburetor heat does.
. The diversion of warm air into the intake reduces the available power from the engine for three reasons: thermodynamic efficiency is slightly reduced, since it is a function of the difference in temperature between the incoming and exhaust gases; the quantity of air available for combustion inside the cylinders is reduced due to the lower density of the warm air; and the previously-correct ratio of fuel to air is upset by the lower-density air, so some of the fuel will not be burned and will exit as unburned hydrocarbon
s. Thus the application of carb heat is manifested as a reduction in engine power. If ice has built up, there will then be a gradual increase in power as the air passage is freed up by the melting ice. The amount of power regained is an indication of the severity of ice buildup.
It must be kept in mind that the ingestion of small amounts of water into the engine following melting in the carburetor may cause an initial period of rough running for as much as one or two minutes before the power increase is noted. Again, the pilot will note this as evidence that icing conditions are present. However, more than one pilot, when confronted with a rough running engine has mistakenly turned the carburetor heat back off, thereby exacerbating the situation.
Applying carb heat as a matter of routine is built into numerous in-flight and pre-landing checks (e.g. see BUMPH and GUMPS
). In long descents, carb heat may be used continuously to prevent icing buildup; with the throttle closed there is a large pressure (and therefore temperature) drop in the carburetor which can cause rapid ice buildup that could go unnoticed because engine power is not used. In addition, the exhaust manifold cools considerably when power is removed, so if carb icing occurs there may not be heat sufficient to remove it. Thus most operational checklist
s call for the routine application of carb heat whenever the throttle is closed in flight.
Usually, the air filter
is bypassed when carb heat is used. If the air filter becomes clogged (with snow or ice), using carb heat allows the engine to keep running. Because using unfiltered air can cause engine wear, carb heat usage on the ground (where dusty air is most probable) is kept to a minimum.
Altitude has an indirect effect on carburetor ice because there are usually significant temperature differences with altitude. Cloud
s contain moisture, and therefore flying through clouds may necessitate more frequent use of carb heat.
The intake air of an aircraft engine equipped with a supercharger is heated through compression, so the air entering the throttle body is already warmed enough that carb heat is unnecessary.
Automobile engines may also use a heat riser, which heats the air/fuel mixture after it has left the carburetor; this is a low temperature fuel economy and driveability feature with most benefits seen at low rpms.
Carburetor
A carburetor , carburettor, or carburetter is a device that blends air and fuel for an internal combustion engine. It is sometimes shortened to carb in North America and the United Kingdom....
, carburettor, carburator, carburettet heat (usually abbreviated to 'carb heat') is a system used in automobile
Automobile
An automobile, autocar, motor car or car is a wheeled motor vehicle used for transporting passengers, which also carries its own engine or motor...
and piston-powered light aircraft
Light aircraft
A light aircraft is an aircraft that has a maximum gross take-off weight of or less.Many aircraft used commercially for freight, sightseeing, photography and scheduled flights are light aircraft.Examples of light aircraft include:...
engines to prevent or clear carburetor icing
Carburetor icing
Carburetor, or carb icing, is an icing condition which can affect any carburetor under certain atmospheric conditions. Carburetor icing occurs when there is humid air, and the temperature drop in the venturi causes the water vapor to freeze. The ice will form on the surfaces of the carburetor...
. It consists of a moveable flap which draws hot air into the engine intake. The air is drawn from the heat stove
Heat exchanger
A heat exchanger is a piece of equipment built for efficient heat transfer from one medium to another. The media may be separated by a solid wall, so that they never mix, or they may be in direct contact...
, a metal plate around the (very hot) exhaust manifold
Exhaust manifold
In automotive engineering, an exhaust manifold collects the exhaust gases from multiple cylinders into one pipe. The word manifold comes from the Old English word manigfeald and refers to the folding together of multiple inputs and outputs.In contrast, an inlet manifold is the part of an engine...
.
Operation
Icing occurs when the atmospheric airEarth's atmosphere
The atmosphere of Earth is a layer of gases surrounding the planet Earth that is retained by Earth's gravity. The atmosphere protects life on Earth by absorbing ultraviolet solar radiation, warming the surface through heat retention , and reducing temperature extremes between day and night...
contains significant levels of water vapor
Water vapor
Water vapor or water vapour , also aqueous vapor, is the gas phase of water. It is one state of water within the hydrosphere. Water vapor can be produced from the evaporation or boiling of liquid water or from the sublimation of ice. Under typical atmospheric conditions, water vapor is continuously...
