Carbon emissions, the depletion of natural resources, traffic congestion and the rising cost of fossil fuels are all issues pushing the world to search for alternative means of transportation. Mass transit buses are now able to reduce emissions by as much as 75% through the use of hybrid-electric propulsion systems. A hybrid-electric (HEV) bus (or plug-in hybrid-electric (PHEV)bus) is powered by an electric motor, and a smaller-than-normal, conventional internal combustion engine that operates at periods of maximum efficiency. In general there are two different types of hybrid systems – parallel and series hybrids. The type of hybrid system employed depends upon the expected duty cycle of the bus; however, in both systems a regenerative braking system is employed. That system is comprised of ultracapacitors that absorb and store virtually all kinetic energy from the braking system so that it can be used later in the propulsion system.