Heavy Duty Hybrid Vehicle Testing:

Carbon Dioxide and Methane GHG

Overview of Greenhouse Gases

Despite the fact that emission rates of most pollutants have been dramatically reduced in newer CNG and diesel buses, they remain a large source of greenhouse gases (GHGs) in many areas. The GHGs most closely identified with the transportation sector include carbon dioxide (CO2), nitrous oxide (N2O) and methane (CH4). These GHGs vary in their "global warming potential." The global warming potential of a greenhouse gas is the ratio of global warming, or radiative forcing (both direct and indirect), from one unit mass of a greenhouse gas to one unit mass of carbon dioxide over a period of time. Global warming potential recommended by the International Panel on Climate Change for nitrous oxide and methane are is shown below.

Overview of Results

The hybrid buses exhibited the lowest total GHG emissions. The Orion-LMCS and Nova-Allison hybrid buses exhibited a 20 to 40 percent, and 10 to 20 percent reduction in GHG emissions, respectively, than a conventional diesel bus. This can be primarily attributed to the capture of energy via regenerative braking to reduce the operating load on the engine.

While GHG reductions are usually associated with a switch to natural gas, the CNG buses do not demonstrate any GHG benefits due to lower fuel economy and high methane emissions that offset the natural gas fuel benefits. When the methane emissions from the CNG buses are converted to carbon dioxide equivalent, the total GHG emission from the CNG buses were comparable to that of conventional diesel buses. This phenomenon reinforces the fact that vehicle GHG emission reductions must be achieved through increased fuel efficiency.

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More on heavy-duty hybrids:

Introduction to Heavy-Duty Hybrid Buses

What is a Hybrid Bus?

Hybrid Bus Demonstrations

Certification of Hybrid Buses