; as the air is drawn into the engine through a carburetor it is cooled sufficiently to cause that vapor to condense out and freeze on the butterfly valve
Butterfly valve
A butterfly valve is a valve which can be used for isolating or regulating flow. The closing mechanism takes the form of a disk. Operation is similar to that of a ball valve, which allows for quick shut off. Butterfly valves are generally favored because they are lower in cost to other valve...
. This cooling has two causes: the air pressure drops due to the expansion caused by engine suction and the increase in speed through the butterfly valve (in part-throttle operation) which drops the air temperature; and the liquid gasoline being introduced into the airstream must evaporate
Evaporation
Evaporation is a type of vaporization of a liquid that occurs only on the surface of a liquid. The other type of vaporization is boiling, which, instead, occurs on the entire mass of the liquid....
, and the heat of evaporation is extracted from the airstream, cooling it. The temperature in the carburetor can drop below the freezing point
Freezing Point
Freezing Point is a news journal in the People's Republic of China which has been the subject of controversy over its criticism of Communist Party officials and the sympathetic ear it lent to a Chinese historian who had criticized official history textbooks...
of water, and if the air is humid
Humidity
Humidity is a term for the amount of water vapor in the air, and can refer to any one of several measurements of humidity. Formally, humid air is not "moist air" but a mixture of water vapor and other constituents of air, and humidity is defined in terms of the water content of this mixture,...
, ice
Ice
Ice is water frozen into the solid state. Usually ice is the phase known as ice Ih, which is the most abundant of the varying solid phases on the Earth's surface. It can appear transparent or opaque bluish-white color, depending on the presence of impurities or air inclusions...
can form inside the carburetor. As the ice builds up, less air can pass through the carburetor, causing a drop in power and, in severe cases, the engine will eventually stall.
Temperature drops of 20°C (36°F) or more are often encountered within the carburetor, so ice can build up even when the outside air temperature is well above freezing. Perhaps paradoxically, cold winter weather is less prone to icing, since cold air contains much less moisture. A warm day with high humidity is considered the most likely scenario for carburetor icing. Since there is less air acceleration through the carburetor at full-throttle
Throttle
A throttle is the mechanism by which the flow of a fluid is managed by constriction or obstruction. An engine's power can be increased or decreased by the restriction of inlet gases , but usually decreased. The term throttle has come to refer, informally and incorrectly, to any mechanism by which...
operation, icing is usually not a problem then.
Carburetor heat uses hot air drawn from the heat exchanger or heat stove (a metal plate around the exhaust manifold
Exhaust manifold
In automotive engineering, an exhaust manifold collects the exhaust gases from multiple cylinders into one pipe. The word manifold comes from the Old English word manigfeald and refers to the folding together of multiple inputs and outputs.In contrast, an inlet manifold is the part of an engine...
) to raise the temperature in the venturi section high enough to prevent or remove any ice buildup. Because hot air is less dense than cold air, engine power will drop when carburetor heat is used.
Engines equipped with fuel injection
Fuel injection
Fuel injection is a system for admitting fuel into an internal combustion engine. It has become the primary fuel delivery system used in automotive petrol engines, having almost completely replaced carburetors in the late 1980s....
do not require carb heat as they are not as prone to icing - the gasoline is injected as a steady stream just upstream of the intake valve, so evaporation occurs as the fuel/air mixture is being drawn into the cylinder, where metal temperatures are higher. The exception is monopoint or TBI injection systems which spray fuel onto the throttle plate.
Some multipoint injection engines route engine coolant through the throttle body to prevent ice buildup during prolonged idling. This prevents ice from forming around the throttle plate but does not draw large amounts of hot air into the engine as carburetor heat does.
In aircraft
In light aircraft, the carburetor heat is usually manually controlled by the pilotAviator
An aviator is a person who flies an aircraft. The first recorded use of the term was in 1887, as a variation of 'aviation', from the Latin avis , coined in 1863 by G. de la Landelle in Aviation Ou Navigation Aérienne...
. The diversion of warm air into the intake reduces the available power from the engine for three reasons: thermodynamic efficiency is slightly reduced, since it is a function of the difference in temperature between the incoming and exhaust gases; the quantity of air available for combustion inside the cylinders is reduced due to the lower density of the warm air; and the previously-correct ratio of fuel to air is upset by the lower-density air, so some of the fuel will not be burned and will exit as unburned hydrocarbon
Unburned hydrocarbon
Unburned hydrocarbons are the hydrocarbons emitted after petroleum is burned in an engine.Any fuel entering a flame will be reacted. Thus, when unburned fuel is emitted from a combustor, the emission is caused by fuel "avoiding" the flame zones...
s. Thus the application of carb heat is manifested as a reduction in engine power. If ice has built up, there will then be a gradual increase in power as the air passage is freed up by the melting ice. The amount of power regained is an indication of the severity of ice buildup.
