The "normal" approach to hybrid trailers is to have an IC engine driving a generator that sends electricity forward to the EV to supply the energy needed for highway cruising.

Generator benefits:

* Energy can be stored in the batteries when IC output is more than needed.
* The IC load is very level as the generator output can be nearly constant.
* No transmission is needed
* Standard trailer dynamics apply

However, this approach suffers from the inherent problem that the energy is transformed from mechanical (at the IC shaft) into electrical, and then back to mechanical (at the EV motor.) The energy will in the best case go through the generator, EV motor controller, and EV motor. In the worst case it will also cycle in and out of the batteries. All of these conversions deal a serious blow to the best efficiency possible. For a typical EV system you will need between 1/2 and 2/3 of the IC horsepower to simply push the car as you will to generate enough electricity to drive it.

Pusher Benefits:

* Fewer energy conversions Chemical--->Thermal--->Mechanical
* Less IC horsepower needed
* Potentially lighter than a gen trailer (no generator, less fuel, less hp)
* EV system does not need to be sized for continuous use
* Cheaper (no generator, more standard system)

But, the pusher trailer is not at all suited for city use. It ONLY makes sense for long distance freeway travel. Since the IC output can't be used unless the car is being pushed, efficiency will fall at slower speeds. However, the whole premise behind the hybrid trailer is that it is only needed for long-range trips (where most of the time is spent on the freeway!)