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The present paper analyses the results from a series of experiments involving tests of catalysts under a stepwise constant engine idle speed schedule in the form of a step pyramid. This test speed schedule consists of an acceleration sequence formed by a number of abrupt speed increments followed by constant speed periods, and a deceleration sequence formed by the same speed changes executed in the reverse order. During the experiments the CO, HC, as well as the catalyst outlet - inlet temperature difference were monitored. The experimental results indicated that as the catalyst efficiency deteriorated with age, the rise of pollution levels was accompanied by significant changes in the CO and temperature difference signals. The HC signal was not as strongly affected. There are positive indications that the outlet - inlet temperature difference signal can be used as an input to a microcontrolled catalyst efficiency assessment system, capable of operating under driving conditions.

The present paper introduces a new method of assessing catalyst performance based on inlet-outlet temperature difference measurements. These preliminary experimental results indicate that at idle speed the age and catalytic efficiency affects this temperature difference reducing outlet temperature as the catalyst ages. The reduced efficiency of the catalyst increases also the time required for the equalization of the inlet and outlet temperature. An additional phenomenon present in these experiments is the gradual appearance of nonlinear time delayed effects as catalyst age increases that are reflected in the autocorrelation function of the temperature difference signal. These effects can be used for a catalyst performance assessment using temperature probes instead of the normal exhaust gas chemical analyzers.