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The ABC's of Fuel Injection

Rochester, Products Div.
Gen. Motors Corp. 

Rochester, N.Y.
The ABC's of Fuel Injection
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What is Fuel Injection?

     Fuel Injection is a means of supplying fuel and air to an engine. Though its job is similar to carburetion it is much more efficient in many ways. From the illustration you can see that the most notable difference from carburetion is that the fuel is not mixed with the air until the air has very nearly reached the cylinder. In the carburetor fuel and air are mixed in the venturi and the intake manifold transports the mixture to the cylinders: in a fuel injection system the manifold carries only air and the fuel is sprayed under pressure into the air stream, either at the intake valve port or in the cylinder itself. The big difference then is in the fact that the manifold is not required to transport fuel/air mixture. It has been manifold limitations which have caused many carburetion problems and which led GM engineers to develop a new fuel injection system.

Why F.I.?  

    Carburetors have come a long way over the past years, and engineering and production methods have combined to produce carburetors which get the most possible out of an engine. Carburetion does, however, have some limitations which can be overcome by fuel injection.


    One of the most important advantages of fuel injection is its ability to divorce the fuel equally between all cylinders. From the illustration showing an exaggerated 8-cylinder manifold, it can be seen that when the manifold carries fuel/air mixture to a variety of sizes and lengths of passages, it is very difficult to feed each cylinder in equal amounts. As a matter of fact, it would not be uncommon to have 15% difference in fuel/air ratio between the leanest cylinder and the richest cylinder of a given engine with a carbureted fuel system. The main difficulty is that air is quite willing to flow around corners and through various shaped passages but the fuel, being heavier, is bothered by obstructions, curves, etc. In fuel injection fuel can be fed under pressure through a set of calibrated nozzles, one for each cylinder so that the fuel charge for each cylinder is virtually equal.

    You can see that in the carbureted system it would be necessary to supply mixtures rich enough so that no cylinders were too lean, which means that there would be waste in the cylinders which were already rich enough. The engine equipped with fuel injection can often be run as much as 10% leaner than it would have to be with a carburetor and manifold.


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