Carb Heater - Automotive Application
Whereas I'm not a "real world" pilot
(like Tango) I have been an auto mechanic & instructor for the past 35 years. This has an automotive application on older engines equipped with carburetors.
It's termed the "venturi effect". As the air moves through the carb venturi;
(a narrow section) it is compressed. As it leaves this area
(to a wider section) the air expands which causes a pressure decrease.
The pressure decrease is what causes the fuel to be drawn through the discharge nozzle(s) and then enter the airstream below the venturi. This also causes a temperature drop; which even in moderately cool conditions can cause ice to form in the barrel(s) of the carb resulting in loss of power or even complete stalling of the engine caused by the lack of the necessary air-fuel mixture ratio due to the restriction or blockage of the "low speed" fuel discharge port(s).
If you look at a carb equipped car;
(generally built after 1967 thru to the early 90's) you'll see a 1"-2" tube running from the exhaust manifold up to the snout
(intake) of air cleaner assembly. Hot air is collected from a tin shield around the exhaust manifold.
(this occurs quickly once the engine is started)
Where the tube joins the air cleaner there is a small vacuum operated & temperature controlled valve
(flapper) which allows the hot air to be drawn into the carb
(while preventing the introduction of unheated outside air) depending upon the intake air temperature that is sensed by a temp. switch located
(generally) on the underside of the air cleaner assembly.
This is the function of all those vacuum hoses on the air cleaner assy.
This was often the cause of stalling & performance complaints when the system did not function properly. The vacuum controlled servo usually developed a split diaphragm; the tube was missing or the tin shield had rusted away & fallen off. This did not allow the hot air into the air cleaner - causing carb icing.
If the valve got stuck in the applied position this can result
(particularly in hot weather) in "pre-ignition" commonly called "pinging".
(this is only one of the many causes of pinging)
In this instance; the incoming air is too hot which causes the temperature in the combustion chambers to exceed the design parameters, resulting in the "pre-ignition" of the air-fuel mixture before the spark plug ignites it.
I was unaware until today of the aircraft applications. I can now fully understand why a heater is used on both takeoff & landing. Two instances where loss of power could really cause you to have a very bad day.
I thought I would add the auto application here as we are heading into the winter months
(in the northern hemisphere) & many people still drive carb equipped vehicles which are prone to this problem.
Personal experience has shown that many so-called "good mechanics" completely miss this problem & only discover it after performing other costly and possibly unneeded repairs.
"Backyard" mechanics are generally completely unaware of this problem.
Many Thanks to Tango for his aircraft oriented explanation & to DD for originally posting this thread.
EXCEPTIONAL THREAD DD!!! 
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