Comparative study of different engine knock metrics for bracketing the octane number of fuels

Abstract

This study presents a comparative analysis of different engine knock metrics used to evaluate the octane number (ON) of fuels in a Cooperative Fuel Research (CFR) engine. The knock metrics examined include knock intensity 20 (KI20), the maximum amplitude of pressure oscillations (MAPO), the maximum pressure rise rate (MPRR), the cumulative knock intensity (CKI), and the wavelet decomposition energy (WDE). Modified versions of standard CFR engine tests were conducted using both liquid and gaseous fuels, covering a range of research octane number (RON) from 60 to 100. The knock data were collected using both a detonation meter and an in-cylinder pressure transducer to compare traditional and pressure-based knock measurement methods. Results indicate that of the metrics investigated, MPRR is the most effective for bracketing octane numbers, showing higher validity and a closer resemblance to knockmeter readings compared to the others analyzed. Furthermore, the study explores the knock resistance of hydrogen, revealing discrepancies with standard RON evaluations. The findings of this work indicates that hydrogen’s RON, evaluated based on MPRR, falls within the range of 98–100. The results provide valuable insights for improving knock measurement accuracy, especially when evaluating fuels with high knock resistance, and for optimizing modern engine designs to meet emerging fuel standards.