The paper presents the development of a methodology for evaluating the torque non-uniformity between the various cylinders of an Internal Combustion Engine (ICE). This non-uniformity can be due, for example, to pathological operating conditions such as misfires or misfuels, as well as to other abnormal operating conditions. Between the nominal torque production and the one corresponding to the absence of combustion there exist, in fact, a series of possible intermediate conditions. Each of them corresponds to a value of produced torque that lies between the nominal value and the one corresponding to the lack of combustion (due for example to statistical dispersion in manufacturing or aging in the injection system).
The diagnosis of this type of non-uniformity is a very important issue in today’s engine control strategies design. The use of the developed methodology should in fact allow the control strategy to adopt the appropriate interventions if the diagnosed non-uniformity is related to different behavior of the injectors.
In order to evaluate this torque production variability between the various cylinders, information hidden in the instantaneous crankshaft speed fluctuations has been processed using a suitable methodology. The procedure has been validated running a supercharged 2.0 liters V6 engine, and a 1.2 liters L4 engine, in a test cell. During the tests, the in-cylinder pressure signal has been acquired together with the instantaneous engine speed, in order to determine a correlation between speed fluctuations and the indicated torque produced by each cylinder. The actual cylinder by cylinder torque non-uniformity can then be evaluated on-board by processing engine speed.
The procedure is able to diagnose the absence of combustion (due for example to a misfire or a misfuel) as well as abnormal combustions that do not necessarily involve lack of combustion, with the accuracy needed for on-board use. Control interventions to injection and ignition time commands of one or more cylinders should in most cases be able to re-establish torque production uniformity.