Trolleybuses vs. Diesel electric hybrid buses
Attempts to improve diesel bus effiency and environmental performance have led to using electric transmissions and, in America, battery storage. Except for trolleybus versions, these experimental vehicles don't stop toxic emissions or provide ride quality advantages.
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Diesel electric hybrid technology
A diesel electric bus is functionally a diesel bus with an electric transmission. It will emit much the same range and amounts of pollutants locally and globally as a diesel bus with a mechanical transmission. Energy efficiency will be similar to a conventional diesel and if anything, probably, somewhat worse. Noise and vibration may be somewhat better than a diesel with a mechanical transmission and so could low floor layout.
Appropriately designed, it can also operate as a trolleybus, with all the environmental pluses of a trolleybus but only while operating in trolleybus mode.
In diesel mode it cannot regenerate energy [no overhead line to put it into, unless there are traction batteries]. Regeneration brings energy savings of around 30% in regenerative trolleybus mode.
A diesel electric will weigh more than a trolleybus [even a trolleybus with an auxiliary power unit]. This can bring lower passenger capacity [if up against a legal weight limit] and also the space required for the diesel generator set may impinge on and reduce the space for passengers.
The greater weight will increase energy costs and wear and tear costs such as tyre wear.
One would expect a diesel electric to be more expensive than an equivalent trolleybus [since it is basically going to be a trolleybus with the extra weight and cost of a diesel generator set].
Performance of a diesel electric will be similar to a conventional diesel and less than a trolleybus, unless a hell of a large diesel engine - 500kW or so, is fitted. Such a large diesel engine [plus generator] will have considerable weight, cost, space and fuel consumption penalties.
Maintenance and therefore maintenance costs of the diesel part of a diesel electric will be similar to that required for a conventional diesel and will be much higher than for a trolleybus. Reliability will be similar to conventional diesel - pretty much the same range of things to go wrong, like cooling systems running out of water - and worse than a trolleybus. The one advantage the diesel electric should have over diesel mechanical and would share with a trolleybus, would be reduced brake maintenance costs, since it could use rheostatic [but not regenerative] electric braking.
Overall, diesel electric offers the possibility of a higher quality diesel bus solution where services are too infrequent to justify overhead, with the possibility of operating in trolleybus mode, with all the advantages of electric operation on busier wired route segments.
One suspects that the lifetime costs of a diesel electric bus will be greater than diesel mechanical - greater capital cost, greater weight, greater fuel consumption, etc. [Yes - in case anybody wants to make the point, diesel electrics seem to work out better than diesel mechanicals in rail traction applications but that is a very different scenario to bus operation e.g. much higher power levels, multiple unit operation, etc.]. About the only thing in its favour cost wise will be maintenance savings from electric braking. Hence lifetime operating costs will probably compare unfavourably than with true trolleybuses, which are likely to be lighter, cheaper, more energy efficient and have lower maintenance costs.