Comparison of real driving emissions from Euro VI buses with diesel and compressed natural gas fuels

Public transport bus service is extended in big cities and cleaner bus fleets are encouraged to reduce greenhouse and air pollutant emissions in high populated areas. In this work, real driving emissions produced by two Euro VI-C buses, one fueled by diesel fuel and one fueled by compressed natural gas, were measured. Both vehicles with different engine technologies but with the same pollutant emission certification, were tested in Madrid (Spain), in order to compare their emissions under the same route. The buses were instrumented with a modular on-board Portable Emissions Measurement System, the OBS-ONE by Horiba. Regulated gaseous emissions and particle number were registered together with different vehicle parameters: speed, exhaust mass flow rate, air and fuel consumption, environmental conditions and global position coordinates during the route. THC emissions from CNG bus doubled those obtained with Diesel bus. Slightly higher NOx emissions from the Diesel bus, with selective catalytic reduction, were obtained during those parts of the circuits (mainly urban) where both the exhaust and the operation temperature of the after-treatment device were below the light-off temperature. However, the CNG bus emitted a greater number of particles, mainly on the rural sector of the circuit. Finally, NOx and particle number concentrations along the route showed that traffic is a determining factor in creating areas with high concentrations of pollutants.