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As far as I know, the mechanics and physics of an aircraft carburetor are the same as those of an automobile carb. (If I'm wrong, there is no basis for this question.) Aircraft carbs are provided with a carb heat option, while no auto carb that I'm aware of ever came with one.

If carb icing only occurred at the significantly colder temperatures experienced at altitude, I'd understand that this is an aircraft-only issue (or, perhaps, one that might effect only land vehicles at very high latitudes or during extreme cold snaps). However, it is a well established fact that carb icing can occur in temps as high as 20-25°C (68-77°F) even in somewhat low relative humidity (as noted in this answer) - conditions in which automobiles are frequently (commonly, even) operated.

While I certainly appreciate that an engine out event is a significantly more critical issue in an aircraft (quick, where the heck do I land before I crash and die) than it is in a Earth-bound vehicle (signal, check mirrors, coast to a safe stop at the side of the road), why is carb icing even an issue in aircraft when it doesn't seem to be an issue at all in autos?

Note: I realize that few to no automobile engines are carbureted anymore, but that was the norm until nearly the turn of the 21st century.

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    $\begingroup$ Wikipedia claims that carb heat was also used on cars and motorcycles, but they don't provide any sources. It might be worth to ask about carb heat on MotorVehicle.SE. $\endgroup$
    – Bianfable
    Commented Jan 23, 2023 at 16:56
  • $\begingroup$ Sounds like this question was asked based on a flawed premise. I should probably close it. $\endgroup$
    – FreeMan
    Commented Jan 23, 2023 at 17:05
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    $\begingroup$ It's a reasonable question @FreeMan, and as I have answered it I have a vested interest in keeping it open ;) $\endgroup$
    – GdD
    Commented Jan 23, 2023 at 17:10
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    $\begingroup$ @Michael see the question/answer I linked for more info on carb icing. Also, there are probably a dozen or more other questions here about it. $\endgroup$
    – FreeMan
    Commented Jan 24, 2023 at 12:25
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    $\begingroup$ I don't know in the US but in germany already in the 90s most new cars had one or another type of injection, mainly due to emissions requirements. $\endgroup$
    – PlasmaHH
    Commented Jan 25, 2023 at 20:34

7 Answers 7

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Carb icing can occur in car engines, I know of at least two cases where it caused rough running, and that's just my own personal experience. If I remember right VW Beetles are susceptible to this, this is because they are air-cooled, liquid cooled engines on cars typically had a hot water jacket or a hot exhaust element routed nearby to keep the carb warm enough to prevent ice buildup.

The reasons you don't hear about it happening are that 1) liquid cooled engines are less prone, so happens infrequently and 2) with a car you pull over to the side of the road, and in an airplane you make an emergency landing. When you think about the myriad of reliability issues you could have with older car engines you realize that we were simply willing to live with them as they weren't generally life-threatening.

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    $\begingroup$ This answer has it, as many airplane engines are air-cooled and many car engines are liquid cooled. Moreover, cars do not descend at reduced power or glide for extended periods of time, which creates a lot of cooling airflow without adequate engine heat to compensate. One can only wonder what all that expansion and contraction does to the life of the engine. Surrounding the carb with warm fluid (oil/antifreeze) and/or burying it in the engine makes a lot of sense. Turbocharging (which compresses and heats) incoming air is even more appealing, and would do wonders for horsepower. $\endgroup$ Commented Jan 23, 2023 at 20:05
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    $\begingroup$ I wonder, @RobertDiGiovanni, whether if you were in a car at the top of a mountain and basically riding the brake for the majority of the downhill, whether you’d end up icing up a car engine. That situation seems roughly similar to a reduced power/glide situation, or as similar as I can imagine getting in a car without a wholly manufactured scenario $\endgroup$ Commented Jan 24, 2023 at 2:29
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    $\begingroup$ @fyrepenguin try I-40 Flagstaff eastbound. It's downhill all the way to Texas. $\endgroup$ Commented Jan 24, 2023 at 2:42
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    $\begingroup$ @fyrepenguin Cars built today have coolant lines through the throttle body for heat. Another key difference is that with modern fuel injection, gasoline is vaporized at the valves or directly in the cylinder, where it is much too hot for ice to form. Icing does occur at air filters and charge air coolers (turbo intercoolers), particularly for diesels which use a large amount of air, but it requires much colder ambient temperatures because you don't have the cooling effect of fuel vaporization. $\endgroup$
    – user71659
    Commented Jan 24, 2023 at 6:20
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    $\begingroup$ @fyrepenguin or, hopefully, engine braking, using the brakes only to slow for turns. Modern brakes and fluids are better for sustained descents, seemingly by more than the amount that modern cars are heavier, but there are still distinct advantages to not riding the brakes. Cars with old air-cooled engines that were apparently more prone to carb icing would have needed care not to overuse the brakes $\endgroup$
    – Chris H
    Commented Jan 24, 2023 at 10:25
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GdD is right. In the dying days of carbureted cars, the engine designers came up with a variety of ways to prevent carb icing. One popular method was to use an "oven" (a sheet metal box enclosing a portion of the exhaust manifold through which outside air would be drawn when the engine was cold) that pre-heated the air entering the engine.

