Fuel Delivery Systems

By SRT-Tom · Jun 30, 2019 ·
  1. SRT-Tom
    A carburetor was the common method of fuel delivery for most US-made gasoline engines until the late 1980s, when fuel injection became the preferred method. This change was dictated by the requirements of catalytic converters and not due to an inherent inefficiency of carburation.

    Basically, a carburetor consists of an open pipe through which the air passes into the inlet manifold of the engine. The pipe is in the form of a venture- it narrows in section and then widens again, causing the airflow to increase in speed in the narrowest part. Below the venturi is a butterfly valve called the throttle valve- a rotating disc that can be turned end-on to the airflow, so as to hardly restrict the flow at all, or can be rotated so that it (almost) completely blocks the flow of air. This valve controls the flow of air through the carburetor throat and thus the quantity of air/fuel mixture the system will deliver, thereby regulating engine power and speed. The throttle is connected, usually through a cable or a mechanical linkage of rods and joints, to the accelerator pedal on a car. Fuel flow is adjusted by means of precisely calibrated orifices, referred to as jets, in the fuel path.

    While basic carburetors have only one venturi, many carburetors have more than one venturi, or "barrel." Two-barrel and four-barrel configurations are commonly used to accommodate the higher air flow rate with large engine displacement.

    The spread-bore four-barrel carburetor, first released by Rochester in the 1965 model year, as the "Quadrajet," has a much greater spread between the sizes of the primary and secondary throttle bores.

    Quadrajet 4-Barrel

    In 1971, a unique carburetor debuted on the 340 engine- the Carter Thermo-Quad. What made it innovative was its phenolic main body which kept the carburetor bowl 20 degrees cooler. Also, like other performance four barrels, it had small 1 ½ inch primaries (for economy and emissions) and huge 2 ¼ inch secondaries. This carburetor remained in use on Chrysler engines until 1985.


    Multiple carburetors were also mounted on engines with progressive linkages, such as two four-barrel carburetors (often referred to as "dual-quads") on 426 Hemi (see photo, below) and three two- barrels on a 440 Six-Pack or a 340 Six-Pack (see photos, below).

    426 Hemi

    440 Six-Pack

    340 Six-Pack

    On gasoline engines, fuel injection replaced carburetors from the 1980s onward. The primary difference between carburetors and fuel injection is that fuel injection atomizes the fuel through a small nozzle under high pressure, while a carburetor relies on suction created by intake air accelerated through a venturi tube to draw the fuel into the air stream.

    The first automotive direct injection system used to run on gasoline was developed by Bosch, and was introduced by Goliath for their Goliath GP700 automobile and Gutbrod in 1952. This was basically a specially lubricated high-pressure diesel direct-injection pump of the type that is governed by the vacuum behind an intake throttle valve. More mainstream applications of fuel injection favored the less-expensive indirect injection methods.

    Chevrolet introduced a mechanical fuel injection option, made by General Motors' Rochester Products Division for its 283 V8 engine in 1956 (1957 model year).

    1959 Corvette small-block 4.6Lwith Rochester mechanical fuel injection

    During the 1960s, other mechanical injection systems such as Hilborn were occasionally used on modified American V8 engines in various racing applications such as drag racing, oval racing and road racing. These racing-derived systems were not suitable for everyday street use, having no provisions for low speed metering, or often none even for starting (starting required that fuel be squirted into the injector tubes while cranking the engine).

    Because mechanical injection systems have limited adjustments to develop the optimal amount of fuel into an engine that needs to operate under a variety of different conditions (such as when starting, the engine's speed and load, atmospheric and engine temperatures, altitude, ignition timing, etc.) electronic fuel injection (EFI) systems were developed that relied on numerous computer sensors and controls. When working together, these electronic components can sense variations and the main system computes the appropriate amount of fuel needed to achieve better engine performance based on a stored "map" of optimal settings for given requirements.

    The first commercial EFI system was the Bendiz "Electrojector," and was offered by AMC, in 1957, in the Rebel’s 327 cubic inch engine.

