TIMED FUEL INJECTION SYSTEM

Timed fuel injection systems for gasoline engines inject a measured amount of fuel in timed bursts that are synchronized to the intake strokes of the engine.   Timed injection is the most precise form of fuel injection but is also the most complex. There are two basic forms of timed fuel injection-mechanical and electronic.

The basic operation of a mechanical-timed injection system (fig. 4-43) is as follows:

• A high-pressure electric pump draws fuel from the fuel tank and delivers it to the metering unit. A pressure relief valve is installed between the fuel pump and the metering unit to regulate fuel line pressure by bleeding off excess fuel back to the tank.
• The metering unit is a pump that is driven by the engine camshaft. It is always in the same rotational relationship with the camshaft, so it can be timed to feed the fuel to the injectors just at the right moment.
• Each injector contains a spring-loaded valve that is opened by fuel pressure, injecting fuel into the intake at a point just before the intake valve.
• The throttle valve regulates engine speed and power output by regulating manifold vacuum, which, in turn, regulates the amount of fuel supplied to the injectors by the metering pump.

The more common type of timed fuel injection is the electronic-timed fuel injection, also known as electronic fuel injection (EFI) (fig. 4-44). Anelectronic fuel injection system can be divided into four subsystems:

1. Fuel delivery system
2. Air induction system
3. Sensor system
4. Computer control system

The fuel delivery system of an EFI system includes an electric fuel pump, a fuel filter, a pressure regulator, the injector valves, and the connecting lines and hoses.

• The ELECTRIC FUEL PUMP draws fuel out of the tank and forces it into the pressure regulator.
• The FUEL PRESSURE REGULATOR controls the amount of pressure entering the injector valves.
• When sufficient pressure is attained, the regulator returns excess fuel to the tank. This maintains a preset amount of fuel pressure for injector valve operation.
• The FUEL INJECTOR for an EFI system is a coil or solenoid-operated fuel valve. When not energized, spring pressure keeps the injector closed, keeping fuel from entering the engine. When current flows through the injector coil or solenoid, the magnetic field attracts the injector armature. The injector opens, squirting fuel into the intake manifold under pressure.

The air induction system for the EFI typically consists of a throttle valve, sensors, an air filter, and connecting ducts.

The throttle valve regulates how much air flows into the engine. In turn, it controls engine power output. Like the carburetor throttle valve, it is connected to the gas pedal. When the pedal is depressed, the throttle valve swings open to allow more air to rush into the engine.

The EFI sensor system monitors engine operating conditions and reports this information to the computer. A sensor is an electrical device that changes circuit resistance or voltage with a change in a condition (temperature, pressure, position of parts, etc.). For example, the resistance of a temperature sensor may decrease as temperature increases. The computer can use the icreased current flow through the sensor to calculate any needed change in the injector valve opening. Typical sensors for an EFI system include the following:

1. Exhaust gas or oxygen sensor
2. Manifold pressure sensor
3. Throttle position sensor
4. Engine temperature sensor
5. Air flow sensor
6. Inlet air temperature sensor
7. Crankshaft position sensor

Since some of these sensors were discussed in the section on computerized carburetor systems, we will only concentrate on the sensors that are particular to the EFI system. These sensors are as follows:

• The THROTTLE POSITION SENSOR is a variable resistor connected to the throttle plate shaft. When the throttle swings open for more power or closes for less power, the sensor changes resistance and signals the computer. The computer can then enrich or lean the mixture as needed
• The AIR FLOW SENSOR is used in many EFI systems to measure the amount of outside air entering the engine. It is usually an air flap or door that operates a variable resistor. Increased air flow opens the air flap more to change the position of the resistor. Information is sent to the computer indicating air inlet volume.
• The INLET AIR TEMPERATURE SENSOR measures the temperature of the air entering the engine. Cold air is more dense, requiring a little more fuel.
• Warm air is not as dense as cold, requiring a little less fuel. The sensor helps the computer compensate for changes in outside air temperature and maintain an almost perfect air-fuel mixture ratio.
• The CRANKSHAFT POSITION SENSOR is used to detect engine speed It allows the computer to change injector openings with changes in engine rpm.

The signal from the engine sensors can be either a digital or an analog type output. Digital signals are on-off signals. An example is the crankshaft position sensor that shows engine rpm. Voltage output or resistance goes from maximum to minimum, like a switch. An analog signal changes in strength to let the computer know about a change in condition. Sensor internal resistance may smoothly increase or decrease with temperature, pressure, or part position. The sensor acts as a variable resistor.

Basic operation of an electronic-timed injection system is as follows:

1. Fuel is fed by a high-pressure electric fuel pump to the injectors that are connected in parallel to a common fuel line.
2. The fuel pressure regulator is installed in-line with the injectors to keep fuel pressure constant by diverting excess fuel back to the tank.
3. Each injector contains a solenoid valve and is normally in a closed position. With a pressurized supply of fuel behind it, each injector will operate individually whenever electric current is applied to the solenoid valve.
4. The electronic computer sends the electric impulses and provides the proper amount of fuel. The computer receives a signal for the ignition distributor to establish the timing sequence.
5. By sending electric current impulses to the injectors in a sequence timed to coincide with the needs of the engine, the system will supply fuel to the engine as it should.

Figure 4-43.—Mechanical-timed injection.

Figure 4-44.—Electronic-timed injection.