Mechanical fuel injection

From Wikipedia, the free encyclopedia

This article may require cleanup to meet Wikipedia's quality standards. Please improve this article if you can. (April 2007)

The introduction to this article provides insufficient context for those unfamiliar with the subject. Please help improve the article with a good introductory style.

[edit] 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 atomisizes 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 milage.

[edit] 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 engines current needs, and to do so, must sense variations in these parameters. The advent of mircoprocessors provided an elegant solution for computing the correct fuel delivery based on data from various sensors on the engine 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 atomised and combustable state. With the gasoline or petrol engines, the carburettor (essentially a venturi with a few holes dilled in it) 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 carburation was the ability of Messerschmidt Bf 109 pilots early in World War II to evade RAF Spitfire I, by manoeuvering their fuel injected Damlier-Benz power aircraft in negative G. The carburetted Rolls Royce Merlin engine would flood and cut out when its pilot tried to follow the manoeuver.

In 1954 Mercedes released its new race-car made sportscar 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 todays 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 with how much the throttle was pushed down. Generally the biggest problem with MFI seemed to be was 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 perfection of electronic fuel injection. A lot of the old American systems were unreliable, and many owners switched them out for carburetors. The European systems came a little later and had the kinks more or less worked out. The main differences between mechanical fuel injection and a carburetor are: no carburetor but instead a fuel line (or two) with an almost microscopic hole dumping into the intake manifold pulled by engine vacuum; airflow is delivered similar to electronic fuel injection, with a big hose going to a plenum that dumps into the intake manifold.

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