Expanding on that waste heat part of the topic for a moment, there really is a lot which can be done with all that heat.
For instance, if you use a high output air pump which engages every time there is a need for braking instead of electromotive brakes, it would fill a very strong air tank which could then later reverse back through that air pump which would then act as a MOTOR used to add a power assist to the drivetrain and the waste heat would maintain the air pressure in the tank because, as we all know, when there is a sharp drop in temp coefficient on a high pressure air tank the pressure DROPS.
So there's that just for starters. Air tanks do not lend a very significant weight source to a vehicle, especially when one considers that semi trucks already HAVE an air tank for the air brake system, so it is conceivable that one could just simply convert the already existing air tank to a unit with stronger cladding for more pressure, and add the waste heat to it to maintain the pressure.
And since semi trucks already HAVE the AIR PUMP which pressurizes the air brake tank, all we're really doing is an upgrade to a better air pump, which can also act as a motor assist.
There's also something to be said for electric braking as an assist, something which has been suggested as a backup to air brakes for decades but which met with stiff resistance from the trucking industry despite being a tried and true mature technology from the train business.
But an upgrade to the air system in which the air pump offers braking power as well as compressed air power assist would already provide a substantial improvement as is.
The diesel engine which powers the truck would be somewhat smaller and lighter too. In fact the powertrain itself would be smaller and lighter because the engine would merely be powering a generator instead of transmitting it's power through a gearbox, drive shaft (net power loss) and differential axle gears.
So one has to include the loss of the gearbox, drive shaft and differentials in any weight calculations. Much of that loss would be offset and neutralized by addition of the generator and the drive motor or motors, plus the addition of the air pumps used in place of electromotive braking.
I actually don't know if it's better to use wheelmotors or a single inline drive motor. The negative talk about wheelmotors is that they add "unsprung weight" to the chassis, but again one must balance that against the fact that with a wheelmotor unit you no longer are using a driveshaft differential gear unit.
The humungous 100 ton diesel-electric quarry trucks that we've all seen do use wheelmotors, so it might be a good idea to investigate the idea of scaling down that technology since it is already very mature and tested under severe service conditions in quarries around the world.
I honestly think that the biggest resistance to diesel-electric taking over the trucking industry has simply been a bunch of stubborn people who insist on adhering to the status quo because it represents a comfort zone.
