As goes Detroit: another "string" (thanks to Prof. Emeritus B.P. Leonard)

B.P. Leonard, Professor Emeritus of Mechanical Engineering at The University of Akron, suggests another important requirement for any new cars to be produced by the US auto industry-in-receivership (or in receipt of bailout funds).

There's a general misconception (in the US) that efficient cars have to be light and small and cramped and unsafe. This need not be the case. Energy is dissipated by vehicles in three ways:

1. Internal inefficiency (poorly designed power-plants; a result of cheap fuel; and the need to cover a wide range of torque in "off- design" conditions).
2. Drag (mostly air resistance, proportional to speed-squared). Important for long trips.
3. Braking (dissipated as heat in the brakes). Important for lots of stopping and starting.

Item 3 is greatly reduced by having a mechanism for regenerative braking (kinetic energy of the vehicle, proportional to mass-times- speed-squared, is efficiently converted and stored as reusable energy): super-capacitors (for fast energy-transfer) plus storage batteries (for slower long-term storage); hydraulic/pneumatic energy storage--ideal for large commuter vehicles (trams, buses). This, of course, is the motivation for purely electric or hybrid vehicles.  There is no limit on size (mass); high-strength materials can be used to increase safety. However, without the regenerative energy storage, dissipation is proportional to mass.

Item 2 is greatly reduced by aerodynamic drag-reduction; i.e., reducing the drag coefficient (CD) by streamlining. Mercedes only recently found that by adding an aerodynamically contoured "belly- pan" underneath the car (covering up all those lumpy bits-and-pieces causing turbulence), the already low CD was further reduced significantly. In addition to speed-squared, aerodynamic drag is proportional to "frontal area"--roughly, the cross-sectional area viewed from the front. So there is some dependence on "size." That's why a low, longish station-wagon is preferable to a high-and-wide SUV, if a lot of space is needed.

Item 1 can be satisfied with a hybrid design--efficient kinetic-to-stored-energy conversion plus an on-board "range-extender," such as a small internal-combustion engine (ICE) driving an electric generator--that can be fully optimized so that the ICE is always running at its most efficient design point while minimizing bad emissions. Using electric motor/generators for (or, possibly, in) each wheel independently allows for ideal stability control, enhancing safety.

All of this technology is available. So I would add to the list of "strings attached"--all future vehicles should be low-drag with regenerative braking. There would be no price disincentive against a hybrid as compared with the analogous non-hybrid version.