Editing Carburetor (section)

== Icing in aircraft engine carburetors ==

[[file:Carburetor Icing diagram.png|thumb|right|upright|The formation of carburetor ice may reduce or block fuel-air flow to the engine.]]

A significant concern for aircraft engines is the formation of ice inside the carburetor. The temperature of air within the carburetor can be reduced by up to 40&nbsp;°C (72&nbsp;°F),<ref name="PHAK">{{cite book |title=Pilots' Handbook of Aeronautical Knowledge |publisher=US Federal Aviation Administration |pages=7–9–7–10 |url=https://www.faa.gov/sites/faa.gov/files/regulations_policies/handbooks_manuals/aviation/phak/09_phak_ch7.pdf |access-date=11 April 2022}}</ref> due to a combination of the reduced air pressure in the venturi and the [[latent heat]] of the evaporating fuel. The conditions during the descent to landing are particularly conducive to icing, since the engine is run at idle for a prolonged period with the throttle closed. Icing can also occur in cruise conditions at altitude.

A carburetor heat system is often used to prevent icing.<ref name="PHAK"/> This system consists of a secondary air intake which passes around the exhaust, in order to heat the air before it enters the carburetor. Typically, the system is operated by the pilot manually switching the intake air to travel via the heated intake path as required. The carburetor heat system reduces the power output (due to the lower density of heated air) and causes the intake air filter to be bypassed, therefore the system is only used when there is a risk of icing.<ref name="PHAK"/>

If the engine is operating at idle RPM, another method to prevent icing is to periodically open the throttle, which increases the air temperature within the carburetor.<ref name="PHAK"/>

Carburetor icing also occurs on other applications and various methods have been employed to solve this problem.  On inline engines the intake and exhaust manifolds are on the same side of the head. Heat from the exhaust is used to warm the intake manifold and in turn the carburetor.  On V configurations, exhaust gases were directed from one head through the intake cross over to the other head. One method for regulating the exhaust flow on the cross over for intake warming was a weighted eccentric butterfly valve called a heat riser that remained closed at idle and opened at higher exhaust flow.  Some vehicles used a heat stove around the exhaust manifold.  It was connected to the air filter intake via tubing and supplied warmed air to the air filter.  A vacuum controlled butterfly valve pre heat tube on the intake horn of the air cleaner would open allowing cooler air when engine load increased.