APC (Engine Knock Control) System

Fig. 3 APC knock control system:

This system automatically adapts to available fuel quality, limiting engine knock by continuously adjusting boost pressure. The APC system, Fig. 3, consists of a knock sensor (1), pressure transducer (2), electronic control unit (3), and solenoid valve (4). The knock sensor, mounted on engine block, senses engine vibration and produces a voltage signal proportional to the vibration. The pressure transducer senses intake manifold pressure downstream of the throttle and produces a voltage signal proportional to boost pressure. The control unit monitors knock sensor and pressure transducer signals and engine RPM, in order to control solenoid valve operation. The solenoid valve regulates control pressure signals to the boost pressure regulator depending upon electrical signals from the control unit.
With engine running, the solenoid valve oscillates at a fixed frequency, opening and closing one time each 1/12 second (one cycle). Boost pressure is regulated by varying pulse width, which is the ratio between the amount of time the valve is closed to the amount of time the valve is open during one cycle. Pulse width is controlled by signals from the control unit.
When the solenoid valve is closed, full control pressure reaches the pressure regulator, and boost is controlled at a basic setting. When valve is open, control pressure is vented through a calibrated orifice to the compressor intake, and the regulator allows boost to increase.