level and higher. References to octane characteristics  above 100 percent iso-octane are made by referencing the  antidetonation properties of the fuel to a mixture of pure  iso-octane and specific quantities of TEL. The specific  mixtures of iso-octane and TEL are assigned arbitrary  octane numbers above 100. In addition to increasing the  antidetonation characteristics of a fuel, TEL also lubricates  the engine valves.  Performance numbers are also used to characterize the  antidetonation characteristics of fuel. A performance number  consists of two numbers (e.g., 80/87, 100/130, 115/145)  in which higher numbers indicate a higher resistance to  detonation. The first number indicates the octane rating of  the fuel in a lean fuel-air mixture, and the second number  indicates the octane rating of the fuel in a rich mixture.  Due to the small size of the worldwide aviation gasoline  market, a single 100 octane low-lead fuel (100LL) is desired  as the only AVGAS for all aircraft with reciprocating engines.  This presents problems in engines originally designed to run  on 80/87 fuel; the low lead 100-octane fuel still contains more  lead than the 80-octane fuel. Spark plug fouling has been  common and lower times between overhaul have occurred.  Other engines designed for 91/96 fuel or 100/130 fuel operate  satisfactorily on 100LL, which contains 2 milliliters of  TEL per gallon (enough to lubricate the valves and control  detonation). For environmental purposes, AVGAS with no  TEL is sought for the aviation fleet of the future. Aircraft and engine manufacturers designate approved fuels  for each aircraft and engine. Consult manufacturer data and  use only those fuel specified therein. The existence of more than one fuel makes it imperative  that fuel be positively identified and never introduced into  a fuel system that is not designed for it. The use of dyes in  fuel helps aviators monitor fuel type. 100LL AVGAS is the  AVGAS most readily available and used in the United States.  It is dyed blue. Some 100 octane or 100/130 fuel may still  be available, but it is dyed green. 80/87 AVGAS is no longer available. It was dyed red. Many  supplemental type certificates have been issued to engine  and engine/airframe combinations that permit the use of  automobile gasoline in engines originally designed for red  AVGAS. A relatively new AVGAS fuel, 82UL (unleaded),  has been introduced for use by this group of relatively low  compression engines. It is dyed purple.  115/145 AVGAS is a fuel designed for large, high  performance reciprocating engines from the World War II  The amount of TEL added to a fuel can be increased to raise  the antidetonation characteristics from 80 to the 100-octane  To increase antidetonation characteristics of fuel, substances  can be added. Tetraethyl lead (TEL) is the most common  additive that increases the critical pressure and temperature  of a fuel. However, additional additives, such as ethylene  dibromide and tricresyl phosphate, must be also be added so  that the TEL does not leave solid deposits in the  combustion chamber. When a fuel has the same critical pressure as a reference  mixture of these two hydrocarbons, i is said to have an  octane rating that is the same as the percentage  of the iso- octane is this reference mixture. An 80-octane fuel has the  same resistance to detonation as an 80 percent  iso-octane, 20 percent heptane mixture; a 90-octane fuel has the same  resistance to detonation as a 90 percent iso-octane, 10  percent heptane mixture; and a 100-octane fuel has the same  resistance to detonation as 100 percent pure iso-octane. So,  by comparing a fuel’s tendency to detonate to reference  mixtures of iso-octane and heptane, octane ratings  from 80  to 100 can be established. The highest-octane fuel possible  with this system of measurement is 100-octane fuel.  Octane ratings and performance numbers are given to fuels  to describe their resistance to detonation. Fuels  with high  critical pressure and high octan or performance numbers  have the greatest resistance. A referencing system is used  to rate the fuel. A mixture of two hydrocarbons, iso-octane  (C$_{8}$H$_{18}$) and heptane (C$_{7}$H$_{16}$), is used. Various ratios of  the two hydrocarbons in a mixture result in proportional  antidetonation properties. The more iso-octane there  is in the  mixture, the higher its resistance is to detonation.  Figure 14-7.  Preignition can cause detonation and damage to  the engine.  Octane and Performance Number Rating Fuel Identification 14-6