very quickly. A low volatility gasoline vaporizes slowly. A good gasoline should have the right volatility for the climate in which the gasoline is used. If the gasoline is too volatile, it will vaporize in the fuel system. The result will be a condition called VAPOR LOCK. Vapor lock is the formation of vapor in the fuel lines in a quantity sufficient to prevent the flow of gasoline through the system. Vapor lock causes the vehicle to stall from lack of fuel. In the summer and in hot  climates,  fuels  with  low  volatility  lessen  the tendency toward vapor lock. Antiknock Quality In modern high compression gasoline engines, the air-fuel  mixture  tends  to  ignite  spontaneously  or  to explode instead of burning rather slowly and uniformly. The result is a knock, a ping, or a detonation. For this reason, gasoline refiners have various ways to make gasoline  that  does  not  detonate  easily. Octane Rating Agasoline  that  detonates  easily  is  called  low  octane gasoline. A gasoline that resists detonation is called high  octane  gasoline. The octane rating of a gasoline is a measurement of the ability of the fuel to resist knock or ping.   A high octane rating indicates the fuel will NOT knock or ping easily. It should be used in a high compression or turbo- charged engine. A low octane gasoline is suitable for a low  compression  engine. Octane  numbers  give  the  antiknock  value  of gasoline. A higher octane number (91) will resist ping better than a gasoline with a low octane number (83). Each manufacturer recommends an octane number for their  engine. AIR-FUEL  RATIO For proper combustion and engine performance, the right amounts of air and fuel must be mixed together. If too much fuel or too little fuel is used, engine power, fuel economy, and efficiency are reduced. For a gasoline engine, the perfect air to fuel ratio is 15:1 (15 parts air to 1 part fuel by weight). Under constant engine conditions, this ratio can help assure that all fuel is burned during combustion. The fuel system must change the air-fuel ratio with the changes in  engine-operating  conditions. Lean Air-Fuel Mixture A lean air-fuel mixture contains a large amount of air. For example, 20:1 would be a very lean mixture. A slightly lean mixture is desirable for high gas mileage and low exhaust emissions. Extra air in the cylinder ensures that all the fuel will be burned; however, too lean of a mixture can cause poor engine performance (lack of power, missing, and even engine damage). Rich Air-Fuel Mixture A rich air-fuel mixture contains a little more fuel mixed with the air. For gasoline, 8:1(8 parts air to 1 part fuel) is a very rich mixture. A slightly rich mixture tends to increase power; however, it also increases fuel consumption  and  exhaust  emissions.  An  overly  rich mixture will reduce engine power, foul spark plugs, and cause incomplete burning (black smoke at engine exhaust). GASOLINE  COMBUSTION For gasoline or any other fuel to burn properly, it must be mixed with the right amount of air. The mixture must then be compressed and ignited. The resulting combustion produces heat, expansion of the gases, and pressure. Normal Combustion Normal gasoline combustion occurs when the spark plug ignites the fuel and burning progresses smoothly through the fuel mixture. Maximum cylinder pressure should be produced after a few degrees of crank rotation after the piston passes TDC on the power stroke. Normal combustion only takes about 3/1,000 of a second. This is much slower than an explosion. Dynamite explodes in about 1/50,000 of a second. Under some undesirable conditions, however, gasoline can be made to bum quickly, making part of the combustion like an explosion. Abnormal Combustion Abnormal combustion occurs when the flame does NOT   spread   evenly   and   smoothly   through   the combustion chamber. The lean air-fuel mixture, high- operating temperatures, low octane, and unleaded fuels used  today  make  abnormal  combustion  a  major problem that creates unfavorable conditions, such as the following: 4-2