Comparison of real-world fuel economy and emissions from parallel hybrid and conventional diesel buses fitted with selective catalytic reduction systems

In this research, two parallel hybrid- and two diesel-buses fitted with selective catalytic reduction systems were tested in real world conditions using portable emission measurement systems. The hybrid buses were chosen to operate in either hybrid mode or diesel mode. In hybrid mode, the buses consumed less fuel, but the brake specific fuel consumptions were higher. The hybrid buses produced less engine-out NOX emissions than diesel buses, but as a result of lower exhaust temperature and lower efficiencies of SCR systems, the tailpipe NOX emissions from hybrid buses were a little higher. The brake specific NOX emissions from hybrid buses were very high and beyond the limit value of Euro-IV standard. History effect was also important for the efficiency of SCR system. The CO emissions from hybrid buses were lower in the unit of g/s, but the fuel-based CO emissions were higher than diesel buses.