FUEL SYSTEMS

The function of the fuel system is to ensure a quantity of clean fuel is delivered to the fuel intake of an engine. The system must provide both safe fuel storage and transfer.

FUEL TANKS

Fuel tanks store fuel in liquid form. The tank may be located in any part of a vehicle that is protected from flying debris, shielded from collisions, and not likely to bottom out (fig. 1-16). Most wheeled vehicles use removable fuel tanks.

Most fuel tanks are made of thin sheet metal coated with a lead-tin alloy to prevent corrosion. Fiber glass and a variety of molded plastics are also popular as corrosion-resistant materials.

The walls of fuel tanks are manufactured with ridges to give them strength and internal baffles that increase internal strength and prevent the fuel from sloshing (fig. 1-17). The filler pipe offers a convenient opening to fill the tank and prevent fuel from being spilled onto the

Figure 1-17.-Fuel tank construction.

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.

Figure 1-18.-Catalytic converter.

NOTE: The cetane number is a measure of the ability of a diesel fuel to provide fast spontaneous combustion with short ignition delay. A knock in a diesel engine is caused by the fuel igniting too slowly.

Jet Fuel

You may be deployed to some sites at which diesel fuel is not available and JET FUEL has to be used. The three major types of jet fuel used by the military are JP-4, JET-A1, and JP-5. DO NOT USE JP-4 IN ANY DIESEL ENGINE. The maintenance supervisor approves the use of JET-A1 and JP-5 and directs the amount of engine oil that must be added to the jet fuel. This must be done to improve the lubricating qualities that prevent the injector pump and injectors from seizing.