Physics

The ideal powerplant for your described application would be a permanent magnet MOGEN originally developed in WWII by a consortium of Kenworth, Eaton University, and MIT. 

The application failed in WWII because of critical shortages of copper and aluminum used for its wiring, and from the need for vacuum tubes in other applications, which denied the technology its use until the development of semiconductors in the 1950's.

Mogens offer surprising advantages in fuel efficiency: hybrid power is only one advantage.  "Hybrid" vehicles include a GENERATOR/CONVERTER so that one can charge the batteries from virtually any household power source.

The next step was to include a GENERATOR/INVERTER which uses the DC storage batteries onboard the vehicle to "repay" power utilities by discharging their stored power back into the power grid, which also maintains the Lithium Ion batteries at peak efficiency.

Mogens offer surprising advantages in terms of efficiency.  Where one would convert kinetic energy to heat energy during braking, the mogen converts kinetic energy to electrical energy during braking, saving fuel and brakes in the same move. 

It has been much publicised that electric cars weighing less than 3,000 pounds operate on hybrid power, but trucking line operators have taken full advantage of work done between PACCAR and Eaton in the last few years that entirely supersedes WWII work using vacuum tubes.

DC motor is best for constant operation.