691 Development of a low-cost piezo lm-based knock sensor E Pipitone* and L D’Acquisto Dipartimento di Meccanica, University of Palermo, Palermo, Italy Abstract: It is well known that spark advance is a key parameter in spark ignition engine manage- ment. Increasing fuel cost and emission regulation strictness require a higher engine eciency, 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 uoride (PVDF ), a uoropolymer characterized by a great piezoelectric eect if polar- ized. 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 depolariz- ing and, above all, owing to the great piezoelectric eect of PVDF, the use of a charge amplier was unnecessary. PVDF proved to have great potential as a knock detector in spark ignition engines at a very low cost. Keywords: knock sensor, spark advance control, spark ignition engine, piezo lm NOTATION CA crank angle ECU electronic control unit IAV integral of the absolute value IFD integral of the rst derivative absolute value KI knock intensity KIR knock intensity ratio MBT maximum brake torque MFD maximum rst derivative PPV peak-to-peak value PVDF polyvinylidine uoride RMS root mean square RPM revolution per minute SNR signal-to-noise ratio WOT wide-open throttle (full load condition) The MS was received on 13 February 2003 and was accepted after revision for publication on 6 March 2003. * Corresponding author: Dipartimento di Meccanica, Viale delle Scienze, University of Palermo, Palermo, 90128, Italy. email: pipitone@dima.unipa.it D02503 © IMechE 2003 Proc. Instn Mech. Engrs Vol. 217 Part D: J. Automobile Engineering 1 INTRODUCTION Today’s spark ignition engines need very accurate spark advance regulation to improve eciency [ 1 –7 ]. The spark advance control can be either in open or in closed loop, the second being more eective in reducing fuel consumption; an accurate control of spark advance (maximum brake torque timing) can cause knocking, an abnormal combustion phenomenon that can seriously damage the engine, so the ECU cannot operate without a knock control system in order to prevent engine failure. A modern, on-board, real-time knock control system needs a sensor that fulls certain requirements: it must be compact, cheap and easy to install, and must ensure reliable knock detection and quantication. Nowadays, two kinds of sensor are mostly used in knock control systems: the pressure sensor and the accelerometer. The rst measures in-cylinder pressure, showing pressure oscillation due to knocking. High costs (especially for multiple-cylinder monitoring ) and mounting diculties (the pressure sensor must be in direct communication with the combustion chamber) are the drawbacks of such a sensor; moreover, piezoelectric pressure transducers DOI: 10.1243/09544070360692087