HYBRID powertrain

A new hybrid electric powertrain developed by US-based Rosen Motors in Woodland Hills, California promises to produce a high acceleration and low emissions vehicle. The design consists of a turbogenerator, a flywheel motor-generator, electric drive motors, and control system (Figure 1).

The gas turbine powered generator is used to satisfy the low average power demands of the vehicle, while the flywheel driven motor-generator is used to supply the surge power demands and recover energy from regenerative braking.

The current turbogenerator provides 30kW of electrical output at 30% thermal efficiency. But a turbogenerator having a maximum power output of 45kW is also planned.

In operation, an electronic controller dynamically selects the blend of flywheel-generated and turbine-generated power to provide the optimum performance.

The electronics controls the four motor-generators (associated with the turbine, flywheel and each of the two drive motors, respectively) in the starting, accelerating, cruising, braking, hill climbing and hill descending operations of the vehicle.

In a prototype version, the flywheel and turbogenerator will produce peak power to the drive motors of 180kW, or 240hp. Combined with the torque characteristic of the electric drivetrain, this will produce acceleration of 0 to 60mph in less than 10s.

One of the most interesting aspects of the design of the hybrid electric powertrain has been the design of the flywheel assembly. The flywheel rotating assembly consists of a carbon-fibre composite cylinder connected to a steel shaft by a titanium hub. The rotor of the motor-generator and the rotating elements of the flywheel magnetic bearing assemblies are mounted on the shaft and supported by magnetic bearings that maintain the clearances between the rotating and non-rotating portions of the flywheel.

Due to the high speeds at the surface of the cylinder, the entire rotating assembly is contained in a vacuum enclosure to prevent aerodynamic drag. The residual gases from the enclosure are pumped by a molecular drag pump into a second higher-pressure chamber containing molecular sieves.

The stator of the flywheel motor-generator is cooled by passing coolant through a jacket surrounding it. This liquid also cools the stators of the magnetic bearings. The rotor of the motor-generator is cooled by radiation, primarily to the stator.

The vacuum housing is supported by a two-axis gimbal system, which isolates the rotor from vehicular angular motions. Without such isolation, resulting gyroscopic forces would prove excessive for the bearing system and the handling of the vehicle.

The vacuum housing is surrounded by a containment system which prevents escape of the rotor in the unlikely event of an accidental burst of the flywheel rotor. This containment system prevents rapid transfer of angular momentum to the vehicle during such an event.

An eventual design objective for the flywheel is for it to exhibit a long self-discharge time of at least several weeks. This has been one of the most challenging of the design issues associated with the flywheel. In the event the flywheel is allowed to run down after long storage without recharge, it can be recharged to its full spin speed by the turbine in about two minutes. In the case of a discharged flywheel, the turbine itself would be started by the 12V battery system.

At its maximum charge, the flywheel can store one kW-hr of energy, of which nearly 80% is available over the flywheel operating speed range of 28,000rpm to 60,000 rpm.

Driving the motors

The two drive motors deliver torque (through a fixed gear reduction) to the drive wheels in response to the positions of the accelerator and brake pedals. They also respond to the position of a hand lever, which is used to select forward or reverse.

The field currents of the drive motors are adjusted to obtain optimum performance for any driving condition. An electronic controller for each provides a pulse-width controlled signal, whose magnitude depends on the desired speed and torque of the drive motor.

The electric drivetrain and fixed gear reduction system results in a vehicle acceleration as smooth as that of a Continuous Variable Transmission (CVT). Moreover, maximum torque is provided by the drive motors instantaneously, in conjunction with the high power flywheel, resulting in a high vehicle acceleration.

Rosen Motors’ hybrid electric powertrain (rear wheel drive version) sports electric drive motors and a flywheel to produce acceleration of 0 to 60mph in under 10s

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