Development of a low-cost piezo film-based knock sensor

It is well known that spark advance is a key parameter in spark ignition engine management. Increasing fuel cost and emission regulation strictness require a higher engine efficiency, which can be improved by an accurate regulation of the spark advance. Under high load conditions, an optimal spark advance choice leads the engine to run next to the knock limit, so the management and control system needs to be equipped with a knock sensor in order to preserve the engine from damage. The authors developed a low-cost knock sensor whose sensing element is a thin washer of polyvinylidine fluoride (PVDF), a fluoropolymer characterized by a great piezoelectric effect if polarized. The sensor has been tested on a spark ignition CFR engine (the standard single-cylinder test engine used by ASTM for octane number determination of spark ignition engine fuel) and compared with a commercial accelerometer and a pressure sensor, in terms of knocking detection capability, measured knock intensity (KI) and signal-to-noise ratio (SNR). Knocking tests have also been carried out on a Renault series production engine. The collected data show that PVDF ensures a reliable detection of knock, a precise measurement of knock energy and accurate information about the frequency content of the perceived vibration. The sensor worked for several hours without depolarizing and, above all, owing to the great piezoelectric effect of PVDF, the use of a charge amplifier was unnecessary. PVDF proved to have great potential as a knock detector in spark ignition engines at a very low cost.