Performance. Some of the fastest and cleanest road vehicles are hybrids.
In the case of the 320s, they already have IC engines driving electric motors - the only components missing are the batteries and charge controller.
Wouldn't even need to be batteries, thinking about it, super-capacitors would do the trick.
OK, lets look at this sensibly...
1 - Mass;
Hybrid drive systems will typically add around 20% - 50% additional mass to the vehicle, although they usually do offset the additional starting mass with the power that is provided, so if one has a rail vehicle, that spends a significant amount of it's time at constant speed rather than starting and stopping, that is most of the mainline, even on services that are very much 'stopping' in nature.
So for example, on the Class 230, assume we have infinite space beneath the solebar, and that we will need to be using some form of electrical regenerative braking recovery system. Lets also assume we're going to be exceedingly clever with how we can regenerate this power, making use of series/parallel techniques and field injection.
Braking at a constant rate down from 60mph, or 27m/s. We have a, lets say, 50,000kg mass to bring to a stop. Next, lets consider that rehostatic braking on the LT118 motors is only effective down to the point at which a D78 Surface stock cuts out this function and moves to pneumatic only braking, that being 22mph, or 9.83m/s.
The total potential energy recovery for this is a relatively simple calculation and yeilds an ideal number of 15.8MJ.
Now one should consider that the efficiency of using this system is around 80% - 85% on a good day, if you're using field variance to get the ideal charging current, with minor boost conversion at the low end. If this isn't possible because the driver is needing to moderate braking, then you're looking more like 60% efficiency because of the boost converter needed at the low end and the various other losses associated with changing current through the armature. Lets assume balanced driving and around 75% efficiency.
Now, we need to be able to store 12MJ. This equates to around 3.3MWh.
Next up, lets consider the volume required. Assuming one is using EDLCs, probably the ideal for this use, optimistically 8Wh/l. So now we need to find around 420ltr of space under the train. Doesn't sound too hard.
Next up, mass... That's around 550kg.
2 - Environment;
Most hybrid drives will use Lithium Ion or Lithium Polymer batteries, these are exceedingly difficult to recycle and have a very limited lifespan, unlike their older, lead acid cousins, where one can remove and recycle the lead, or repair the battery and continue using it.
3 - Complexity;
Take what is a very simple, and familiar diesel-electric drive, that you can pretty much buy off the shelf, it's likely this will use some form of either SCR (Silicon Controlled Rectifier) or Uncontrolled Rectifier and Chopper circuit, both available off the shelf and using a rather limited amount of this precious semiconductor that everyone is under delusion that we have an unlimited supply of. To put regenerative systems on this, you will at least double if not quadruple the number of these switching devices.
Not to mention the increased maintenance cost associated with this complexity, and the additional, significant hazards presented to the maintainers when servicing these energy storage devices. A number of significant injuries have occurred with maintainers servicing input filters without sufficient bleed off time being allowed, imagine how bad this could be with a true charged device? The additional training, time and complexity of maintenance will far outweigh any savings.
4 - Lifespan;
As has been demonstrated by 1992TS, the lifespan of a DC motor that is continuously driven in either motoring or braking is significantly shortened, and the service intervals shortened. These motors where not designed for intensive regenerative braking, with anything other than a basic field injection system.
Not to mention the additional performance one wants to see when setting off will further stress these components.
And it's getting late in the evening, so I'll stop for now.
