RTA Hybrid Buses, In-depth

Regional Transit Authority's South County Area Transit hybrid-electric bus operates on a "series" drive system, in which the engine is completely decoupled from the driveline and is used only to generate electrical power. This type of hybrid architecture is especially attractive for large vehicles that perform large amounts of stop-and-go driving, such as urban transit buses and delivery trucks. Conventional buses and trucks of this type are highly inefficient and produce high levels of toxic emissions because they have large (typically diesel) engines that are constantly ramping up and down - the least efficient way to operate a power source. In the series hybrid system, a smaller engine is mated to a generator and operated at a constant, efficient speed and power output level. When vehicle power requirements temporarily increase - such as during acceleration or hill-climbing - additional power is drawn from an onboard energy storage system comprised of batteries or ultracapacitors (a new type of energy storage device). When vehicle power requirements are low, the energy storage system is recharged. Not only is engine efficiency increased, but the vehicle is able to recapture energy whenever it slows down through a process called "regenerative braking."

In the RTA-SCAT hybrid-electric bus' drive system, the auxiliary power unit and energy storage systems work in harmony to supply power to the motive drive system. During braking, the motive drive system recaptures energy and sends it back into the energy storage system - a process that can increase overall vehicle energy efficiency by 25% or more. The operating system constantly monitors vehicle power needs and the status of all of the vehicle's components, assuring that these power needs are met in the most efficient way possible, and provides early warning of any potential problems with vehicle components.

The main obstacle to the adoption of series hybrid drive systems has been the technological complexity of managing the interaction of the three separate power sources in such a system: the engine-generator, energy storage system, and the electric drive motor (which acts as a generator during regenerative braking). This problem is particularly challenging in heavy-duty buses and trucks, where the power levels to be managed are significantly higher than in smaller vehicles such as passenger cars. In the RTA-SCAT hybrid-electric bus, the control system uses a network architecture similar to an Ethernet computer network to monitor and control all of the hybrid system components. Each major hybrid component is controlled by a microcomputer that acts as a "gateway" to the overall network, which utilizes an operating system that uses standard automotive industry Controller Area Network (CAN) protocols. This gives the hybrid architecture greater flexibility and upgrade potential than competing architectures, as any engine or other component using CAN-based controls can be plugged into the system with a minimum of new development.