Cycle-by-Cycle Variations in Exhaust Temperatures Using Thermocouple Compensation Techniques

Exhaust gas temperatures in a 1.4 L, sparked ignition engine have been measured using fine wire thermocouples at different loads and speeds. However the thermocouples are not fast enough to resolve the rapid change in exhaust temperature. This paper discusses a new thermocouple compensation technique to resolve the cycle-by-cycle variations in exhaust temperature by segmentation. Simulation results show that the technique can find the lower time constants during blowdown, reducing the bias from 28 to 4%. Several estimators and model structures have been compared. The best one is the difference equation-least squares technique, which has the combined error between -4.4 to 7.6% at 60 dB signal-to-noise ratio. The compensated temperatures have been compared against combustion parameters on a cycle-by-cycle basis. The results show that the cycle-by-cycle variations of the exhaust temperatures and combustion are correlated. For instance, the mass fraction burnt is positively correlated with cycle-averaged temperature. When there is a larger variability in combustion, the exhaust temperature also exhibits higher cycle-by-cycle variations. Overall, the results have confirmed that the segmentation technique has successfully resolved the cyclic variations in exhaust temperature, and that the cycle-by-cycle variations in the exhaust temperature can be used to infer the combustion variability.