What is a Hybrid Bus? | Demonstrations | Emissions | CertificationNAVC Hybrid Transit Bus Emissions Project
In February, 2000, the NAVC released the results of the year-long Hybrid-Electric Drive Heavy-Duty Vehicle Testing Project which put hybrid-electric, compressed natural gas, and low-sulfur fuel diesel buses through extensive tests to demonstrate energy efficiency and emission performance in comparison with conventional diesel heavy-duty vehicles. Two hybrid bus models, three CNG bus models, and one diesel bus model were selected for testing, with each model representing the most current, commercially-available version of each technology.
The buses were evaluated in over six different emission test cycles with average speeds ranging from 3 to 17 mph and with duty cycles ranging from 4 to 18 stops per mile. In addition, various fuel types were used in order to evaluate the effects of fuel sulfur levels with respect to particulate emissions. The tests measured nitrogen oxides (NOx), carbon monoxide (CO), carbon dioxide (CO2), volatile organic compounds (VOC) and particulate matter (PM). Fuel economy for each vehicle was calculated on a mile per gallon basis.
Overview of Results
The program results demonstrate that hybrid-electric buses offer significant emission reductions from that of conventional diesel buses. Hybrids showed the greatest benefits when they were operated on low-sulfur fuel. PM emissions from the low-sulfur diesel hybrids were 50 to 70 percent lower than conventional diesel while NOx emissions were 30 to 40 percent lower than conventional diesel. The hybrid electric buses also exhibited the lowest CO emission of any of the buses tested, with a 70 percent reduction from a conventional diesel bus. The hybrid results are especially encouraging since heavy-duty hybrid technology is relatively new and will improve as the technology matures.
The project demonstrated significant fuel economy benefits for hybrids with 30 to 65 percent fuel economy improvements over conventional diesel and as much as 100 percent over a comparable CNG bus when operated on severe duty cycles. The hybrid vehicle also demonstrated significantly lower total greenhouse gas emissions than that of a conventional diesel or CNG bus.
Why are hybrid bus emissions different?
There are several reasons for the reductions. Regenerative braking contributes significantly to reducing fuel consumption and thereby improving efficiency. Regenerative braking takes advantage of the energy storage system to capture the kinetic energy of the vehicle during braking. This is accomplished by using the drive motors as generators during braking to recapture the vehicle's kinetic energy and restore a portion of this energy back to the energy storage device to be used later -- for example, during acceleration.
Another contributing factor is the fact that, on a series hybrid, the engine is not directly coupled to the vehicle drivetrain (i.e., the electric drive motor alone drives the wheels). This allows the auxiliary power unit (APU) to operate independently from the vehicle. This would theoretically allow the engine/generator to operate at peak efficiency and optimized emission load points. Series hybrid control strategies typically prevent the engine from operating in zones where its efficiency may be low and its emissions high.
Detailed information on the project methodology and on buses that were tested.
In-depth review of NOx and NMOC, particulate matter, carbon monoxide, fuel economy, CO2 and greenhouse gas emissions.
You can download a PDF file of the NAVC heavy-duty hybrid report here.