It must be kept in mind that the ingestion of small amounts of water into the engine following melting in the carburetor may cause an initial period of rough running for as much as one or two minutes before the power increase is noted. Again, the pilot will note this as evidence that icing conditions are present. However, more than one pilot, when confronted with a rough running engine has mistakenly turned the carburetor heat back off, thereby exacerbating the situation.
Applying carb heat as a matter of routine is built into numerous in-flight and pre-landing checks (e.g. see BUMPH and GUMPS
GUMPS
GUMPS is an acronym widely used by fixed wing aircraft pilots as a mental checklist to ensure nothing critical has been forgotten before landing...
). In long descents, carb heat may be used continuously to prevent icing buildup; with the throttle closed there is a large pressure (and therefore temperature) drop in the carburetor which can cause rapid ice buildup that could go unnoticed because engine power is not used. In addition, the exhaust manifold cools considerably when power is removed, so if carb icing occurs there may not be heat sufficient to remove it. Thus most operational checklist
Checklist
A checklist is a type of informational job aid used to reduce failure by compensating for potential limits of human memory and attention. It helps to ensure consistency and completeness in carrying out a task...
s call for the routine application of carb heat whenever the throttle is closed in flight.
Usually, the air filter
Air filter
A particulate air filter is a device composed of fibrous materials which removes solid particulates such as dust, pollen, mold, and bacteria from the air. A chemical air filter consists of an absorbent or catalyst for the removal of airborne molecular contaminants such as volatile organic compounds...
is bypassed when carb heat is used. If the air filter becomes clogged (with snow or ice), using carb heat allows the engine to keep running. Because using unfiltered air can cause engine wear, carb heat usage on the ground (where dusty air is most probable) is kept to a minimum.
Altitude has an indirect effect on carburetor ice because there are usually significant temperature differences with altitude. Cloud
Cloud
A cloud is a visible mass of liquid droplets or frozen crystals made of water and/or various chemicals suspended in the atmosphere above the surface of a planetary body. They are also known as aerosols. Clouds in Earth's atmosphere are studied in the cloud physics branch of meteorology...
s contain moisture, and therefore flying through clouds may necessitate more frequent use of carb heat.
The intake air of an aircraft engine equipped with a supercharger is heated through compression, so the air entering the throttle body is already warmed enough that carb heat is unnecessary.
In automobiles
In cars, carburetor heat may be controlled automatically (by a wax-pellet driven flap in the air intake) or manually (often by rotating the air cleaner cover between 'summer' and 'winter' settings). The air filter bypass found on aircraft engines is not used, because the air filter on automobiles is not exposed to the elements, and because the carburetor is usually mounted closer to the cylinder block, so that enough engine heat is transmitted through the structure to keep the venturi walls warm. However, this is not always sufficient, and some automobiles have a history of temporary engine failure (power output drops to zero until the engine sits long enough for the engine heat to melt the accumulated ice) during rain or snow conditions.Automobile engines may also use a heat riser, which heats the air/fuel mixture after it has left the carburetor; this is a low temperature fuel economy and driveability feature with most benefits seen at low rpms.
See also
- FogFogFog is a collection of water droplets or ice crystals suspended in the air at or near the Earth's surface. While fog is a type of stratus cloud, the term "fog" is typically distinguished from the more generic term "cloud" in that fog is low-lying, and the moisture in the fog is often generated...
- Dew pointDew pointThe dew point is the temperature to which a given parcel of humid air must be cooled, at constant barometric pressure, for water vapor to condense into liquid water. The condensed water is called dew when it forms on a solid surface. The dew point is a saturation temperature.The dew point is...
- CarburetorCarburetorA carburetor , carburettor, or carburetter is a device that blends air and fuel for an internal combustion engine. It is sometimes shortened to carb in North America and the United Kingdom....
- Carburetor icingCarburetor icingCarburetor, or carb icing, is an icing condition which can affect any carburetor under certain atmospheric conditions. Carburetor icing occurs when there is humid air, and the temperature drop in the venturi causes the water vapor to freeze. The ice will form on the surfaces of the carburetor...
- Heated air inlet
- Fuel injectionFuel injectionFuel injection is a system for admitting fuel into an internal combustion engine. It has become the primary fuel delivery system used in automotive petrol engines, having almost completely replaced carburetors in the late 1980s....
External links
- Picture of automobile engine exhaust manifold heat stove. http://www.widnerindustries.com/product4.htm