142 Multipoint Injection

In petrol injection systems it is often the practice to supply the injectors with petrol, under pressure, through a fuel gallery or 'rail'. Each injector is connected to this gallery by a separate pipe, as shown in Fig. 1.15.

The pressure of the fuel in the gallery is controlled by a regulator of the type shown in Fig. 1.16. This particular pressure regulator is set, during manufacture, to give a maximum fuel pressure of 2.5 bar.

In operation the petrol pump delivers more fuel than is required for injection and the excess pressure lifts the regulator valve (5) off its seat to allow the excess fuel to return to the fuel tank via the return connection (6). The

1. Injector

4. Fuel gallery

5. Retaining clip

1. Injector

4. Fuel gallery

5. Retaining clip

Fig. 1.15 The fuel gallery

Fig. 1.16 The fuel pressure regulator

internal diaphragm (2) of the regulator is subject to inlet manifold pressure (vacuum) and this permits the diaphragm and spring to regulate fuel pressure to suit a range of operating conditions. Raising the diaphragm, against the spring, lowers the fuel pressure and this permits a low pressure of approximately 1.8 bar. Lowering the diaphragm, in response to higher pressure in the inlet manifold (wider throttle opening), gives a high fuel pressure of approximately 2.5 bar.

With this arrangement, the amount of fuel that each injector sprays into the inlet manifold is determined by the length of time for which the injector valve is opened by its operating solenoid. By varying the length of time for which the injector valve remains open, the amount of fuel injected is made to suit a range of requirements.

Fuelling requirements for a particular engine are known to the designer and they are placed in the ECM memory (ROM). In operation, the ECM receives information (data) from all of the sensors connected with the engine's fuel needs. The ECM computer compares the input data from the sensors with the data stored in the computer memory. From this comparison of data the ECM computer provides some output data which appears on the injector cables as an electrical pulse that lasts for a set period. This injector electrical pulse time varies from approximately 2 milliseconds (ms), to around 10 ms. The 'duty cycle' concept is based on the

A typical square waveform is shown in the figure, a single cycle is indicated by 'C', which consists of an ON time 'A', and an OFF time 'B'.

Duty cycle is the length of the ON time 'A' compared to the whole cycle 'C', expressed as a percentage. (Please note, On time can be High and Low on certain systems.)

Using the figure and the time periods, the duty cycle is 25%.

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