A supplier of digital power components and systems products, Enova Systems offers Electric, Hybrid Electric and Fuel Cell Drive Systems in three sizes 60-90 kW, 120 kW and 240 kW.
Some time ago this blog noted that a converted microcar used a cast aluminum gearbox with 10:1 gear reduction, differential, and parking pawl. The reduction gearset featured lightweight housing, helical gears, low noise and long life. It is the type of reduction gears / differential for today’s electric vehicles.
Coming back to the present, Green Car Congress noted that Enova Systems had announced a post transmission parallel hybrid system available for OEM or retrofit applications.
In addition to motor and controller, a core component of the Enova drive system is a gear reduction / differential unit. Rather than removing the OEM transmission and replacing it with one with electric assist, the electric drive comes after the transmission. How Enova Systems possibly offers such a parallel hybrid system may be to provide a double differential.
In a vehicle drivetrain the differential is after the transmission; it transfers power from the drive shaft to the wheels. If one adds a second drive gear to the differential then additional power from a different source can be applied to propel the vehicle.
Not only can such a double differential allow for electric assist, but also a suitable electric motor / controller can allow for regenerative braking. According to a quite knowledgable person on EV List, add electronic pressure-sensing pads on the brake and accelaerator pedals, which signal the motor controller, and you have a parallel system.
In their recent press release Enova Systems describes their Post Transmission Parallel Hybrid Drive as a non-invasive system. Little to no modifications to chassis, body, instrument panel, etc. is required, nor does the Enova system intrude upon, or require any alteration to, the engine control / communication system. (This avoids any emissions certification issues.) The Enova controller monitors, but avoids any direct control over the existing engine.
While it was difficult to find a better adjective this blog quibbles with use of the term “non-invasive”. Obviously, the system must be invasive for its power transmission to interface with an existing drivetrain.
Also, when the driver presses the brake pedal, engaging the regenerative braking, then the vehicle is slowed, which has a similar impact upon engine RPM as downshifting would. Regenerative braking is when the electric motor switches to become a generator; the torque turning the wheels, axle and gears in the differential then drives the generator; and, the controller shunts the electricity to batteries for storage, so that it can be used later to power the electric drive when the generator again switches to become a motor.
At the ceremony (Macromedia Flash required) announcing the first hybrid school bus, Enova Systems President Edwin Riddell claimed 40% improvement in mileage with a “blended” system that captures kinetic energy from frequent stop and go driving and returns it as electric propulsion during normal vehicle operation. Both vendors and buyers were clear about the objective: cleaner, cheaper transportation for “the kids”.
Enova Systems also is willing to offer a plug-in version, i.e., add extra batteries and the means to charge the batteries while the bus is connected to a charging station. It is unclear whether driving in an all-electric mode requires driver intervention or whether what Enova refers to as Charge Depletion operation somehow occurs automatically.
