141 Singlepoint Injection

In its simplest form, petrol injection consists of a single injector that sprays petrol into the induction manifold in the region of the throttle butterfly valve, as shown at (4) in Fig. 1.13.

Finely atomized fuel is sprayed into the throttle body, in accordance with controlling actions from the engine computer (EEC, ECM), and this ensures that the correct air-fuel ratio is supplied to the combustion chambers to suit all conditions. The particular system shown here uses the speed density method of determining the mass of air that is entering the engine, rather than the air flow meter that is used in some other applications. In order for the computer to work out (compute) the amount of fuel that is needed for a given set of conditions it is

Fig. 1.13 Single-point injection details

necessary for it to have an accurate measure of the air entering the engine. The speed density method provides this information from the readings taken from the manifold absolute pressure (MAP) sensor, the air charge temperature sensor, and the engine speed sensor.

1 Housing

2 Fuel duct

3 Solenoid

4 Jet needle

5 O-ring

Fig. 1.14 The single point CFI (central fuel injection) unit

The actual central injector unit is shown in Fig. 1.14. The injector valve is operated by the solenoid (3) which receives electric current in accordance with signals from the engine control computer. When the engine is operating at full or part load the injector sprays fuel during each induction stroke. When the engine is idling the injector operates once per revolution of the crankshaft. Because the fuel pressure regulator maintains a constant fuel pressure at the injector valve, the amount of fuel injected is determined by the length of time for which the solenoid holds the valve in the open position.

The throttle plate (butterfly valve) motor is operational during starting, coasting, when shutting down the engine, and when the engine is idling.

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