Correlation of Air Fuel Ratio with ionization signal metrics in a multicylinder spark ignited engine

Document Type

Conference Proceeding

Publication Date



Department of Mechanical Engineering-Engineering Mechanics


Accurate individual cylinder Air Fuel Ratio (AFR) feedback provide opportunities for improved engine performance and reduced emissions in spark ignition engines. One potential measurement for individual cylinder AFR is in-cylinder ionization measured by employing the spark plug as a sensor. A number of previous investigations have studied correlations of the ionization signal with AFR and shown promising results. However the studies have typically been limited to single cylinders under restricted operating conditions. This investigation analyzes and characterizes the ionization signals in correlation to individual AFR values obtained from wide-band electrochemical oxygen sensors located in the exhaust runners of each cylinder. Experimental studies for this research were conducted on a 2.0L inline 4 cylinder spark ignited engine with dual independent variable cam phasing and an intake charge motion control valve. Nineteen metrics characterizing the ionization signal were determined on an individual cylinder and cycle basis. Linear and quadratic regression analysis was performed between the ionization metrics and the AFR values for the four cylinders. The results show that a number of good correlations exist between select ionization signal metrics and AFR values on individual cylinder basis. However when all four cylinders are combined, the correlations deteriorate reducing the accuracy in which AFR estimates can be made. Additionally, it is shown that various independent factors such as engine speed and load, in-cylinder charge motion, ignition timing, dilution, and type of fuel caused additional variation in the correlations.

Publisher's Statement

Copyright © 2009 SAE International. Publisher’s version of record: https://doi.org/10.4271/2009-01-0584

Publication Title

SAE Technical Papers