Individual Cylinder Knock Detection Based on Ion Current Sensing: Correlation Analysis

Abstract

This paper addresses issues related with the measurement, analysis and real-time control of knocking combustions in high-performance spark-ignition engines. In particular, the relationship between output torque and knock intensity has been investigated. Issues examined include a methodology for identifying target knocking levels, and a critical comparison of different signals for extracting knock-related information. When considering high-performance spark-ignition engines, individual cylinder spark advance management that allows maximum output torque while protecting engine components from knock-related damage, is particularly complex. The first part of the activity is focused on an analysis aimed at the identification of a knocking level that allows reaching maximum performance while protecting engine components: For a given engine operating condition, such knocking level is shown to be constant for all the engine cylinders, and it is directly measurable through knock intensity indexes obtained by post-processing the in-cylinder pressure signal. If such knocking level is to be achieved during on-board operation, it is necessary to real-time reconstruct individual cylinder pressure-based knock indexes values. One of the main objectives of this work is therefore the evaluation of the impact that the adoption of an ion current sensing system would have on the performance of such a spark advance controller. The background of the second part of the work is the knocking-related information that can be extracted by real-time processing engine block vibration signals. The main drawbacks of such approach are related to the definition of the minimum number of sensors to be installed, to the evaluation of their optimal position, and to the signal-to-noise ratio typical of such systems, which becomes critical especially at high engine speeds. Possible solutions are the use of in-cylinder pressure or ion current sensors installed on board the vehicle. This work is mainly focused on ion sensing application, due to the still existing cost and reliability problems associated with the onboard application of in-cylinder pressure measuring systems. The second part of the work therefore deals with the correlation analysis between pressure based and ion current based knock intensity indexes. The experimental tests have been performed on a V12 6.0 liter and on a V12 6.2 liter high performance engines: Large spark advance sweeps were performed for each speed breakpoint, while acquiring 6 in-cylinder pressure and 6 ion current signals. Several indexes were extracted from both type of signals, in order to achieve both maximum correlation levels and physical consistency with knock-related damage. The results are particularly encouraging, since the correlation levels between pressure-based and ion current-based knock indexes are very high, thus allowing the definition of a closed-loop individual cylinder spark advance controller able to achieve the target knocking level.