5. Conclusions
Our experimental results indicate that emissions from both spark ignition (including common gasoline, flex-fuel and hybrid vehicles) and compression ignition vehicles are strongly and negatively affected by low ambient temperatures. Higher emissions of THC, CO, NOx, SPN and NH3 were observed when vehicles were tested at 7 C einstead of 23 C. These pollutants are important sources of the most problematic pollutants in terms of harm to human health in Europe: PM, ground-level O3 and nitrogen dioxide (NO2). However, they are not properly addressed for modern vehicles in the current EU vehicle emissions regulation for cold temperature testing (Type 6 test). For that reason, vehicular emissions of THC, CO, NOx, SPN and NH3 should be addressed in the next revision of the EU legislation of light-duty vehicle emissions at cold temperature for all vehicle technologies.
NO2 ratio (NO2/NOx) in diesel exhaust and NH3 ratio (NH3/NOx) in gasoline exhaust are higher than those observed for pre-Euro 6 vehicles. These pollutants are involved in fundamental chemical processes in the atmosphere. Thus, this strong variation of their vehicular emissions may have a strong impact on urban air quality.
CO2 and N2O (GHGs) emissions were found to be higher (9e30% higher for CO2 and up to 1.9 times for N2O) when vehicles were tested at 7 C than at 23 C. CO2 and N2O emissions are not measured or regulated under the Type 6 test. Therefore, the contribution of the transport sector to the GHG budget may be underestimated, highlighting the importance of a new and representative procedure that enables the authorities to assess the emissions from vehicles at cold ambient temperatures.
It has been observed that a large amount of emissions can take place during the last two phases of the new type approval cycle (i.e., WLTC), and not only during the cold start. This study suggests that vehicles should be tested over the entire WLTC to be able to properly assess their emissions at cold temperature.
