Performance and regulated emissions from a Euro VI-D hybrid bus tested with fossil and renewable (hydrotreated vegetable oil) diesel fuels under urban driving in Bilbao city, Spain.

Renewable liquid fuels are a promising alternative solution to decarbonize the transport sector. However, the analysis of emissions of local contaminants coming from the use of such fuels relevant to preserve air quality and related human health issues. Even though the advantages in reducing climate change using renewable fuels appear to be clear, there are still discrepancies on the effect of fuel composition on engine performance, power, and emissions of local pollutants. In the present work, the performance and regulated emissions from a Euro VI-D hybrid bus using conventional fossil diesel fuel and renewable fuel (hydrotreated vegetable oil (HVO)) under urban driving conditions have been studied. The tests were carried out using an on-board modular measurement system that registers emissions, exhaust gas parameters and records data from the engine, as well as environ-mental conditions. The registered CO2 emissions and calculated mass, volumetric and energy fuel consumptions were lower when using the renewable fuel, despite the lower volumetric heating value compared to fossil diesel fuel, which is partly attributed to the lower EGR rate with renewable fuel that leads to a higher energy efficiency. Additionally, CO and THC emissions coming from the combustion process of renewable fuel are lower than those of the fossil fuel, given the paraffinic structure of renewable fuel that favors a cleaner combustion. During cold operating conditions lower NOx emissions were registered using the renewable fuel in comparison with the fossil diesel fuel, although slightly lower EGR rates were observed. Finally, the average particle number obtained in the tests is higher when using renewable fuel, but differences are not considered as statistically significant. The results obtained regarding performance and emissions suggest that the renewable fuel could be used as partial or total substitute of diesel fuel with a reduction of direct CO2 and gaseous pollutant emissions in most of the conditions. The developed methodology highlights the relevance of analyzing cumulative values, at least for CO2 and particle number concentrations, before analyzing all the results after testing a vehicle under driving con-ditions, allowing the detection of any anomaly in the measurements of the different parameters. Future work may be addressed for electrified powertrains combined with renewable fuels and ad-hoc optimization of ECU settings for renewable fuels to improve pollutant emissions performance.