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Fuel System

by Fred Bordoff

The function of the fuel system is to store and supply fuel to the cylinder chamberwhere it can be mixed with air, vaporized, and burned to produce energy. The fuel, whichcan be either gasoline or diesel is storedin a fuel tank. A fuel pump draws the fuel from the tank through fuel lines and delivers itthrough a fuel filter to either a carburetor or fuel injector,then delivered to the cylinder chamber for combustion.



Gasoline is a complex blend of carbon and hydrogen compounds. Additives are then addedto improve performance. All gasoline is basically the same, but no two blends areidentical. The two most important features of gasoline are volatility and resistance toknock (octane). Volatility is a measurement of how easily the fuel vaporizes. If thegasoline does not vaporize completely, it will not burn properly (liquid fuel will notburn).

If the gasoline vaporizes too easily the mixture will be too lean to burn properly.Since high temperatures increase volatility, it is desirable to have a low volatility fuelfor warm temperatures and a high volatility fuel for cold weather. The blends will bedifferent for summer and winter fuels. Vapor lock which was a persistent problem yearsago, exists very rarely today. In today's cars the fuel is constantly circulating from thetank, through the system and back to the tank. The fuel does not stay still long enough toget so hot that it begins to vaporize. Resistance to knock or octane is simply thetemperature the gas will burn at. Higher octane fuel requires a higher temperature toburn. As compression ratio or pressure increases so does the need for higher octane fuel.Most engines today are low compression engines therefore requiring a lower octane fuel(87). Any higher octane than required is just wasting money. Other factors that affect theoctane requirements of the engine are: air/fuel ratio, ignition timing, enginetemperature, and carbon build up in the cylinder. Many automobile manufacturers haveinstalled exhaust gas recirculation systems to reduce cylinder chamber temperature. Ifthese systems are not working properly, the car will have a tendency to knock. Beforeswitching to a higher octane fuel to reduce knock, make sure to have these other causeschecked.




Diesel fuel, like gasoline is a complex blend of carbon and hydrogen compounds. It toorequires additives for maximum performance. There are two grades of diesel fuel used inautomobiles today: 1-D and 2-D. Number 2 diesel fuel has a lower volatility and is blendedfor higher loads and steady speeds, therefore works best in large truck applications.Because number 2 diesel fuel is less volatile, it tends to create hard starting in coldweather. On the other hand number 1 diesel is more volatile, and therefore more suitablefor use in an automobile, where there is constant changes in load and speed. Since dieselfuel vaporizes at a much higher temperature than gasoline, there is no need for a fuelevaporation control system as with gasoline. Diesel fuels are rated with a cetane numberrather than an octane number. While a higher octane of gasoline indicates resistance toignition, the higher cetane rating of diesel fuel indicates the ease at which the fuelwill ignite. Most number 1 diesel fuels have a cetane rating of 50, while number 2 dieselfuel have a rating of 45. Diesel fuel emissions are higher in sulfur, and lower in carbonmonoxide and hydrocarbons than gasoline and are subject to different emission testingstandards.




Tank location and design are always a compromise with available space. Most automobileshave a single tank located in the rear of the vehicle. Fuel tanks today have internalbaffles to prevent the fuel from sloshing back and forth. If you hear noises from the rearon acceleration and deceleration the baffles could be broken. All tanks have a fuel fillerpipe, a fuel outlet line to the engine and a vent system. All catalytic converter cars areequipped with a filler pipe restrictor so that leaded fuel, which is dispensed from athicker nozzle, cannot be introduced into the fuel system. All fuel tanks must be vented.Before 1970, fuel tanks were vented to the atmosphere, emitting hydrocarbon emissions.Since 1970 all tanks are vented through a charcoal canister, into the engine to be burnedbefore being released to the atmosphere. This is called evaporative emission control andwill be discussed further in the emission control section. Federal law requires that all1976 and newer cars have vehicle rollover protection devices to prevent fuel spills.




Steel lines and flexible hoses carry the fuel from the tank to the engine. Whenservicing or replacing the steel lines, copper or aluminum must never be used. Steel linesmust be replaced with steel. When replacing flexible rubber hoses, proper hose must beused. Ordinary rubber such as used in vacuum or water hose will soften and deteriorate. Becareful to route all hoses away from the exhaust system.




Two types of fuel pumps are used in automobiles; mechanical and electric. All fuelinjected cars today use electric fuel pumps, while most carbureted cars use mechanicalfuel pumps. Mechanical fuel pumps are diaphragm pumps, mounted on the engine and operatedby an eccentric cam usually on the camshaft. A rocker arm attached to the eccentric movesup and down flexing the diaphragm and pumping the fuel to the engine. Because electricpumps do not depend on an eccentric for operation, they can be located anywhere on thevehicle. In fact they work best when located near the fuel tank.

Many cars today, locate the fuel pump inside the fuel tank. While mechanical pumpsoperate on pressures of 4-6 psi (pounds per square inch), electric pumps can operate onpressures of 30-40 psi. Current is supplied to the pump immediately when the key isturned. This allows for constant pressure on the system for immediate starting. Electricfuel pumps can be either low pressure or high pressure. These pumps look identical, so becareful when replacing a fuel pump that the proper one is used. Fuel pumps are rated bypressure and volume. When checking fuel pump operation, both specifications must bechecked and met.




The fuel filter is the key to a properly functioning fuel delivery system. This is moretrue with fuel injection than with carbureted cars. Fuel injectors are more susceptible todamage from dirt because of their close tolerances, but also fuel injected cars useelectric fuel pumps. When the filter clogs, the electric fuel pump works so hard to pushpast the filter, that it burns itself up. Most cars use two filters. One inside the gastank and one in a line to the fuel injectors or carburetor. Unless some severe and unusualcondition occurs to cause a large amount of dirt to enter the gas tank, it is onlynecessary to replace the filter in the line.


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