Today, lithium-ion battery technology is more beneficial for transit bus application and continues to surface in the marketplace. Lithium-ion batteries are available at lower costs than ever before and changing all the rules behind what is possible with hybrid bus drivetrain design. Newer, “large-format” lithium batteries have cell sizes much larger than previously available, resulting in much lower battery pack integration costs, higher reliability and higher energy densities. Using a common engineering rule of thumb, it’s a much better design and more reliable approach to utilize hundreds of batteries vs. thousands when possible. The resulting pack generally will have more energy for the weight and be more compact.
It is these new large format lithium-ion batteries that are finally providing the necessary building blocks to transit vehicle OEMs to construct viable battery-dominant approaches to the hybrid bus drivetrain.
In the battery-dominant hybrid systems, the engine size is reduced much more dramatically vs. a non-hybrid vehicle. While highway speeds are always a must, with some flexibility on the fleet operator’s continuous highway speed requirements (i.e., 30 minutes to 1 hour continuous highway speed vs. sustained), the transit bus engine size can be reduced even more dramatically by a factor of up to four times. In compensation, the sizing of the battery in a battery-dominant system will be two to four times larger than that found in today’s engine dominant-hybrid drivetrains. A 60 kilowatt hour lithium-ion battery pack based on large format lithium cells and considered large enough for a 40-foot battery-dominant hybrid transit bus can weigh approximately 1,800 pounds. It is batteries of this size, where very high energy density is a requirement, that make large-format lithium-ion the battery of choice, whereas other more traditional advanced batteries — such as nickel metal hydride — could weigh up to 3,500 pounds for the same amount of energy.
Plug-in Hybrid Approach
So now that we know the differences between an engine-dominant and battery-dominant hybrid bus, what are the benefits of going with the more advanced drivetrain?
Running a smaller engine supplemented by a much deeper reservoir of battery power on the battery-dominant drivetrain will provide even further fuel economy and emissions benefits than possible with the engine-dominant approach. Likewise, the much larger battery will allow a measurably larger percentage of regenerative breaking to be absorbed and then discharged than the smaller battery on the engine-dominant drivetrain. This is because batteries operate more efficiently when the ratio of power being pushed in and out of them vs. their total energy capacity is lower, allowing the hybrid to be even more “hybrid.”
However, one of the even bigger drivers behind increasing fuel economy and lowering tailpipe emissions in favor of the battery-dominant hybrid is the ability of these buses to run in pure electric mode for very significant periods of time. Because the battery has a much greater reserve capacity to drive the bus in pure electric mode with the engine off, a “plug-in” hybrid approach can be utilized by fleet operators as an option. This plug-in hybrid approach would involve charging the vehicle battery at night or in between stops with “plug-in” power from the electric utility grid. Today’s lithium-ion batteries can be fully recharged in as little as two hours, with a significant amount of charge possible in as little as one hour.
This plug-in hybrid approach enables a portion of the hybrid transit bus’s “fuel” to be derived from grid charging vs. the diesel fuel tank. It has been well documented that increasing the mix of vehicle energy derived from grid electricity vs. an onboard fueled engine leads to dramatically higher fuel efficiencies, lower fuel costs and reduced tailpipe emissions and CO2 generation.
If this “plug-in” hybrid approach sounds resoundingly familiar, it should — think of the Chevy Volt. Just as the battery-dominant Chevy Volt has recently been released by Detroit as one of the most evolved of the various hybrid automobile platforms, so too is the battery-dominant approach to hybrid city buses now becoming available to fleet operators as the most evolved of the heavy-duty hybrid drivetrain architectures.
Other benefits can be specific to the fleet operator. For example, the ability to shut down the vehicle engine for significant periods of time enables fleet operators to tailor the noise performance of the buses when traveling through pedestrian districts.