Hybrid Drive Systems

The essentials of their operation, care and safety


It bears mentioning that last year, Mack Trucks delivered a diesel-electric hybrid Mack TerraPro Low Entry model refuse truck to the city of New York. It is the first production intent parallel diesel-electric hybrid truck in the U.S. designed specifically for Class 8 heavy duty applications and meeting the U.S. EPA's 2010 diesel emission regulations.

The Mack diesel-electric hybrid powertrain features an integrated tarter, alternator and electric motor. The system captures energy from braking in the form of electricity (regenerative braking), stores the electricity in a 600-volt lithium ion battery pack and uses it to power the electric motor, which assists the Mack MP7 diesel engine with propulsion of the truck.

The Mack system is what is known as a parallel hybrid because the diesel engine and electric motor can operate either in unison or as separate units. There are three possible modes: pure diesel power, such as at highway speeds; pure electric power, useful for quiet operations in residential neighborhoods; and combined diesel-electric mode, when maximum power is desired, such as accelerating from a stop with a full load.

The combined diesel-electric mode improves performance and requires less fuel than diesel-only.

When the parallel hybrid vehicle decelerates, electric energy is returned to the battery pack using the same hybrid system.


Azure Dynamics Corporation is a company that develops electric and hybrid electric drive technology for light to medium duty commercial vehicles. It offers a range of hybrid systems, including its Balance Hybrid Electric drive system on Ford E450 cutaway and strip chassis trucks.

This system, which manages the conventional 5.4-liter Triton gasoline engine and 5-speed automatic TorqShift transmission, features electric-launch assist, engine-off at idle and regenerative braking, all of which combine to improve fuel economy and reduce idling for fewer greenhouse gas emissions, explains Bill Nash, Azure Dynamics' product support manager.

This hybrid system runs on either the electric motor or the gas engine. Output to the drive wheels is through a common driveshaft. Although the gas engine will start and run at the beginning of a daily work cycle, it will shut off automatically after warm up.

The vehicle operates on the electric motor at speeds below about 25 mph with the gas engine starting automatically as needed for high-voltage battery charging or acceleration demands. At highway speeds, the gas engine provides driving power and continues to charge the high-voltage battery pack as needed.

Azure Dynamics' CitiBus product is a series hybrid electric system on a Workhorse chassis that comes in a gas or diesel version. In this type of system, the conventional transmission and driveline is replaced with an electric motor to drive the rear wheels. The engine is not coupled directly to the drive wheels and serves only to power a generator to charge the high-voltage battery back which powers the electric motor. On deceleration, the electric drive motor serves as a generator and converts rolling energy into high-voltage battery charging ability.

Regenerative energy is stored in the high-voltage battery. This energy is reused by the electric motor while driving the vehicle. When the vehicle comes to a stop, the engine will often shut off, Nash explains. When this happens, the energy stored in the battery pack runs the auxiliary drives, such as the power assist systems (brakes and power steering), electric air conditioning system and the DC/DC converter which charges the 12-volt batteries.

This CitiBus also has the capability to start the engine and charge the high-voltage battery using the generator or to start the engine with the 12-volt starter as a back-up.

Another product is the Force Drive battery electric drivetrain which will be integrated on the Ford 2010 Transit Connect van - a full electric, plug-in system that is much less complex systems than other hybrid systems, says Nash. According to Ford, the Transit Connect BEV model has a targeted range of 80 miles minimum on a full charge and zero tailpipe emissions.

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