    Chrysler offered Electrojector on the 1958 Chrysler 300D, DeSoto Adventurer, Dodge D-500 and Plymouth Fury- arguably the first series-production cars equipped with an EFI system. The early electronic components were not equal to the rigors of underhood service, however, and were too slow to keep up with the demands of "on-the-fly" engine control.


    In the 1970s and 1980s, in the U.S. and Japan, the respective federal governments imposed increasingly strict exhaust emission regulations. During that time period, the vast majority of gasoline-fueled automobile and light truck engines did not use fuel injection. To comply with the new regulations, automobile manufacturers often made extensive and complex modifications to the engine carburetor(s). While a simple carburetor system is cheaper to manufacture than a fuel injection system, the more complex carburetor systems installed on many engines in the 1970s were much more costly than the earlier simple carburetors. To more easily comply with emissions regulations, automobile manufacturers began installing fuel injection systems in more gasoline engines during the late 1970s.

    Fuel injection is the introduction of fuel in an internal combustion engine, most commonly automotive engines, by the means of an injector.


    All diesel engines use fuel injection by design. Gasoline engines can use gasoline direct injection, where the fuel is directly delivered into the combustion chamber, or indirect injection where the fuel is mixed with air before the intake stroke.

    The functional objectives for fuel injection systems can vary. All share the central task of supplying fuel to the combustion process, but it is a design decision how a particular system is optimized. There are several competing objectives such as:

    • Power output
    • Fuel Efficiency
    • Emissions performance
    • Running on alternative fuels
    • Reliability
    • Driveability and smooth operation
    • Initial cost
    • Maintenance cost
    • Diagnostic capability
    • Range of environmental operation
    • Engine tuning
    Fuel is transported from the fuel tank (via fuel lines) and pressurized using fuel pump(s). Maintaining the correct fuel pressure is done by a fuel pressure regulator. Often a fuel rail is used to divide the fuel supply into the required number of cylinders. The fuel injector injects liquid fuel into the intake air (the location of the fuel injector varies between systems).

    Mopar Electric Fuel Pump

    Unlike carburetor-based systems, where the float chamber provides a reservoir, fuel injected systems depend on an uninterrupted flow of fuel. To avoid fuel starvation when subject to lateral G-forces, vehicles are often provided with an anti-surge vessel, usually integrated in the fuel tank, but sometimes as a separate, small anti-surge tank.

    Multipoint fuel injection (MPI), also called port fuel injection (PFI), injects fuel into the intake ports just upstream of each cylinder's intake valve, rather than at a central point within an intake manifold. MPI systems can be sequential, in which injection is timed to coincide with each cylinder's intake stroke; batched, in which fuel is injected to the cylinders in groups, without precise synchronization to any particular cylinder's intake stroke; or simultaneous, in which fuel is injected at the same time to all the cylinders. The intake is only slightly wet, and typical fuel pressure runs between 40-60 psi.

    Many modern EFI systems use sequential MPI; however, in newer gasoline engines, direct injection systems have replaced sequential ones. In a direct injection engine, fuel is injected into the combustion chamber as opposed to injection before the intake valve (gasoline engine) or a separate pre-combustion chamber (diesel engine).

    In a common rail system, the fuel from the fuel tank is supplied to the common header and is then sent through tubing to the injectors, which inject it into the combustion chamber. The header has a high pressure relief valve to maintain the pressure in the header and return the excess fuel to the fuel tank. The fuel is sprayed with the help of a nozzle that is opened and closed with a needle valve, operated with a solenoid. When the solenoid is not activated, the spring forces the needle valve into the nozzle passage and prevents the injection of fuel into the cylinder. The solenoid lifts the needle valve from the valve seat, and fuel under pressure is sent in the engine cylinder. Third-generation common rail diesels use piezo-electric injectors for increased precision, with fuel pressures up to 26,000 psi.

    6.1L Fuel Rail

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