Carb ice

I have a 914 for a while now and I did some technical analysys for the installation.

First, the temperature rise in the manifold is driven by three things:
-temperature of the air in to the loader of course.
-the pressure rise by the turbocharger
-the warm turbocharger itself.

Primary plan is to have cold, outside air into the charger to have maximum power. Second is, that the hot air unter the cowling leads to inacceptable hot manifold temperatures at high power settings, so let's assume inlet temperature is outside temperature.

During idle speeds there is not pressure rise, so the temperature increase is only a few degress. I see about 5°C which seems to be enought to prevent carb icing.

At higher RPM the charger increases temperature and this is about 15°C by pressure, rest by the warm charger. I see 20-30°C temperature rise in normal operation. Hottest airbox temp in summer was 45°C

In summary: a fairly good installation with breathing fresh air does not need an intercooler and without intercooler you do not need to worry about carb ice.

Carburettor icing, is usually a feature of low powered engine operation eg descent power. It can occur during taxying (Lycoming, personal experience).

I am have no experience with Rotax 914's but have some understanding of turbo charging.

Carburettor icing should be almost impossible in a turbo charged system. The addition of an intercooler, is highly unlikely to alter my speculation on this.

The turbo is designed to raise air pressure to improve volumetric efficiency (burn more fuel for a given combustion chamber volume). Some of the efficiencies derived from higher air pressure is lost due to air compression and heat transfer, principally,from the turbos exhaust side.

The intercooler (a heat exchanger) is designed to  reduce some of the heat gained by turbo charging. By doing so it restores some of the lost potential, achieved by pressurising, for higher volumetric efficiency. The air entering the carburettors will still be a lot hotter than ambient.

The temperature of the incoming (to turbo) air will influence the resultant temperature rise in the pressurised air but unless you are flying in extraordinarily cold climate (say -45C) the air entering the carb's is still going to be well above ambient. In my limited experience, very cold weather has very dry (low moisture) air further reducing the chance of icing. 

I can absolutely confirm Sean‘s findings. In flight temp underneath the cowling is by far not as much higher to OAT as one may guess. Consequently the effect on carb ice is limited.
Humidity is a strong factor, too. That’s why  carb ice may occur at OAT of more than 10 and even 20 degrees Celsius - it‘s not necessarily a cold days issue.

Sorry, but the temperature under the cowling is really important.

With air inlet under the cowling I had around 90C airbox temperature.

After changing to a NACA scoop with plenum to the turbo inlet, it was 45C.

at idle I see airbox a few degrees over outside temperature, I see the risk of being cold enough for ice, when an intercooler is mounted, because airbox temperature will be close to the outside then.

Hi Steffen,

you sure have a point there. What I meant was that there may be a tendency to guess a higher temp underneath the cowling than it actually is (under normal circumstances). The difference to OAT rather is in the range of 8 to 12C than 20 to 25C when there is adequate airflow e.g. when cruising or climbing (and I admit I never measured temps on a turbo).

Of course once one stops a plane after landing the temps underneath the cowling soar and the OAT-diff increases a lot but not in flight.

Google „Carburettor icing-probability chart“ and check out the pics you find. They show the story (in my opinion).

Carb icing is not necessarily a cold days issue.