Electronic fuel injection in automotives
Fuel Injectors
Fuel injectors are electromechanical devices thatmeter and atomize fuel so it can be sprayed into theintake manifold. Fuel injectors resemble a spark plug in size and shape. O-rings are used to seal the injector at the intake manifold, throttle body, and/or fuel rail mounting positions. These O-rings provide thermal insulation to prevent the formation of vapor bubbles and promote good hot start characteristics. They also dampen potentially damaging vibration. When the injector is electrically energized, a fine mist of fuel sprays from the injector tip.
Most injectors consist of a solenoid, a needle valve, and a nozzle (Figure 30–4). The solenoid is attached to the nozzle valve. The PCM controls the injector by controlling its ground circuit through a driver circuit. When the solenoid winding is energized, it creates a magnetic field that draws the armature back and pulls the needle valve from its seat. Fuel then sprays out of the nozzle. When the solenoid is de-energized, the magnetic field collapses and a helical spring forces the needle valve back on its seat, shutting off fuel flow .
Another injector design uses a ball valve and valve
seat. In this case, the magnetic field created by the solenoid coil pulls a plunger upward, lifting the ball valve from its seat. A spring is used to return the valve to its seated or closed position. Each fuel injector has a two-wire connector. One wire supplies voltage to the injector. This wire may
connect directly to the fuse panel or to the PCM, which, in turn, connects to the fuse panel. In a few
systems, a resistor under the hood or in the PCM is used to reduce the 12-volt battery supply voltage to
3 volts or less. The second wire is a ground wire. This ground wire is connected to the driver circuit inside the PCM .
Figure 30–4 A typical fuel injector used in multiport fuel injection systems. Courtesy of Ford Motor Company
The amount of fuel released by an injector depends on fuel pressure and the length of time the injector is energized. Fuel pressure is mainly controlled by a pressure regulator, and the injector’s pulse width is controlled by the PCM. Typical pulse widths range from 1 to 10 milliseconds at full load.
The PCM controls the pulse width according to various input sensor signals, operating conditions, and its programming. The primary inputs are related to engine load and engine coolant temperature. Cold starting requires the longest pulse width. Different engines require different injectors.
Injectors are designed to pass a specified amount of fuel when opened. In addition, the number of holes at the tip of the injector varies with engines and model years. Fuel injectors can be top fuel feeding, side feeding, or bottom feeding (Figure 30–5). Topand side-feed injectors are primarily used in port injection systems that operate using high fuel system pressures. Bottom-feed injectors are used in throttle body systems. Bottom-feed injectors are able to use fuel pressures as low as 10 psi.
There have been some problems with deposits on injector tips. Because small quantities of gum are
present in gasoline, injector deposits usually occur when this gum bakes onto the injector tips after a hot engine is shut off. Most manufacturers use fuel injectors designed to reduce the chance of deposit buildup at the tips. Also, oil companies have added a detergent to their gasoline to help prevent injector tip deposits.
Figure 30–5 Examples of top feed and bottom feed injectors.
