Original submission by T Mar 5 2006:Back to Fuel System Index
What Is Carburettor Ice?:
Just as the Freezer section of a Refrigerator shows evidence of Water Vapour from the air freezing, Fuel Systems can suffer similarly.Carburettor Ice in a Rotax Engine.
Water droplets appear in Fuel Tanks as a result of the water vapour in the air being condensed by the cooling effect of the Fuel in the Tank. Manufacturers provide Carburettor Cars with a means of heating the Carburettor to prevent the buildup of Carburettor Ice. Engines without Carburettor Heat are exposed to the possibility of rough running followed by Engine stoppage as in the following images.
Carburettor Heat And The Hot Spot:
In Old Holdens, Carburettor Heat may be provided from a Flap Vane built into the Exhaust Manifold or a Coolant Source that allows hot Coolant to flow through the Inlet Manifold. Not only does this heat aid in Fuel Vapourisation, but it guards against Carburettor Ice. Carburettor Ice forms inside the carburettor of a running Engine when the humidity of the day is high, that is there is a lot of Water Vapour in the air. As the Fuel leaving the Main Jet is converted into Fuel Vapour, its temperature drops. The drop in temperature is sufficient to cause any incoming Water Vapour to form Ice Crystals over the Main Jet and Throttle Butterfly. Eventually the Ice buildup becomes so great that it will stop the Engine. You see Top Fuellers getting Alcohol sprayed on their intake Butterflies for the same reason. The Alcohol prevents Ice buildup.
Carburettor Ice Duration:Carburettor Ice doesn't stick around very long after the Engine has stopped. It's usually there just long enough to wreak havoc. Evidence of its presence will have to be collected rapidly because the Engine's heat will begin to make it melt and the absence of vapourising Fuel will remove its prime cause. It's for the above reasons that it has been such an evasive cause of Engine stoppage.
Fuel Injected Engines:
Fuel Injected Engines do not suffer from ice because the Fuel leaving the Injectors does not drop in temperature. Also, the fuel is introduced to the air downstream of the throttle butterfly in contrast to a carburettor where the fuel enters the air upstream of it.
In Aviation, all Carburettor equipped aircraft have a control on the Cockpit Dashboard called Carburettor Heat. In the event of Carburettor Ice forming, the Pilot can activate this control and cause exhaust heated air to be forced down the Carburettor intake to blow any ice away. Carburettor Ice causes a number of Forced Landings in Aircraft every year sometimes because the symptoms of a dying Engine are recognised too late and sometimes because particular Engines are very unforgiving of Carburettor Ice and simply quit too rapidly for the Pilot to be able to take action.