Page 42 - Instrumentation and Measurement

3.4 Case Studies

The following are examples of closed-loop control systems to illustrate how, despite the different
forms of control being exercised, the systems all have the same basic structural elements.

3.4.1 Control of Fuel Pressure

The modern car involves many control systems. For example, there is the engine management system
aimed at controlling the amount of fuel injected into each cylinder and the time at which to fire the
spark for ignition. Part of such a system is concerned with delivering a constant pressure of fuel to
the ignition system. Figure 3.11A shows the elements involved in such a system. The fuel from the
fuel tank is pumped through a filter to the injectors, the pressure in the fuel line being controlled to
be 2.5 bar (2.530.1 MPa) above the manifold pressure by a regulator valve. Figure 3.11B shows the
principles of such a valve. It consists of a diaphragm which presses a ball plug into the flow path of
the fuel. The diaphragm has the fuel pressure acting on one side of it and on the other side is the
manifold pressure and a spring. If the pressure is too high, the diaphragm moves and opens up the
return path to the fuel tank for the excess fuel, so adjusting the fuel pressure to bring it back to the
required value.

FIGURE 3.11 (A) Fuel supply system and (B) fuel pressure regulator.

The pressure control system can be considered to be represented by the closed-loop system shown in
Figure 3.12. The set value for the pressure is determined by the spring tension. The comparator and
control law is given by the diaphragm and spring. The correction element is the ball in its seating and
the measurement is given by the diaphragm.

FIGURE 3.12 Fuel supply control system.

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