Awareness of icing in the carburettor and intake is very important for safe operation and knowledge of the current Aeronautical Information Circular on induction system icing is of importance.
When liquid evaporates it absorbs heat from its surroundings. Therefore, when fuel is sprayed into the induction system, it extracts heat from its surroundings. This causes the water vapour to condense and it will freeze if the temperature is low enough. Six evaporation factors have an influence on the rate of formation of ice in carburettors, and these are:
- Temperature increases.
- Surface area of the liquid increases.
- Atmospheric pressure decreases.
- Humidity of the air decreases.
- A direct function of the volatility of the exposed liquid.
- Air flow across the surface of the liquid increases.
Carburettor Ice Formation
As fuel is sprayed into the low pressure area of the carburettor venturi it rapidly evaporates cooling the air, the wall of the induction system, and the water vapour. If the air humidity is high and the metal of the carburettor is cooled to below 0°C, ice will form and operation of the engine will be affected this is due to the size and shape of passages being changed by a coating of ice.
Carburettor ice formation may occur by any one of three processes:
- The freezing of the condensed water vapour of the air at or near the throttle forms ice known as throttle ice or expansion ice and is the most likely form of icing.
- The cooling effect and the evaporation of fuel after being introduced into the airstream may produce what is known as fuel ice or fuel evaporation ice.
- Water in suspension in the atmosphere coming into contact with engine parts at a temperature below 0°C, may produce impact ice or atmospheric ice.
With the throttle in a partially closed position, such as when descending, then throttle icing is most likely. In this position the air velocity at the edge of the throttle valve will be increased, thus a pressure and temperature drop will occur, causing ice formation at the throttle. It is important to remove throttle ice as quickly as possible as, due to ice build-up on the throttle, the venturi effect between the throttle and the wall of the inlet manifold will increase causing an even greater pressure and temperature drop, therefore making the situation worse.
When water droplets impact with the intake and throttle body wall they freeze, which is most likely to occur at temperatures of between 0°C and -7°C. The ice builds up around the air intake disturbing the airflow, and alters the fuel/air ratio, causing a loss of power or engine failure.