adric22 wrote:As any Leaf driver will tell you, if you hop on the highway and hit 65-75 mph, your range drops nearly in half. I've noticed after driving the Volt for a while that the highway doesn't really seem to affect it nearly as much. I seem to get 40 miles in town or 40 miles on the highway.
So the question is, why? Is it a matter of aerodynamics? Or is there something about the Leaf drivetrain that makes it inefficient at those speeds? I remember reading something about Tesla having to make modifications to the model-S to improve high speed efficiency. The Volt's drivetrain is considerably more complicated to include the I.C.E., however I believe I read somewhere that it does use both of the electric motors at times even when the I.C.E. is powered down. Is that the key?
I was just thinking if the Leaf could improve the highway efficiency where one could reliably get 80 or more miles per charge on the highway that would actually make a pretty big difference in the acceptance in the USA market.
It is more sophisticated.
There are 4 modes in the Volt. 2 of the modes are for electric ONLY driving
Electric Low-Speed [in Charge-Depleting (CD) mode]: The simplest and most well-known of the Volt's four modes, this calls for the vehicle to use the charge from its 435-lb lithium-ion battery pack to power a 111-kW ac traction motor. When the traction motor turns, it transmits power to a sun gear in a planetary gear set, which turns the drive axle. Volt owners are expected to make heavy use of this mode - many will drive the car to work and back, charge it at night, and never use a drop of gasoline in the process.
Electric High-Speed [in Charge-Depleting (CD) mode]: The Volt typically hits this mode at about 70 mph. Then, the supervisory controller splits the power between the large traction motor and a smaller 54-kW generator-motor (which is still operated by the battery). The planetary gear set blends the power from the two motors and sends it to the drive axle. Even as it switches to this mode, however, the Volt still isn't employing its internal combustion engine - and therefore still isn't burning any gasoline.
A second description:
Mode 1: Low-speed EV Propulsion (Engine Off). In this mode, the ring gear is held (locked) by clutch C1. With clutch C2 and C3 disengaged, the generator-motor is decoupled from the engine as well as the planetary gearset. As the traction motor is permanently coupled to the sun gear, the planetary carriers must rotate when the traction motor rotates. Since the planetary carriers are permanently coupled to the final drive, the traction motor propels the vehicle. The generator-motor and the engine are idle during this mode, although the engine is free to start if necessary (example: engine maintenance mode).
Virtually all of the vehicle’s motive power is therefore delivered by the traction motor in this mode, including hard accelerations, using power supplied by the battery pack. With this configuration, the traction motor can produce up to 111 kW (149 hp) and deliver up to 370 N·m (273 ft-lb) of torque.
Mode 2: High-Speed EV Propulsion (Engine Off). As vehicle speed increases, motor speed and losses also increase. To engage both motors and preserve motor efficiency, clutch C1 is disengaged, allowing the ring gear to rotate. At the same time, clutch C2 is engaged, connecting the ring gear to the generator-motor. The generator-motor is then fed current from the inverter, and runs as a motor. The engine remains disengaged from the generator-motor.
This mode allows the two electric machines to operate in tandem at a lower speed than if the traction motor alone was providing torque. The speed of the traction motor in this mode drops to about 3250 rpm from 6500 rpm in the 1 motor mode, according to Fletcher.
This strategy allows the Volt to wring out as much as two extra miles of all-electric operation out of its battery pack, depending on operating conditions. However, switching from low-speed to high-speed EV mode requires the simultaneous operation of two clutches. GM’s experience with simultaneous clutch operation in their two-mode transmissions and transaxles was key to the development of the Volt’s transaxle control strategy.