Protection against stalling due to carburetor icing is a measurable quality feature of motor gasolines. Carburetor icing occurs when air temperature is low and humidity high. Ice is formed in critical areas of the carburetor after enough fuel has been evaporated to cool the carburetor and condense water from the intake air. Stalling and rough idling continue until the engine has warmed up sufficiently to melt the ice. Engine design characteristics and gasoline volatility affect the icing tendencies of cars; several makes avoid icing by applying more heat to the carburetor early in the warm-up period.
Icing increases as gasolines become more volatile in their mid-boiling range. Because the more volatile gasolines give much better warm-up in the absence of carburetor icing, optimum performance must be achieved through the use of effective and safe anti-icing additives. Such additives are usually either depressors, which reduce the freezing point of the condensed water to less than the carburetor temperature, or “surfactants”, which reputedly wet carburetor surfaces with a slippery film to which ice cannot adhere easily. Newer evidence shows the actual mechanisms to be more complex. The development of anti-icing additives is complicated by differing responses of cars to additives.