Hybrid transit buses have become a valuable and proven choice for fleet operators looking to reduce maintenance costs, increase fuel economy and lower emissions. The marketplace has responded to the benefits of these high-tech vehicles with a very strong vote of approval.
Nearly one-third of all new transit bus orders have a hybrid drivetrain specified, and as of 2010 almost every major city bus OEM has a hybrid offering in its city bus lineup.
With hybrid technology now having gone mainstream in the public transit industry, one of the questions forward-looking fleet operators are asking is if there are substantial improvements yet remaining ahead from the current production models.
We see, in fact, that the hybrid transit bus still has considerable continued evolution and improvement ahead which will result in even more dramatic operating cost reductions, fuel efficiency improvements and continued emissions and noise reductions. This evolution is being driven by improvements in what has always been the weakest link in hybrid drive technology: the energy storage system (or in normal-person speak, “the batteries”).
Engine Dominant Hybrids vs. Battery Dominant Hybrids
To understand how improvements in battery technology can enable even lower operating costs, increased fuel efficiency and quieter hybrid vehicles, it’s necessary to first understand the difference between engine-dominant hybrid and battery-dominant hybrid bus architectures.
The majority of production hybrid transit bus vehicles that have been fielded to date have engine-dominant hybrid drivetrains. An engine-dominant system utilizes the smallest possible battery system for storage of energy in the drivetrain and is typically equipped with the same size, or only a slightly smaller engine than a non-hybrid vehicle.
Engine-dominant hybrid systems have been the lowest first cost and technologically the most conservative approach toward introducing hybrid drive-train technology onto transit bus platforms. Engine-dominant hybrid designs have also provided a vehicle that was more “familiar” to fleet maintenance personnel, with engines that had the same maintenance requirements and replacement parts as pre-existing fleets.
The engine-dominant approach was the most appropriate design architecture for the OEM’s who first introduced heavy-duty hybrid drivetrain technologies more than 10 years ago, since historically batteries were by far the weakest portion of the technology available at the time. The original Orion hybrid buses had lead-acid batteries, creating significant weight restrictions for moving to all but the smallest energy capacity battery possible (back then advanced batteries such as lithium-ion were not even an option). Allison Transmission, with its first production hybrid debut in 2003, went the path of nickel metal hybrid batteries, which were a proven advanced battery of the time.
Lithium-Ion Batteries — Changing the Rules in Drivetrain Design
In the nearly 10 years since these original hybrid buses demonstrated their basic value to the marketplace, significant progress has been made in commercializing more advanced batteries suitable for heavy-duty transportation application, such as lithium-ion. Lithium-ion batteries have long been viewed as a breakthrough technology for vehicle electrification. BAE Systems was one of the first major drivetrain providers in the heavy-duty hybrid industry to embrace the benefits of lithium-ion and switched over to lithium-ion batteries for its production hybrid drive trains in 2008. This new lithium pack was based on aggregating many thousands of small flashlight-sized lithium batteries into a single battery pack, while still keeping with the overall engine-dominant architecture of the drivetrain.