Presumably, at some point, all this will have been researched quite thoroughly so if nobody ‘knows’ the ‘answer’ it must be locked away in some corporate filing cabinet, or we’re not looking in the right place. Or perhaps it was all sorted pre-war and we’ve carried on using designs that simply work, without worrying too much about the minutiae.
That’s sure interesting about the mogas… What’s ‘needed’ is a complete coupled thermal/CFD (computational fluid dynamic or color for dollars) analysis, performed over a range of conditions. Yours for $500K or so, I know just the guys to do it. 
I believe there’s a fair amount of water in the air in northern Scotland 
With a Continental O-200 in a Jodel DR1050, using mogas, in north Scotland, I have carb ice very often. Frequently it will stop the engine immediately after start. (Checked by engineer many years ago.) With a wet runway, I have carb heat on when I open the throttle for take-off, closing it as I gain speed. A Canadian CAA document says carb icing can occur at much higher temperatures on mogas. I’ve encountered it at temperatures which seem unlikely from the diagram, on a Konsin treated runway. Carb icing is just something I have to live with, and when a student, a carb heat check every 10 minutes was hammered in.
My carb never shows icing, not even in IMC.
I’ve been flying around in light aircraft since the 70s, and have experienced carb icing twice, once in about 1979 flying with my dad, and once a year or so ago taxiing out to the runway. I have also been a pilot in IMC exactly once… and appreciate the attractive simplicity of the fixed jet, one moving part carburetor. Obviously it’s a good idea to choose the right tool for the job, and the simple aircraft is a good tool for my job.
It’d be interesting to look at the problem of induction air cooling ‘properly’ and figure it out.
I don’t understand why the pressure drop after the throttle plate should be such a low factor, compared to evaporation and pressure drop in the venturi.
At 50% power/50% ambient pressure, dry air would cool by around 45 degrees, and saturated air by around 20-25, i calculate that just from applying the adiabatic laps rates – 50% pressure is approximately equivalent to going up to FL 180.
In a venturi, the gas speeds up, so its temperature drops (straight thermodynamics). That is why carbs suffer icing. To a first order approximation, the icing isn’t anything to do with the presence of fuel. Without fuel you would still get ice buildup. The fuel evaporation just makes it happen a bit sooner.
You are right about venturi, but wrong about carb icing. The reason is the actual values you get. The pressure drop is way too small to have a large effect. The cooling due to fuel evaporation is an order of magnitude larger.
Throttle Body Injectors (TBI)s are much less susceptible to icing. The reason is supposedly that they have no butterfly valve for the ice to form.
did not have the ability to feed back fuel into the tanks (a requirement for fuel injection)
It isn’t. I don’t have any return to tanks.
All you need to avoid a tank return is a constant pressure fuel pump. It is systems that use a fuel pump without a pressure relief that need to return fuel.
I would never buy a plane with a carburettor. It is completely pointless and just a big extra risk. Loads and loads of people have been killed by carb icing, and the evidence is gone by the time they get to you.
It highly depends on the installation. My carb never shows icing, not even in IMC. Other installations ice up on a CAVOK day during taxi. With the “I would never buy an airplane that has…” you end up building your own. Life is a series of compromises In my case, the only disadvantage I can see is not being able run LOP (only a bit) because of inaccurate fuel distribution. The 182 airframe did not have the ability to feed back fuel into the tanks (a requirement for fuel injection) and Cessna was in a big hurry when they brought the 182 RG to market. The restart 182 have that capability and come with injection engines but no more retractable gear and bad useful load.
In a venturi, the gas speeds up, so its temperature drops (straight thermodynamics). That is why carbs suffer icing. To a first order approximation, the icing isn’t anything to do with the presence of fuel. Without fuel you would still get ice buildup. The fuel evaporation just makes it happen a bit sooner.
The reason you get less carb icing at larger throttle openings is because the velocity increase is smaller so the temperature drop is smaller.
The reason why fuel servos (fuel injection) don’t normally suffer icing is because there is no venturi so the gas (the air) doesn’t cool down on the way through. You can still get icing on the fuel injection nozzles; right on their ends, but this is rare and is too small to cause a blockage of the entire port.
A fuel servo doesn’t use vacuum to meter the fuel/air ratio. A carb does that by sucking the fuel out of the chamber underneath, using the vacuum created by the venturi. A fuel servo does it by metering the air velocity and opening a valve for the fuel; this needs the fuel to be available under pressure otherwise it won’t come out.
At least that’s how I understand it…
I would never buy a plane with a carburettor. It is completely pointless and just a big extra risk. Loads and loads of people have been killed by carb icing, and the evidence is gone by the time they get to you. Even if you come out of it OK, you will never really know if the engine stoppage was due to carb icing, or due to some mysterious fuel system blockage which cleared itself, perhaps by the obstruction having moved elsewhere where it will block the flow another time.
Incidentally there is a famous carb icing diagram, all over the internet, which has the incorrect axes on it, but for years or decades nobody realised it and just kept reproducing it. It even featured in flying theory textbooks
Just read CAA safety sense 14, which claims that icing is less common at high power settings primarily because the carburettor is likely to be hotter
b) Engines at reduced power
settings are more prone to icing
because engine induction
temperatures are lower. Also, the
partially closed butterfly can more
easily be restricted by the ice
build-up. This is a particular problem
if the engine is de-rated as in many
piston-engined helicopters and some
aeroplanes.
It also mentions carb surface roughness as a factor in icing.
