Reliability of the current models of instantaneous pollutant emissions

Pollutant emissions and fuel consumption of passenger cars are usually asse ssed as a function of average speed. Nevertheless a number of attempts were made to take into account the driving pattern using emission vs. instantan eous speed and acceleration models. Five quite similar models were develope d in Europe. To which extent these instantaneous models are innovative as c ompared to conventional models? In a first step the various development sta ges of an emission-based model and the various associated errors are presen ted. Among the possible sources of error, we selected modelling-induced err ors, enabling to compare rigorously the various model types in terms of per formance. The analysis was performed using the Modem model, developed from the measurement results obtained over a sample of 150 vehicles, representat ive of French, English and German fleets, over 14 representative driving cy cles under urban conditions. Average emissions as measured over these 14 cy cles are compared to emissions calculated over same cycles using this model . Modelling errors range from -51% to +57% as a function of the considered cycle and the vehicle type. This inaccuracy is quite similar for the other four European models. This demonstrates the low reliability of the models u sed, which cannot be used to assess the impact of slight changes in the dri ving pattern, sometimes leading to completely false conclusions. These mode ls are barely more precise than average speed based models. In a second ste p, a number of alternatives liable to improve the reliability of instantane ous models are contemplated: for example using another method for calculati ng acceleration, or increasing the number of speed and acceleration classes significantly. But this does not improve significantly the model reliabili ty. Limited measurements performed on a catalyst vehicle demonstrated that very high engine loads, even if not frequent, play a significant role in em issions: sometimes they yield emission values a thousand times higher. It i s therefore of prime interest to study them in a comprehensive manner. In a ddition, this demonstrates that each model should be developed from measure ments carried out over a set of representative driving cycles under real-wo rld driving conditions. (C) 1999 Elsevier Science B.V. All rights reserved.