be produced without the fuel detonating. Larger, high- compression engines require fuel with a greater amount of  potential power production without detonation than smaller  low-compression engines. As fuel vaporizes, it draws energy from its surroundings to  change state from a liquid to a vapor. This can be a problem  if water is present. When fuel vaporizes in the carburetor,  water in the fuel-air mixture can freeze and deposit inside  the carburetor and fuel induction system. The fuel discharge  nozzle, throttle valve, venturi, or simply the walls of the  induction system all can develop ice. As the ice builds, it  restricts the fuel-air flow and causes loss of engine power.  In severe cases, the engine stops running.  [Figure 14-5] One of the most important characteristics of an aircraft fuel  is its volatility. Volatility is a term used to describe how  readily a substance changes from liquid into a vapor. For  reciprocating engines, highly volatile fuel is desired. Liquid  gasoline delivered to the engine induction system carburetor  must vaporize in the carburetor to burn in the engine. Fuel  with low volatility vaporizes slowly. This can cause hard  engine starting, slow warm-up, and poor acceleration. It  can also cause uneven fuel distribution to the cylinders and  excessive dilution of the oil in the crankcase in engines  equipped with oil dilution systems. However, fuel can also  be too volatile, causing detonation and vapor lock. AVGAS is a blend of numerous hydrocarbon compounds,  each with different boiling points and volatility. A straight  chain of volatile compounds creates a fuel that vaporizes  easily for starting, but also delivers power through the  acceleration and power ranges of the engine.  Carburetor icing is most common at ambient temperatures  of 30–40 °F but can occur at much higher temperatures,  especially in humid conditions. Most aircraft are equipped  with carburetor heating to help eliminate this threat caused  by the high volatility of the fuel and the presence of moisture.  [Figure 14-6]  The aviation gasoline market is a relatively small part of the  overall gasoline market. AVGAS producers are few. In years  past, when this was less the case, considerable quantities of  aromatic hydrocarbons were sometimes added to increase  the rich mixture performance of AVGAS. It was used mainly  in high horsepower reciprocating engines, such as military  Vapor lock is a condition in which AVGAS vaporizes in the  fuel line or other components between the fuel tank and the  carburetor. This typically occurs on warm days on aircraft  with engine-driven fuel pumps that suck fuel from the tank(s).  Vapor lock can be caused by excessively hot fuel, low  pressure, or excessive turbulence of the fuel traveling through  the fuel system. In each case, liquid fuel vaporizes prematurely  and blocks the flow of liquid fuel to the carburetor. Aircraft gasoline is refined to have a vapor pressure be  between 5.5 pounds per square inch (psi) and 7.0 psi at 100  °F. At this pressure, an aircraft fuel system is designed to  deliver liquid fuel to the carburetor when drawn out of the  tank by an engine-driven fuel pump. But temperatures in the  fuel system can exceed 100 °F under the engine cowl on a  hot day. Fuel may vaporize before it reaches the carburetor,  especially if it is drawn up a line under a low pressure, or  if it swirls while navigating a sharp bend in the tubing. To  make matters worse, when an aircraft climbs rapidly, the  pressure on the fuel in the tank decreases while the fuel is  still warm. This causes an increase in fuel vaporization that  can also lead to vapor lock. Various steps can be taken to prevent vapor lock. The use of  boost pumps located in the fuel tank that force pressurized  liquid fuel to the engine is most common.  14-4 Figure 14-5.  An example of common areas where ice can form on  a carburetor. The evaporation of volatile fuel takes energy from  its surroundings to change state. As it does, water in the fuel-air  mixture condenses and freezes. Carburetor Icing Volatility Aromatic Fuels Vapor Lock Fuel/air mixture To engine Ice Ice Ice Incoming air Venturi