Heavy Duty Hybrid Vehicle Testing:

Fuel Economy

Why is fuel economy important?

The United States imports more than half of the oil it consumes. The trade imbalance created by oil imports can pose a major threat to the nation's economy should foreign sources of fuel oil become disrupted. Congress has directed federal agencies to implement programs to reduce the nation's consumption of imported fuels. One of the primary strategies has been the development of advanced vehicle technologies, like hybrid-electric, that reduce fossil fuel consumption. Another strategy is to utilize domestically produced fuels such as natural gas.

Overview of Results

There were consistent fuel economy improvements of nearly one mile per gallon for the Orion-LMCS Hybrid and nearly one half of a mile per gallon for the Nova-Allison Hybrid over conventional diesel buses on the NY Bus, CBD and Manhattan cycles. While these may seem like small numbers, bear in mind that, on the NY Bus cycle, the best performing diesel buses only achieved 1.4 mpg fuel economy versus 2.3 mpg for the Orion-LMCS hybrid-electric bus. This equates to about a 65 percent fuel economy improvement for the Orion-LMCS Hybrid on the NY Bus cycle over a conventional diesel.

Fuel economy from CNG buses was 10% to 20% lower than conventional diesel buses. This fuel economy penalty is attributed to both lower CNG engine efficiency (when compared to diesel) and higher overall vehicle weight (due to added CNG storage tanks).

Weight vs. regen braking

The hybrid-electric vehicles tested under the NAVC project are essentially conventional buses with hybrid-electric drive systems. As a result, the hybrid-electric buses weigh more than conventional diesel buses (CNG buses are heavier as well) due to the extra weight associated with the batteries (or CNG tanks in a CNG bus). However, much of the additional energy used for accelerating this weight can be recovered via regenerative braking in the hybrid-electric vehicle, although inefficiencies in the drive motors, differential and batteries prevent the capture of all of this energy.

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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