Fuel Consumption and Exhaust Emissions From a Heavy-Duty Hybrid Bus

The introduction of prototype heavy-duty hybrid vehicles introduces a number of challenges in assessing emissions performance compared to conventionally powered diesel or gasoline-fueled, heavy-duty vehicles. Difficulty is encountered because the engine may be operated on an intermittent basis (as a function of load or state of charge of the energy storage system) and in a narrow speed/load range. An engine test alone would not characterize the hybrid vehicle's emissions or fuel economy. Therefore, in this project, heavy-duty chassis dynamometer emission measurements were used to benchmark the fuel consumption and exhaust emissions of a heavy-duty hybrid vehicle against a conventionally powered vehicle. The hybrid bus was powered with a CNG-fueled, VW 2.0L engine. Exhaust emissions from the hybrid bus were compared to a 1996 model year diesel-powered bus operated over the same driving cycles, and using the same inertia weight and road load as the hybrid bus. SwRI noted that the aftermarket CNG fuel system installed on the 2.0L VW engine did not function adequately; therefore, the exhaust emissions from the hybrid bus could have been much better if detailed optimization had been performed. In fact, NOx emissions were 25 to 30 percent higher than for the diesel bus. However, even with the non-optimum CNG fuel system, the exhaust emissions of NMHC and CO were significantly lower than observed for a comparable diesel bus. Although not directly measured, PM emissions from the hybrid bus were assumed to be essentially zero. Another significant finding was that the fuel consumption of the hybrid bus was 13 to 30 percent better than the diesel bus over the CBD-14 cycle, and 38 to 45 percent better than the diesel bus over the HDCC.