53 Combustion knock sensors

A knock sensor that is commonly used in engine control systems utilizes the piezoelectric generator effect, i.e. the sensing element produces a small electric charge when it is compressed and then relaxed. Materials such as quartz and some ceramics like PZT (a mixture of platinum, zirconium and titanium) are effective in piezoelectric applications. In the application shown, the knock sensor is located on the engine block adjacent to cylinder number 3 (Fig. 5.10). This is the best position to detect vibrations arising from combustion knock in any of the four cylinders.

Because combustion knock is most likely to occur close to TDC in any cylinder, the control program held in the ECM memory enables the processor to use any knock signal generated to alter the ignition timing by an amount that is sufficient to eliminate the knock. When knock has ceased the ECM will advance the ignition, in steps, back to its normal setting. The mechanism by which vibrations arising from knock are converted to electricity is illustrated in Fig. 5.11.

The sensor is accurately designed and the center bolt that pre-tensions the piezoelectric crystal is accurately torqued. The steel washer that makes up the seismic mass has very precise dimensions. When combustion knock occurs,

Fig. 5.10 The knock sensor on the engine
Fig. 5.11 The principle of the piezoelectric combustion knock sensor

the resulting mechanical vibrations are transmitted by the seismic mass, to the piezoelectric crystal. The 'squeezing up' and relaxing of the crystal in response to this action, produces a small electrical signal that oscillates at the same frequency as the knock sensor element. The electrical signal is conducted away from the crystal by wires that are secured to suitable points on the crystal.

The tuning of the sensor is critical because it must be able to distinguish between knock from combustion and other knocks that may arise from the engine mechanism. This is achieved because combustion knock produces vibrations that fall within a known range of frequencies.

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