Counter Pro Course #6
Fuel Injection #1
What fuel injection systems are and how they work:
We assume you have read and passed the Carburetor/Fuel Pump Counterman Training Course. Using what you learned in that course, we will begin this course by making a number of comparisons between carburetor systems and fuel injection systems. This will help you understand fuel injection systems more easily.
Like carburetor systems, fuel injection systems must be able to perform under all engine conditions, including cold and hot starting, idle, off idle, cruise, wide-open throttle and acceleration.
Both carburetor systems and fuel injection systems handle the blending of fuel and incoming air in an attempt to create that "perfect" stoichiometric ratio of 14.7 pounds of air to one pound of fuel, which is necessary for complete combustion in the chamber. Also, both systems must handle this blending of fuel and air in a wide variety of engine operating speeds, loads, etc. However, carburetor systems and fuel injection systems differ significantly in the method used to introduce the fuel.
To push fuel into the intake airstream of the engine, a carburetor uses static air pressure (ordinary barometric air pressure which results from the layer of air extending from the earth into the atmosphere). As explained in the carburetor/fuel pump training course, a venturi in the carburetor speeds up the flow of air entering the carburetor throat. This venturi action produces a pressure differential between the float section and various other air entry points in the carburetor. Since the intake manifold pressure becomes less than atmospheric air pressure, it is typically considered a negative pressure differential. This pressure differential causes the fuel and air to become mixed. This air/fuel mixture is then forced by atmospheric pressure to various fuel delivery passages and ports, which feed the mixture into the engine.
Unlike a carburetor, a fuel injection system needs no venturi action or drop in air pressure within the intake system to get air and fuel into the system. Instead, fuel is delivered into the airstream by a high-pressure fuel pump, which provides a very high positive pressure differential to force fuel into the airstream. The basic air metering device is a throttle plate, which is controlled by the accelerator pedal system.
In both systems, there is a "push" which causes the fuel flow. In a carburetor system, the push is a differential between atmospheric air pressure and lowered air pressure within the carburetor. If there is no pressure differential, the fuel will not flow.
In a fuel injection system, the push comes from a pump, which provides a positive pressure source. The pump has the power to force fuel into the manifold, whether or not there is a pressure differential between the outside air pressure and the air pressure within the intake manifold.
There are two types of true fuel injection systems in use on today's cars and light trucks. One type is called "mechanical" or "continuous," and the other is called "electronic". Both types are composed of two primary mechanisms:
1. A basic control unit, which determines where, when and how much fuel will be delivered at any given instant.
2. The fuel injector. The fuel injector will be discussed first.
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