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Automobile Fuel System

Updated on August 23, 2010

The fuel system consists of a fuel tank, fuel pump, carburetor, intake manifold, and exhaust manifold. The fuel tank, ranging in capacity from 14 to 26 gallons, generally is located beneath the body or in the fenders at the rear of the automobile. A filler pipe with removable cap extends to the outside of the body for convenient refueling, and the interior of the tank is fitted with baffles to restrict the surging of gasoline due to car movement. A float within the tank operates an electrical gauge on the instrument panel to indicate the fuel level.

The flexible diaphragm of a mechanical fuel pump is actuated by a special cam on the camshaft, maintains a constant level of gasoline in the carburetor float chamber at all road speeds. A sediment bowl and filter keep foreign matter out of the carburetor.


The purpose of the carburetor is to meter and atomize the fuel, mix it with a suitable quantity of air, and deliver it to the engine cylinders via the intake manifold. The theoretical air-fuel ratio that provides the exact quantity of air required to burn the fuel is about 14.7:1. However, the carburetor has provisions for varying the mixture ratio automatically for different operating conditions. For example, a rich mixture is required for acceleration, hill climbing, and high speeds; for level driving, maximum economy is ensured by running the engine on a leaner mixture. Fig. 5 shows the automotive fuel system, and Fig. 6 the basic carburetor. Air enters the top of the air horn, flows down through the venturi, past the throttle valve, and into the intake manifold. With the throttle valve closed, only a very limited amount of air is drawn past it. This creates a vacuum on the engine side of the throttle valve as the pistons move down on the intake stroke. However, an idle discharge hole is located below the throttle valve, and enough fuel is drawn to run the engine at idling speeds. As the throttle valve opening is increased, the valve uncovers a second idle discharge hole, and additional fuel is supplied to the engine. When wider throttle openings further increase the flow of air through the carburetor throat, the vacuum created by the venturi also will draw gasoline from the main nozzle.

When the throttle is opened quickly, the flow of gasoline from the carburetor lags behind the rapid increase in air flow and causes a momentary lag in engine power. An accelerating pump connected to the throttle linkage provides the necessary temporary enrichment by injecting a stream of gasoline directly into the carburetor air horn.

Gasoline does not vaporize readily in cold weather, and the air-fuel ratio often must be reduced to as low as 1:1 to start the engine. To provide this ratio, a choke valve in the upper part of the air horn limits the amount of air entering the carburetor, and a richer mixture is drawn into the engine cylinders. In most automobiles, the choke valve functions automatically under the control of a thermostat.

A combination air cleaner and silencer, connected to the carburetor intake, keeps abrasive dirt particles out of the engine by trapping them in a filtering element. Filter elements may be made of pressed paper, polyurethane foam, or metal gauze. Pressed-paper elements are discarded at recommended mileage intervals, and new paper elements installed; elements of polyurethane foam or metal gauze may be cleaned, reoiled, and used indefinitely. Incorporated in the air-cleaner design is a resonance muffler, tuned to reduce the noise of air rushing into the carburetor intake.

Superchargers, available as optional equipment on some American passenger car engines, improve engine performance by packing more air into the cylinders than normally would be drawn in by the pistons. Basically an air compressor, the supercharger may be driven by a gear train, a V-belt, or a turbine wheel spun by engine exhaust gases.

Fuel Injection

Fuel injection was first offered by an American automobile maker in 1957. Instead of the conventional carburetor, this system utilizes a high-pressure gear pump and a metering/distributing system to deliver gasoline to the intake port of each cylinder. Metered air is introduced into the cylinder through an intake manifold. See also fuel injection.

Intake Manifold, Exhaust Manifold, and Muffler. The gasoline and air mixture formed by the carburetor is conducted to each cylinder of the engine by a ducting system called the intake manifold. The intake manifold generally is made of cast iron or aluminum and has flanged connections for the carburetor and cylinder ports.

Exhaust gases are conducted from the exhaust ports of the engine by a manifold connected to the exhaust pipe and the muffler. The exhaust manifold is made of cast iron and provided with the necessary flanges for connection to the cylinder block or head. It usually contains a section that surrounds the portion of the intake manifold immediately below the carburetor mounting flange. This section, or "stove" as it sometimes is called, heats the incoming air-fuel mixture in the intake manifold during engine warm-up. A thermostatically controlled valve permits heat to be added to the air-fuel mixture when the engine is cold and also prevents heat from being added after the engine has warmed up to operating temperature. The engine exhaust, leading from the exhaust manifold to the muffler, usually is made of steel tubing.

Mufflers are cylindrical or oval chambers of sheet steel installed between the exhaust pipe and tailpipe. They contain perforated baffles that effectively reduce the noise of engine explosions without raising the exhaust-system back pressure sufficiently to cause engine overheating and loss of power. The environment of the automotive muffler is highly corrosive and coatings of zinc, aluminum, or ceramic often are used to extend muffler life. Increasingly, cars are being equipped with mufflers made wholly or in part of stainless steel. Exhaust gases are led from the muffler by a tailpipe and discharged into the atmosphere at the rear of the vehicle.


Gasoline, which first became available in the 1860's, is a complex mixture of hydrocarbons that meets all the requirements of a satisfactory automotive engine fuel: it has a high heat content per unit of weight or volume, high volatility, and good antiknock characteristics. Although the volatility of gasoline must be sufficiently high to ensure its combining with air to form a good combustible mixture, it also must be kept low enough to minimize evaporation from the tank and carburetor float chamber, both of which are open to the atmosphere. Equally important are antiknock characteristics of a. gasoline. Any engine knocking caused by the spontaneous ignition of portions of the fuel charge will result in increased engine temperatures, loss of power, and objectionable noise. In general, gasolines of high octane number have less tendency to cause engine knock.

The term "compression ratio" expresses the relation of cylinder volumes when the piston is (1) at the bottom of its stroke and (2) at the top of its stroke. Fuel burns more efficiently at higher compression ratios; for example, with a compression ratio of 6:1 only 25 percent of the potential energy of the fuel is converted into mechanical power by the engine, but with a compression ratio of 12:1 the efficiency is close to 32 percent. As engine compression ratios go higher, however, it becomes increasingly difficult to control the burning of the air-gasoline mixture within the combustion chamber. The addition of various chemicals to both gasoline and lubricating oil has made possible the design of passenger-car engines with compression ratios as high as 11.1:1, and they generally vary from 8:1 to slightly more than 11:1. Some manufacturers also build special-purpose automobile engines with compression ratios up to 13.5:1.


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