Electronic fuel injection (EFI) system is widely used in modern automobiles. An EFIsystem can be divided into four subsystems: fuel delivery system, air induction system,sensor system and computer control system.
Fuel delivery system
The fuel delivery system of an EFI system includes an electric fuel pump，a fuel filter，a pressure regulator, fuel injectors，and connecting lines and hoses，shown as Fig. 4. 7.
The electric fuel pump draws gasoline out of tank and forces it into the pressureregulator. The fuel pressure regulator controls the amount of pressure entering theinjector valves. When sufficient pressure is attained, the regulator returns excess fuel tothe tank. This maintains a preset amount of fuel pressure of injector operation. Enginevacuum is ported into the fuel pressure regulator. This allows the pressure regulator tolower fuel pressure slightly at idle speed and increase it with higher engine speed.
A fuel injector for an EFI system is a coil-or solenoid-operated fuel valve (Fig. 4. 8)．When not energized，spring pressure holds the injector closed, keeping fuel from enteringthe engine. When current flows through the injector coil，the magnetic field attracts theinjector armature and the injector valve opens. Fuel then squirts into the intake manifoldunder pressure.
Air induction system
An air induction system for an EFI system typically consists of an air filter, throttlevalves，sensors, and connecting ducts. Fig. 4. 9 pictures the air induction system for onetype of EFI-equipped engine.
Airflow enters the inlet duct and flows through the air filter. The air filter traps dustand debris and prevents it from entering the engine. Plastic ducts then route the clean air into the throttle body assembly. The throttle body assembly in multiport injection systemssimply contains the throttle valve and idle air control device. After leaving the throttlebody, air flows into the engine intake manifold. The manifold is divided into runners thatroute the air to each cylinder head intake port.
Sensor system
The EFI sensor system monitors engine operating conditions and sends information about these conditions to the control module. An engine sensor is an electrical device that
changes its electrical output（resistance, voltage，current) with a change in a condition,such as temperature，pressure, or position. For example, a temperature sensor'sresistance may decrease as temperature increases. The control module analyzes theincreased current flow through the sensor to determine if a change in injector valve openingis needed.
Computer control system
The computer control system uses electrical data from the sensors to control theoperation of the fuel injectors and other engine performance-related devices. A wiringharness connects the sensors to the input of the computer（control module). Anotherwiring harness connects the output of the computer to the fuel injectors.
The engine control module is the brain of an electronic fuel injection system. It is apreprogrammed microcomputer（Fig. 4. 10). The control module uses sensor inputs tocalculate when and how long to open the fuel injectors. To open an injector, the controlmodule completes，or closes，the circuit between the battery and injector coil. To close theinjector，the module disconnects，or opens，the circuit between the battery and theinjector coil.
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