Diesel loco providing full DC power to an EMU

[mr_jrt]

Reading the Voyager thread something popped back into my mind that also cropped up when thinking about the IEP concept.

If acceleration is the issue against a diesel loco hauling an EMU, is it not viable to have the diesel power plant generating DC power for the EMU's electric motors and not just hotel power? As has been pointed out in these discussions before, there are few 125mph+ sections of line that aren't going to be wired up (and it figures...if the line is busy enough to warrant 125mph running, wires are probably justified!), so the arrangement probably only needs to be able to maintain 90-110mph, if that, and if the loco is only having to accelerate it's own mass (the EMU accelerating itself), then it surely wouldn't be much of an issue?

Just wondering if anyone more knowledgeable could chip in a few thoughts on the matter.

 

[HSTEd]

Means you either need to have 25kV capable auto couplers and then an transformer on the locomotive which weighs a tonne and has technical issues or you need an enormous power bus the entire length of the multiple unit which has its own set of weight and technical issues.

I've been thinking about it alot and its quite iffy really.

 

[jopsuk]

the auto coupling traction power connector would be the hardest thing to arrange safely

 

[mr_jrt]

Means you either need to have 25kV capable auto couplers and then an transformer on the locomotive which weighs a tonne and has technical issues or you need an enormous power bus the entire length of the multiple unit which has its own set of weight and technical issues.

I've been thinking about it alot and its quite iffy really.

Is it really so difficult to have a DC bus the length of the unit? Taking the class 377s as an example, with the pantograph in the 3rd carriage, then surely there is a DC bus running the length of the unit powering the motors in carriages 1,2 and 4 from the transformer. As you can probably tell I'm not well versed in the more technical side of things, but I am curious.

the auto coupling traction power connector would be the hardest thing to arrange safely

Indeed, there is obviously a safety issue with the coupling required, but this should be a trivial problem to solve in a safe manner, surely?

 

[HSTEd]

Is it really so difficult to have a DC bus the length of the unit? Taking the class 377s as an example, with the pantograph in the 3rd carriage, then surely there is a DC bus running the length of the unit powering the motors in carriages 1,2 and 4 from the transformer. As you can probably tell I'm not well versed in the more technical side of things, but I am curious.

Indeed there is, but it is normal to not rate one "arm" of the the power bus for the entire rating of the system.
This becomes an extreme problem when you look at things like Cl390s which have four DC Low Voltage busses drawing power from two transformers.

You couldn't single end feed the entire thing without quadroupling the size of the conductors throughout the train since you would have to provide for feeding from both ends.

 

[mr_jrt]

Indeed there is, but it is normal to not rate one "arm" of the the power bus for the entire rating of the system.
This becomes an extreme problem when you look at things like Cl390s which have four DC Low Voltage busses drawing power from two transformers.

You couldn't single end feed the entire thing without quadroupling the size of the conductors throughout the train since you would have to provide for feeding from both ends.

Cheers, that's the sort of insight I was fishing for. It's basic physics that you have to increase the cross-section to overcome the resistance at higher current, but given the 3rd rail network manages it with the cross-section of standard 3rd rail, a bus with similar dimensions (after all, it would only have to handle the current to power 12 carriages, maximum, rather than however many units the 3rd rail sections currently have to handle) surely it shouldn't be too hard to fit one of those safely into the undersides of an EMU?

 

[DXMachina]

Given that whats discussed here is using a diesel to power a high performance EMU, is the power capacity of a 750v rail (heavy, inflexible) enough? I dont think DC EMUs have anything like the performance of 25kv sets

 

[Kali]

They do, when they can draw enough power - the limitation is the 3rd-rail supply infrastructure rather than anything inherent in it being 750v DC. IIRC a 450 should perform the same as a 350 if it can pull enough juice out of the supply.

The problem is mostly because of the low voltage - the diesel engine can supply whatever you like ( within reason ), so you don't have to make it 750v if the EMU can take a higher voltage supply. Aren't modern DC EMUs built with at least some capability of taking HVAC?

 

[mr_jrt]

Given that whats discussed here is using a diesel to power a high performance EMU, is the power capacity of a 750v rail (heavy, inflexible) enough? I dont think DC EMUs have anything like the performance of 25kv sets

Well, as I mentioned above, I don't imagine they would need to be operating at full-spec. 90mph for example would probably be a reasonable limit for the sections off-wire. DC EMUs do tend to have lower performance, but it is my understanding a large part of that is primarily due to the mechanics of a physical-contact shoe, the other main factor being the current available to power multiple units in a section.