passenger, engine, or cargo compartments. The fuel outlet pipe is located inside the tank and its opening is about one-half inch above the bottom. This location allows sediment to fall to the bottom of the fuel tank without being drawn into the fuel system. Most fuel tanks have a position on top to install a fuel gauge sending unit. This is usually a flanged hole. A threaded drain plug is normally located at the bottom of the tank and is used for draining and cleaning of the tank. Gasoline Fuel Gasoline,  a  by-product  of  petroleum,  contains carbon and hydrogen. This factor allows the fuel to burn freely and to create extensive heat energy. Two types of gasoline are used: leaded and unleaded. Leaded gasoline  has  a  higher  octane  rating  than  unleaded gasoline and is more effective as a valve and valve seat lubricant; however, leaded gasoline has almost been discontinued, because engines that use it emit a great amount  of  harmful  hydrocarbons  that  pollute  the atmosphere.  Engines  that  use  unleaded  gasoline  emit fewer  hydrocarbons,  have  fewer  combustion  chamber deposits, and provide a longer life for spark plugs, exhaust  systems,  and  carburetors;  however,  unleaded gasoline  emits  about  the  same  amount  of  carbon monoxide and nitrogen oxide as leaded gasoline. NOTE: The octane number in gasoline is a measure of its ability to burn evenly and resist spontaneous combustion. A knock in a gasoline engine is caused by gases  burning  too  rapidly. Catalytic  Converter A  catalytic  converter  is  positioned  in  the  exhaust system, usually between the engine and the muffler, to control   the   emission   of   carbon   monoxide   and hydrocarbons  produced  from  burning  gasoline.  As  the engine  exhaust  passes  through  the  converter,  carbon monoxide and hydrocarbons are oxidized (combined with oxygen), changing them to carbon dioxide and water. This oxidation causes the outer shell of the converter  to  operate  consistently  at  temperatures  that  are several hundred degrees higher than the rest of the exhaust  system.  The  outer  shell  of  the  catalytic converter is normally made of stainless steel to cope with the high operating temperatures. A  chemical  catalyst  is  an  element  or  chemical compound that increases the reaction between two other chemicals without reacting with them. In this case, the catalyst in the catalytic converter increases the reaction between oxygen and the harmful carbon monoxide and hydrocarbons  to  produce  harmless  carbon  dioxide  and water   emissions. Platinum and palladium are precious metals often used as catalysts in catalytic converters. Small amounts of the catalysts are used to coat the surfaces of the material  in  the  converter.  Two  common  types  of converters are shown in figure 1-18. NOTE: The use of leaded gasoline is destructive to a catalytic converter. The lead in the exhaust can coat the catalyst as it passes through the converter, and this coating  can  completely  halt  catalytic  converter operations. Diesel Fuel Diesel fuel comes from the residue of the crude oil after  the  more  volatile  fuels,  such  as  gasoline  and kerosene,  are  removed  during  the  petroleum  refining process. As with gasoline, the efficiency of a diesel fuel varies with the type of engine. The refining and blending process can produce a suitable diesel fuel for almost any engine  operating  conditions.  Using  a  contaminated  fuel or an improper grade of fuel can cause hard starting, incomplete combustion, a smokey exhaust, or cause an engine to knock. Cleanliness  of  diesel  fuel  is  important  because  fuel containing more than a trace of foreign substances can cause fuel pump and injector problems to develop. Diesel fuels can hold dirt particles in suspension longer than gasoline because it is heavier and more viscous. In refining, not all foreign materials can be removed, and harmful matter, such as dirt and water, can get into the fuel during the handling process, Water can rust an injection system and cause it to fail. Dirt clogs injectors and spray nozzles and can cause an engine to misfire or stop  altogether.  To  be  safe,  remember  to  take precautions when refueling and try to prevent foreign matter from entering the fuel tank. High-cetane diesel fuels allow diesel engines to be started at low temperatures, provide fast warmups without misfiring or producing white smoke, reduce the formation  of  carbon  deposits,  and  eliminate  diesel knock. However, a too high cetane number can lead to incomplete combustion and exhaust smoke if the delay is too short to allow for proper mixing of fuel and air. Most diesel fuels range from 33 to 64 in cetane number, with 40 the minimum for military grades DF-1 and DF-2. 1-14