The Orion VII buses with the BAE HybriDrive have such superior performance over other hybrid buses, especially GM's, I wonder why other cities purchase the GM buses?

Please unconfuse me on the series parallel distinction. Does the Orion Bus use the diesel to run a genarator that just feeds electricity to the motors? What are the Prius and the Honda Civic? Which is better? for what?

At the heart of the Orion VII hybrid bus is the HybriDriveTM propulsion system provided by BAE Systems. The system propels the bus with a single electric motor that is powered by a diesel-driven generator and an energy storage unit. Among the system's benefits:

* The engine is smaller than that used in conventional buses and runs at optimum speed for clean operation and efficiency.
* The design offers quicker acceleration, helping drivers merge into heavy traffic.
* Customers enjoy a quieter ride than on a conventional diesel bus.
* The system design eliminates the transmission, thereby removing a major maintenance item on vehicles operated in heavy stop-and-go conditions.
* A regenerative braking system uses the drive motor to slow the bus, effectively turning the motor into a generator to help recharge the energy storage system. This feature saves energy and also reduces brake wear by about one-third, reducing the frequency of brake maintenance. -- From BAE Web site

Robert, in a series hybrid such as the Orion, the engine serves only to power a generator. The generator provides the electricity that in turn powers the electric motors (or recharges the batteries), which propel the vehicle. There is no transmission. The engine runs fairly constantly, and at a constant speed.

In a parallel hybrid, such as that produced by GM in its version of a diesel-hybrid bus, both the engine AND the motor provide motive power. Both engine and motor, in other words, connect to a transmission, switching back and forth as condition vary. Although the parallel hybrid can run just on electric power at low speed for a short period of time, most often the electric motor is used in support of the engine.

The Prius and Hondas are all parallel hybrids.

As to what’s better in terms of the transit buses, part of the answer—as always—depends on what the driving conditions might be. But we’re also gaining an increasingly large body of operational data that seems to be pointing more in the direction of the series hybrid than the parallel.

There have been reports from cities (notably Seattle, with the largest deployment of GM hybrid buses to date) that the expected fuel economy improvements just are [not] materializing. On the other hand, the operational experience with the Orions seems consistently postive. Hence the upsurge in order over the past few months.

The parallel hybrids are clearly more complex mechanically, as they have to deal with integrating two sources of tractive power in the transmission.

There are also a number of different approaches to delivering that capability.

The idea behind GM’s parallel design, I think, was to optimize what each power source does best--the engine for higher-speed, longer-range runs, the electric motor for low speed and acceleration and assistance for the engine under different driving conditions.

That’s why GM is carrying over the same type of parallel design (two mode compound split) into the hybrid system it says it plans to deploy on its SUVs in another year or so.

Based on what Mike stated, I guess it boils down to:
The MTA's of cities should chose series hybrids, and the long haul bus lines should use present Diesel engine and transmission.
Any bus service in between is a toss up between Pure series and parallel.

Above, Mike meant to write "[in the GM parallel drive hybrid bus] the expected fuel economy improvements just are not materializing". In practice the BAE system is proving much superior. Without substantial changes I expect that the GM system will be as disappointing in cars as it has been in busses.

Also, one should note the significant differences between the Honda "mild" hybrid and the Toyota "full" hybrid systems. The Toyota system gives better economy and lower emissions while eliminating the conventional transmission.

Ooops. Thanks, richard. :-)

The Toyota/Ford system is actually a series-parallel design: during acceleration or going up a hill the engine will operate the generator to power the traction motor (as well as provide direct motive power) in order to provide the necessary torque (electric motors provide excellent torque) but when cruising at a steady speed the engine provides almost all the motive power directly. Honda uses a parallel system where the motor can only assist the engine, it cannot provide motive power without spinning the engine.