In 2009 we acquired a 2009 hybrid school bus driven by a six liter MaxxForce Diesel Engine. It’s still running around today and I want to go through how it operates and describe some of the components that make this system work.
The whole idea behind a hybrid is to assist the launch of the school bus out of the gate using battery power (kilowatts) and a generator that is integral to the drive shaft. This is the main method to save fuel since full throttle acceleration from a standing start uses a lot of diesel over a period of time.
The hybrid system also runs up to 30 miles an hour assisting the torque load required during driving mode. The generator starts the engine after a short delay in the crank position on the key switch. The hybrid also helps with braking (retarder) reversing current and also recharging the batteries at the same time.
The generator is water cooled and there is a cooling fan to keep battery temperatures down as well. The generator cooling system consists of a water pump, radiator and reservoir similar to a standard automotive design. The reservoir level has to be maintained at all times to keep a consistent and safe generator temperature when it’s working consistently driving the bus.
The computerized control unit (CEU) monitors the speeds and loads and supplies amperage as needed and controls battery recharging as the bus is coasting downhill. Light braking recharges the battery pack as well. The coolant water pump is computer controlled to keep up with demands from the generator which creates more heat as it charges batteries and retards the forward progress.
I can’t say that this hybrid bus is better than our diesel buses because of the lack of power compared to a diesel engine. That’s the sacrifice that needs to be made to save on NOx from diesel emissions. They are very expensive and no one in our shop has been trained to repair a hybrid vehicle.
We were instructed on the safety aspects of the hybrid and general knowledge on how this system works by the factory representative but any trouble experienced will require the dealer alone to find the problem. One of the main safety requirements is to shut down the main switch on the battery pack since there is a very high voltage in the lithium ion batteries. Any orange coloured cables are designated as high voltage.
Thanks for reading this information and I hope it enlightened you on a 2009 medium duty hybrid school bus works. Leave your comments if you desire or leave a question.
