Why is carburetor heat unnecessary at full throttle?

Question

Questions like How does aircraft design affect carburetor ice? show how carburetor mount point affects the relative passive heating of the carburetor by the engine, but I've never learned why full power eliminates the need for carb heat.

Looking at how Wikipedia depicts carb ice:

Regardless of the carb's mount point on the engine, high throttle wouldn't get rid of the ice on the throttle valve, heat generated by high throttle doesn't affect the temperature of the air flowing in.

Further, when the throttle is full open, there is a greater airflow speed across the neck of the carb and hence a greater temperature drop.

So, what explains why high throttle eliminates the need for carb heat in Lycoming/Continental engines?

Thanks!

Answer 1

The icing you are talking about is called "throttle ice" and actually just forms around the edges and back side of the throttle butterfly. It's not really related to the venturi that is upstream; it's from the pressure drop caused by the restriction of the throttle plate (the high vacuum on the back side you might say) along with the presence of vaporizing fuel, which massively boosts the temperature reduction caused by the pressure drop because the evaporating fuel is absorbing a lot of heat from the air stream.

It's the temperature drop from this pressure drop at the butterfly, plus the effect of evaporating fuel, that turns ambient humidity into carb ice. On a fuel injected airplane you only have the butterfly valve without fuel present, and the temperature drop from that alone isn't sufficient, and therefore you don't need carb heat on injected engines (just an alternate air source in case of impact ice on the air intake).

Anyway, when the throttle is wide open, the pressure drop downstream of the butterfly caused by the throttle restriction is minimal. As a result, the chilling effect of the pressure drop and of evaporating fuel is also minimal. Because of this you won't get ice formation as readily (not impossible, just relatively unlikely) when the passage has say only a 5 or 10% restriction at WOT, vs say, an 80% restriction at a low power setting. For a given ambient temperature/humidity, carb ice always forms more easily the more the throttle is closed.

One of the worst airplanes for carb ice was the 65 hp J-3 cubs where the carb receives very little conductive or radiant heat from the engine due to the carb mounting and the cowl with the exposed cylinders. Descending with the engine at or near idle on a 65 deg F high humidity evening was begging for a carb ice related stoppage, and liberal use of carb heat was advised even if there were no signs of icing. Carb ice at WOT wasn't too much of a problem, but you can never be sure. Always err a little on the overuse side with carb heat, especially on Continental powered airplanes which are a bit more sensitive to all this due to the way the carb is insulated from the crankcase, at least until you get to know the engine's quirks.

Answer 2

Having the throttle at full doesn't eliminate the need for carb heat. It's actually the opposite: with the throttle at full, the drop in air temperature through the Venturi is greatest, so you're (slightly) more likely to develop ice, all else being equal.

In general, he reason you're not supposed to use carb heat at full throttle is because it reduces the density of air going into the engine, so it can't produce quite as much power. Presumably you pushed the throttle forward for a reason, so reducing the amount of power right when you need it doesn't make much sense.

If you're in a situation where carb ice is a factor, then use the carb heat before you start your climb, so that you have full power available to you during the climb itself. But if you notice that your engine power has started to drop, by all means, go ahead and pull the knob, even if you're in a full-power climb.

Answer 3

There is a small risk of carb icing at full throttle but it is a much narrower range of conditions than at partial throttle. Pressure drop causes temperature drop. The purpose of the throttle is to control manifold pressure, a more open throttle creates less pressure drop from ambient outside to inside the manifold so less temperature decrease. The temperature drop must also be enough to condense moisture from the air, if the dew point is much below freezing then there is no point where liquid water forms. Direct formation of ice is like snow it just gets sucked into the hot cylinders, you need a liquid droplet phase for the ice to adhere.

So with high dew point and a large pressure drop you get super cooled liquid water droplets that freeze on impact. With low humidity the range for liquid condensation is reduced and with low pressure drop the temperature doesn't drop below freezing.

Injected engines actually can develop intake ice however they do not have a venturi and without the additional pressure drop in the venturi the total temperature drop is not as substantial or it occurs in a less critical section so problems caused by icing are not as common. Venturis are more sensitive to ice as they are part of the calibrated fuel metering and mixing system. Fuel evaporation is a contributing factor to the difference in temperature between carbureted and injected systems but it is not a major factor when comparing part throttle to open throttle within a system as the mass of fuel is directly proportional to the mass of air and less fuel evaporates at low temperatures.

Also high power settings create more cylinder heat, this heat increases engine temperature and the increased temperature increases the heat transmitted to the intake components both by increased conduction through the manifold and increased infrared radiation.(proportional to the fourth power of absolute temperature[kelvin^4])

Answer 4

One of the reasons for not using carb heat at full throttle is due to the increased risk for detonation due to the temperature increase and reduction in density of the air charge.