Individual cylinder Air-to-Fuel Ratio (AFR) control has been proposed by many authors in recent years as a technique of controlling the AFR of the various cylinders individually, based on a single lambda measurement for each engine bank. Most of such works describe theoretical and experimental efforts to develop and identify an observer, able to estimate the AFR of each cylinder separately.
In this paper, a simple individual cylinder AFR controller is described, based on the observation that any type of AFR disparity between the various cylinders is reflected in a specific harmonic content of the AFR signal spectrum. In particular, any type of AFR disparity will be reflected on a limited number of frequencies, or harmonics, multiple of the engine cycle frequency. As a consequence, a way to reduce such disparity is to use a closed-loop controller (for example a PI controller) that modifies each cylinder injection duration until the amplitude of the harmonics that are excited by the AFR disparity is reduced under a certain level. At the same time, a second closed-loop controller, slower and identical for all the engine cylinders, guarantees the desired AFR mean value.
The proposed AFR individual cylinder closed-loop controller has been tested in real time, by implementing it in a virtual Electronic Control Unit, using rapid control prototyping techniques. The results observed on a 4 cylinder Spark Ignition 1.2 liter engine are encouraging, since in most of the engine operating conditions the controller is able to guarantee AFR inequality below 0.01 lambda.