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Mechanical fuel injection

  • Basic Concept of Mechanical Injection

    All forms of fuel injection (FI) are designed to achieve the delivery of a correct fuel/air mix into an internal combustion engine for the most fuel efficient burn. All forms of fuel injection do so by forcing the fuel under pressure through nozzles (injectors) that breaks up or atomizes the fuel into a fine spray. This spray when mixed in the correct ratio with the air drawn into the engine (or forced into the engine by a turbo or supercharger) burns very completely creating maximum power and minimum emissions. Fuel injection not only provides power improvements over the rpm range of an engine, it can also provide better gas mileage.

    Mechanical vs Electronic Fuel Injection (EFI)

    The term Mechanical when applied to fuel injection is used to indicate that metering functions of the fuel injection -how the correct amount of fuel for any given situation is determined and delivered- is not achieved electronically but rather through mechanical means alone. All the components in both 'mechanical' or Electronic Fuel Injection (EFI) systems that deliver the fuel in to the engine -the pumps, plumbing, valves, injectors- are mechanical in operation. The difference being that in EFI systems the injectors are switched on and off electronically by the EFI control electronics. This ability to control the injectors electronically allows far more variation in when and how the fuel is injected.

    The correct ratio of fuel to air varies continuously with changes in engine temperature, air temperature, altitude, engine speed and load, and with gasoline engines, ignition timing. The fuel injection system must be able to adjust the amount of fuel it is injecting to meet the engine's current needs, and to do so, must sense variations in these parameters. The advent of microprocessors provided an elegant solution for computing the correct fuel delivery based on data from various sensors on the engine to continuously make metering adjustments. This system of electronic control was further developed to integrate control of ignition timing, and in some cases valve timing into a sophisticated engine management system. Some of these engine management systems now direct injector units to give multiple squirts of fuel on each cylinder induction stroke to create a stratified charge within the combustion chamber which will burn even more efficiently.

    Fuel injection itself has been around almost as long as the diesel engine, as fuel-injection provides the only practical way of presenting the combustion chamber with diesel oil in an atomized and combustible state. With the gasoline or petrol engines, the carburetor (which uses venturis and jets to perform the basic fuel metering functions) was a cheaper and practical alternative to the expensive precision pumps and injectors needed for fuel injection and thus gained supremacy over fuel injection until the 1980s and 1990s.

    A notable example of the advantages of mechanical fuel injection over carburetion was the ability of Messerschmidt Bf 109 pilots early in World War II to evade RAF Spitfire I, by maneuvering their fuel injected Damlier-Benz power aircraft in negative G. The carburetted early Rolls Royce Merlin engine would drain its carb float bowls and stall the engine when its pilot tried to follow the maneuver.


    In 1954 when Mercedes released its new race-car made sports-car to the general public, it was one of the first vehicles with Mechanical Fuel Injection, producing an impressive 215 BHP and reasonable mileage even by today's standards. The essence of MFI was easy, fuel pressure and valves (depending on the maker deemed what style) shot in a given amount of fuel usually measured by the revs of engine and by how much the throttle was pushed down. Generally the biggest problem with MFI seemed to be the engine's desire to keep running or pumping fuel in to the cylinders after the car was intended to be turned off. After 1957 however Bosch released its secrets to the rest of the world just after Mercedes decided to kill the 300SL Gullwing.

    Mechanical fuel injection was used with engines made by the likes of 1950s GM and Mopar high performance vehicles, 1960s Corvettes, Audi and Mercedes-Benz, but fell out of favor with the perfection of electronic fuel injection. A lot of the old American systems were unreliable or poorly understood, and many owners switched them out for the familiarity and reliability of carburetors. The European systems came a little later and had the kinks more or less worked out. See Fuel Injection for more detail on the history of automobile fuel injection systems, both mechanical and electronic.

    Early mechanical fuel injection was employed mostly for racing, and land speed records in the 1950s and 1960s. Enderle and Hilborn were two of the early innovators. These systems were used because EFI (electronic fuel injection) in the day was unreliable due to the electronics involved.

  • Created on Tuesday, 14 October 2008 19:54   print
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