Today the focus regarding Hydrid Vehicles centers on
mileage and emmission - importan factors indeed.
However most forget another major factor, the flexibility inherent in modularization.
In this regard Hybrid Technology opens up many new possibilities in vehicle design.
Just think if you could exchange your diesel engine for a "plug-in" Hydrogen engine, because you are moving to a state where Hydrogen is in more supply and cheaper. As Fuel Cells become more compact you could even "plug-in" a Fuel Cell in stead of your Diesel!
In Hybrid Vehicles, modularity is the name of the game.
This kind of modular approach is not a far flung fantasy.
Take a look at the serial-hybrid.
As you can see there is no mechanical connection between the ICE and the wheels. This is pure plug-and-play, providing of course the form of the ICE packet is the same as the one you are exchanging.
Ok! if you are exchanging a diesel ICE with a petrol (gasoline for you in the US) ICE you may have to flush the diesel tank as petrol engines are not all that happy with diesel.
The other way around is however less of a hassle as the diesels don't mind a couple of liters of petrol in a whole tank of diesel.
The potentially exchangeable "modules" are illustrated on this x-ray drawing of a Hybrid Vehicle (courtesy of IIT). The modules are:
The Hybrid Power Unit. his Power Unit could be Any ICE (diesel, petrol, hydrogen, biogas, biodiesel etc.) or a Fuel Cell.
Traction Motor can be electric and driven directly by the Hybrid Power Unit or the Traction Motor can be hydraulic or a mechanical drive driven indirectly by the Hybrid Power Unit v.i.a. an electric motor.
When designers are free of the mechanical coupling and the mechanical constraints inherent in most of our current Automobiles, Power Units can be mounted low (Center of gravity) and out of the way, Wheels can be placed a the extreme corners of a vehicle. Their location in reality only constrained by the available suspension technology.
Putting this flexibility into a practical setting, one could imagine that a Truck Driver could for example add extra driven wheels and an extra Power Unit to his heavy hybrid hauler to tackle special terrain and extra heavy loads.