62 Petrol engine fuel injectors

In the overview of systems given in Chapter 1, it was shown that there are two basic petrol injection systems in common use. These are single-point (or throttle body) injection and multi-point injection. More recently the concept of direct petrol injection into the combustion chamber has been developed. It is the single-point and multi-point systems only that are covered here.

6.2.1 SINGLE POINT INJECTION

As the name suggests, there is a single injector. This injector is placed at the throttle body, on the atmospheric side of the throttle valve, as shown in Fig. 1.17. The fuel pressure at the injector is controlled by the fuel pressure regulator and the amount of fuel injected is determined by the length of time for which the injector valve is held off its seat. In this particular system, the fuel is injected towards the throttle butterfly where the air velocity helps to mix the fuel spray with the air. Figure 6.1 shows a typical single-point injector.

The injector valve is designed to weigh as little as possible so that it can be opened and closed rapidly. The magnetic field caused by electric current in the solenoid winding opens the valve and when the current is switched off the injector valve spring returns the valve to its seat.

6.2.2 MULTI-POINT PETROL INJECTION

In these systems, there is an injector for each cylinder. The injectors are normally placed so that they spray fuel into the induction tract, near the inlet valve. Figure 6.2 shows the construction of a common type of fuel injector as used for multi-point injection systems.

Multi-point petrol injection systems normally use a fuel gallery to which the fuel pipes of all the injectors are connected. The pressure in this gallery is controlled by the fuel pressure regulator. This means that the quantity of fuel that each injector supplies is regulated by the period of time for which the control computer holds the injector open. This time varies from approximately 1.5 ms at low engine load, up to approximately 10 ms for full engine load. Naturally, these figures will vary

1 Housing

2 Fuel duct

3 Solenoid

4 Jet needle

5 O-ring

Fig. 6.1 A single point, or throttle body injector

Fig. 6.2 A typical injector for a multi-point injection system

from engine to engine; larger capacity and more powerful engines will require greater amounts of fuel than small capacity and low powered engines.

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