There have been many many types of hybrid power source vehicles suggested and some have been demonstrated to work.
Batteries
One frequently-suggested hybrid combines an internal combustion
engine powered generator with a storage battery to drive electric
traction motors. Sometimes the object is to relieve the power unit of
the necessity to cope with large surges of power for starting,
sometimes it is to enable the vehicle to operate over a limited range
with the benefits of electric power.
In either case, the extra weight of the batteries increases the power
requirements and their inefficiency increases still further the load
on the generator during charging periods.
Like most battery-powered vehicles, these usually have a life of about three years. At this point the battery capacity begins to reduce and the cost of replacement dictates that the only economic course of action is to scrap the vehicle.
Capacitors
Capacitor storage in combination with a fuel cell has recently
appeared in an experimental car, but the technology is prohibitively
expensive and unlikely to become affordable in the forseeable
future.
Flywheel
The flywheel is more promising than batteries because it allows
energy to be put in and taken out quickly. A smaller total capacity
can be used more effectively. A flywheel-driven bus was produced by
Oerlikon some years ago and experimental flywheel-powered trams have
been tested in Britain and more recently in the Netherlands.
Bristol Railbus Eindhoven experimental Date 1998-9 2002 Total capacity 1 kWh 4 kWh Maximum energy rate ? 300 kW Weight of flywheel assembly 1 tonne ? Maximum working speed 1,500 rpm 15.000 rpm Energy input Motor 70vDC Motor/Generator from 750vDC Energy output Mechanical transmission Motor/Generator to 750vDC Flywheel material Steel Composites
Flywheels have been used for
energy storage at tramway power stations
for many years, but these are not restricted to a size and weight
which could fit on a vehicle.
flywheel tram
A hybrid vehicle is at its best in stop-start traffic conditions. When continuous high power is required, such as on hills or long fast journeys, it is disadvantaged by the limited engine output and the extra weight. Every one of the ingenious combinations proposed, suffers from this fundamental problem of multiple power sources. All sources except the one in use at any particular moment are 'dead weight' to be carried around uneconomically.