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There have been many many types of hybrid power source vehicles suggested and some have been demonstrated to work.

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.

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.

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
flywheel tram




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



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.

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.

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