Patrick -

trucking is an industry operating on paper-thin margins defined largely by fuel economy and payload. If there were even small savings to be had by further aerodynamic sculpting (beyond the wind guide above the driver cabin), you can be sure they would be implemented. Just compare US 18-wheeler and European truck designs. The difference is due mostly to the different price of diesel fuel.

Rafael-

Walmart has implemented various changes to their truck fleet that have improved the aerodynamics of their vehicles (talking about 18 wheelers).

http://www.greencarcongress.com/2005/12/walmart_seeks_t.html

The largest challenge to improving fuel ecomony in the U.S Heavy Duty (class 8 - 80,000 lbs GVW) trucking market is to get drivers & fleets to accept aerodynamic vehicles. Most drivers do not "like" the Aero look, they prefer the boxy, Peterbilt 379 and Kenworth W900 with a wide trailer gap. Switching to a Pete 357 or Kenworth T2000 a very small (>18") trailer gap can improve fuel ecomony by 1 MPG. The acute driver shortage prompts smaller fleets to use Truck Model style & features to keep drivers. The rising cost of fuel will hopefully push the industry towards sensible trucks with better economy. A properly configured aero package with the correct geatrain spec can get 7-8 MPG. Most fleets cannot get out of the 6 MPG range.

Truck stop overnite idling is a big consumer of fuel. Implementation of external power interface, or a small clean secondary engine would make a big difference in fuel and $$$ spent. Not to mention air quality and quiet. Parts of the hybrid concept described above may enable doing away with the small engine or external power.

Given the cost of a Class 8 truck, I don't see fleets rushing to replace what they already have.
There'll have to be some Carrot & Stick program at state or federal level.

Tom -

for substantial reductions in aerodynamic drag at the tail end, you'd have to approximate an eleongated teardrop shape. Failing that, a vortex-shedding cut-off is actually better than a half-way house. Some improvement may be possible without changing total vehicle length, but not all that much.

More significant opportunities are related to airflow under the vehicle. But as I pointed out before, commercial vehicles are built first and foremost for payload. The cost of diesel would have to be quite high before payload weight would be sacrificed for additional aerodynamic cowlings.

The regenerative braking seems like an interesting partial solution to stupid small towns' most annoying trait: the banning of jake brakes. Many little mountain towns prohibit the use of an engine brake because of the noise (never mind the 80,000 lb of truck and trailer trying to slow down with drum brakes). Could the regenerative braking system be used to slow the truck in a way similiar to a jake? Also, could the trailer be fitted with it's own baterries and electric motor's to provide boost and additional regeneration?

John -

recuperative retardation on downhill descents is a major motivation for hybrid HDVs. It may not be feasible or economical to install batteries large enough to handle really long descents but a hybrid arrangement would reduce the thermal load on the retarder. This quiet component is found in HDV but not in LDV drivetrains, perhaps you are familiar with it:

http://en.wikipedia.org/wiki/Retarder_%28mechanical_engineering%29
http://www.voithturbo.com/533863.htm

As for electric motors and batteries in a trailer, in principle that would be possible. However, you'd have to be careful about torque levels. Go too far and you could end up destabilizing the vehicle dynamics.

On a more general point, electric motors can drive axles that are not connected to the mechanical drivetrain. This would be beneficial in several respects: greater traction for safety on wet roads, reduced smoke puff and tire noise during acceleration, increased tire life expectancy (and hence, safety).

http://www.airtab.com/