On the Behavior of the Start and Stop System in European Real Driving Emissions Tests and Its Effect on Greenhouse and Tailpipe Emissions

The Start/Stop (S/S) system is a technology that switches off the engine without the intervention of the driver when the vehicle is stopped. The goal of this device is to eliminate the consumption of fuel associated with the idling of the engine and, consequently, save carbon dioxide (CO₂) and pollutant emissions. However, its effectiveness is related to the percentage of the total driving time with the vehicle stopped. Moreover, even if the S/S system is installed and the vehicle is stopped, the S/S system can be inhibited by the condition of the vehicle like, for example, a too low state of charge of the battery. This investigation evaluates the actual effect of S/S on tailpipe gaseous emissions in Real Driving Emissions tests compliant with the new European Regulations (E-RDE). The investigation is based on data from on-road and on-track RDE tests performed with a Portable Emission Measurement System on a diesel sports utility vehicle (SUV). From the analysis of these data, the reduction of emission guaranteed by the S/S system was found to be quite lower than the potential in the New European Driving Cycle (NEDC) test due to the limited activation of the S/S system in real driving tests. Moreover, the analysis put into evidence that the saving associated with the S/S could be counterbalanced by the engine restart especially if the stop time is shorter than a certain threshold.