The Use of Piezoelectric Washers for Feedback Combustion Control 2020-01-1146
The use of piezoelectric cylinder pressure sensors is very popular during engine testing, but cylinder pressure information is becoming mandatory also in several on board applications, where Low Temperature Combustion (LTC) approaches require a feedback control of combustion, due to poor combustion stability and the risk of knock or misfire.
Several manufacturers showed the capability to develop solutions for cylinder pressure sensing in on-board applications, and some of them have been patented. The most straight-forward approach seems the application of a piezo-electric washer as a replacement of the original washer equipping the spark plug; the injector could also be used to transfer the cylinder pressure information to the piezoelectric quartz, in diesel or GDI engines.
The paper describes the features of signals acquired using piezoelectric washers, discussing possible applications, highlighting the factors which impact the sensors accuracy, and proposing algorithms to compensate potential errors in the evaluation of combustion metrics. The sensors have been first tested on a press, then in two different gasoline engines: a naturally aspirated V12 and a turbocharged 2 cylinders equipped with a VVL system. Signals have been compared to those obtained with lab-grade cylinder pressure sensors, focusing on peak pressure location and amplitude, knock intensity and other useful information, such as inlet valve closing position.
The main issue affecting the accuracy of cylinder pressure measurement using piezoelectric spark plug washers is the effect of temperature variations both on the force transmitted by the thread to the washer and on the washer sensitivity (pyroelectricity): an algorithm to compensate these effect has been developed, with the aim of aligning the combustion metrics obtained from the washer with those evaluated using lab-grade sensors.
Enrico Corti, Marco Abbondanza, Fabrizio Ponti, Lorenzo Raggini