My 1973 Subaru had a thermostatically-controlled preheat oven which was broken, but since the car was from California it didn't matter. When I bought it and moved to western Oregon, the first time I drove it through freezing fog it promptly iced up.

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The main reason is the same reason carb icing is much less common on Lycoming engines than on Continentals. The carb in Lycomings is bolted directly to the bottom of the oil pan and so the carb body is relatively warm from heat conduction (I've never experienced carb ice in my plane's Lycoming O-290, although I use carb heat religiously anyway, because the potential is always there)

Continental engines have the carb on a spider assembly in front of the oil tank and so it runs colder and has less margin before icing conditions in the throat can be achieved. Piper Cubs with the open cylinders are the worst for this, because there is no cylinder heating of the airflow in the lower cowl to help warm the carb body. They will develop carb ice if you look at them with a mean face on some days.

With cars, the carb is on top, usually bolted right to the engine, so the sweet spot conditions for icing to start are very small, although it can still happen, depending on the carb mounting and the overall amount of heat under the hood. That occasional time when the engine would die mysteriously for no reason, especially after sitting idling for a while, where it starts normally after a few minutes, was probably carb ice, but it would almost certainly take just the right conditions of moderate temperature (60-70F) and very high humidity to overcome all the heat the carb is being bathed in.

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Modern cars don't have carburetors. With older cars, it is a problem. A W124 Mercedes of ours inexplicably went out in medium wet/cold weather (typically 5°C to 10°C) at the first stoplight and refused starting up again. By the time mechanics arrived, it was fine to go again. Symptom was not reproducible in car hall conditions.

Took an old mechanic hand to figure out that the "pointless" torn off thin-walled thick hose inside of the motor was for prewarming the carburetor in colder conditions with exhaust heat. Replacing that hose gave the car a lease back on life.

So from my own experience, carburetor icing definitely is a thing with cars.

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Since the answers so far address cars, I'll address motorcycles. A few of the higher end motorbikes of the 1980s-1990s have electrically heated carburetors. For example, the Yamaha VMax has 4 Mikuni BVS34 carburetors, each one having a small threaded cartridge heater powered by 12v. I couldn't find any information on exactly how much power they consume, but they generally thread into the body of the carburetor

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    $\begingroup$ and why aren't these electrically heated carburetors present or enough for aviation? $\endgroup$
    – Federico
    Commented Jan 25, 2023 at 16:22
  • $\begingroup$ @Federico A possible reason is that aircraft in the size range of likely interest replace a dumb "good enough" solution with a human-brain driven system which can be optimised for best results. Considering that multiple factors may be life-impacting, a human brain adds both better potential capability and also a degree of danger. $\endgroup$ Commented Feb 17, 2023 at 11:05
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Aircraft engines are all about reliability, which is often best achieved through simplicity. There is also the matter of approvals for anything fitted to an aircraft. As a result, many technologies common on cars are slow to arrive on aircraft, if ever. Both car and airplane engines solve the carb icing problem in the same way - preheat the air. The difference is that car engines typically have a thermostatic control to automatically regulate the amount of heating, where an airplane engine leaves that task to the pilot. Hence as a pilot you have to be aware of the issue and proper use of the control that mitigates it, but as a motorist you don't even need to know the problem exists.

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  • $\begingroup$ Curious as to why designers didn't have an electronic trace heating that keeps the carb at a set temperature? $\endgroup$
    – user13555
    Commented Jan 25, 2023 at 11:53
  • $\begingroup$ @user13555 Warm air is less dense & so has less oxygen. It makes combustion less efficient as can be seen by the revs drop when you apply carb heat. Related:mechanics.stackexchange.com/questions/11097/… $\endgroup$ Commented Jan 26, 2023 at 11:37
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Cars did have carb heat. It was just automatic.

Inside the air cleaner assembly, there was a flapper valve that admitted air either from ram air, or from a heat stove on the exhaust manifold. There was a bi-metal thermostatic strip that either actuated the flapper door directly, or acted as a pilot valve for a vacuum motor. It would operate the flapper in an analog manner assuring a minimum temperature for intake air.

GM called this the Thermostatic Air Cleaner system or ThermAC. I have fixed several foul-running cars where the riser pipe or heat stove baffle itself had rotted out.

Obviously this was phased out on newer cars like the Chevy Bolt - seriously though, in the 1980s decade, cars went from 98% carbureted, to 98% fuel injection under computer control (no mechanical backup). Achieving smog standards requires extremely precise fuel control, and nothing less than digital control will do. That eliminated fine passages prone to icing. There's nothing delicate in the injector assembly, it blasts fuel at about 2-4 atmospheres of pressure, and it sits very close to the cylinder head.

There is one small passage to a mini-throttle controlled by the computer, called the Idle Air Control valve. That is typically heated by a coolant loop.

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