passenger, engine, or cargo compartments. The fueloutlet pipe is located inside the tank and its opening isabout one-half inch above the bottom. This locationallows sediment to fall to the bottom of the fuel tankwithout being drawn into the fuel system. Most fueltanks have a position on top to install a fuel gaugesending unit. This is usually a flanged hole. A threadeddrain plug is normally located at the bottom of the tankand is used for draining and cleaning of the tank.Gasoline FuelGasoline, a by-product of petroleum, containscarbon and hydrogen. This factor allows the fuel toburn freely and to create extensive heat energy. Twotypes of gasoline are used: leaded and unleaded. Leadedgasoline has a higher octane rating than unleadedgasoline and is more effective as a valve and valve seatlubricant; however, leaded gasoline has almost beendiscontinued, because engines that use it emit a greatamount of harmful hydrocarbons that pollute theatmosphere. Engines that use unleaded gasoline emitfewer hydrocarbons, have fewer combustion chamberdeposits, and provide a longer life for spark plugs,exhaust systems, and carburetors; however, unleadedgasoline emits about the same amount of carbonmonoxide and nitrogen oxide as leaded gasoline.NOTE: The octane number in gasoline is a measureof its ability to burn evenly and resist spontaneouscombustion. A knock in a gasoline engine is caused bygases burning too rapidly.Catalytic ConverterA catalytic converter is positioned in the exhaustsystem, usually between the engine and the muffler, tocontrol the emission of carbon monoxide andhydrocarbons produced from burning gasoline. As theengine exhaust passes through the converter, carbonmonoxide and hydrocarbons are oxidized (combinedwith oxygen), changing them to carbon dioxide andwater. This oxidation causes the outer shell of theconverter to operate consistently at temperatures that areseveral hundred degrees higher than the rest of theexhaust system. The outer shell of the catalyticconverter is normally made of stainless steel to copewith the high operating temperatures.A chemical catalyst is an element or chemicalcompound that increases the reaction between two otherchemicals without reacting with them. In this case, thecatalyst in the catalytic converter increases the reactionbetween oxygen and the harmful carbon monoxide andhydrocarbons to produce harmless carbon dioxide andwater emissions.Platinum and palladium are precious metals oftenused as catalysts in catalytic converters. Small amountsof the catalysts are used to coat the surfaces of thematerial in the converter. Two common types ofconverters are shown in figure 1-18.NOTE: The use of leaded gasoline is destructive toa catalytic converter. The lead in the exhaust can coatthe catalyst as it passes through the converter, and thiscoating can completely halt catalytic converteroperations.Diesel FuelDiesel fuel comes from the residue of the crude oilafter the more volatile fuels, such as gasoline andkerosene, are removed during the petroleum refiningprocess. As with gasoline, the efficiency of a diesel fuelvaries with the type of engine. The refining and blendingprocess can produce a suitable diesel fuel for almost anyengine operating conditions. Using a contaminated fuelor an improper grade of fuel can cause hard starting,incomplete combustion, a smokey exhaust, or cause anengine to knock.Cleanliness of diesel fuel is important because fuelcontaining more than a trace of foreign substances cancause fuel pump and injector problems to develop.Diesel fuels can hold dirt particles in suspension longerthan gasoline because it is heavier and more viscous. Inrefining, not all foreign materials can be removed, andharmful matter, such as dirt and water, can get into thefuel during the handling process, Water can rust aninjection system and cause it to fail. Dirt clogs injectorsand spray nozzles and can cause an engine to misfire orstop altogether. To be safe, remember to takeprecautions when refueling and try to prevent foreignmatter from entering the fuel tank.High-cetane diesel fuels allow diesel engines to bestarted at low temperatures, provide fast warmupswithout misfiring or producing white smoke, reduce theformation of carbon deposits, and eliminate dieselknock. However, a too high cetane number can lead toincomplete combustion and exhaust smoke if the delayis 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 andDF-2.1-14
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