Electric vs Diesel

[Waddon]

I was reading somewhere on here about plans to put extra carriages into voyagers with pantographs so the units could run on electric when under the wires...

and it got me wondering, has it ever been done the other way round... i.e. a primarily third rail or AC emu carrying a diesel generator, maybe in its own carriage, to power the traction motors when it's away from the juice? So, for example on a route like London to Uckfield it could use the electric until the third rail runs out and then use the generator to power it's electric motors...

Has it ever been tried, or is such a thing even possible or practical?

 

[Intermodal]

I'm not sure about units, but certainly there are locomotives that do this.

• Electro-diesel locomotives in the UK
• Class 73
• Class 74

 

[Waddon]

Yes, but do the Class 73's run their electric motors off of the diesel or does the diesel engine run the wheels directly? I was kind of wondering if diesel generator-powered electric motors had ever been tried in a multiple unit which could also take power from a third rail or overhead wire...

 

[Intermodal]

Yes, but do the Class 73's run their electric motors off of the diesel or does the diesel engine run the wheels directly? I was kind of wondering if diesel generator-powered electric motors had ever been tried in a multiple unit which could also take power from a third rail or overhead wire...

This is how I believe class 220/221 work, a diesel generator powering electric motors, hence why they are being considered for conversion to electric, because it would be a "fairly" straightforward operation.

Wikipedia says:

Each coach is equipped with a Cummins QSK19 diesel engine producing 560 kW (750 hp) at 1,800 rpm driving an electrical generator which powers two motors, each driving one (inner) axle per bogie each via a cardan shaft and final drive.

 

[The Barlow Boy]

Yes, but do the Class 73's run their electric motors off of the diesel or does the diesel engine run the wheels directly? I was kind of wondering if diesel generator-powered electric motors had ever been tried in a multiple unit which could also take power from a third rail or overhead wire...

The 73s are classified as Electro-Diesels which is a slightly confusing title. When they're in diesel mode the engine powers an electric generator which supplies power for and to the electric motors at a somewhat reduced power.
In diesel mode they're rated at 650hp, in electric mode they're rated at 1,600hp.

 

[Hydro]

That's correct, the diesel engine generates electricity which is fed then to the traction motors. When the shoes are down and it's running off the juice, the current goes direct to the motors (via all the electrical gubbins, of course).

73's aren't the best performers when on diesel, unless you have a pair of them. I remember having one in multiple with a 31 in push pull working off the juice, the blue star went down and with the 73 leading and 5 coaches plus a dead 31, we never got over 20mph. Oh, and it exploded after about 15 miles.

 

[jopsuk]

Bombardier's "AGC" range of regional MUs for France includes bi-mode versions- even including one version that can run on DC, AC and diesel.

 

[MCR247]

Well once you have AC and diesel, it should be too hard to get 3rd rail should it if you can have DC/AC and AC/diesel combos?

 

[dubscottie]

Well once you have AC and diesel, it should be too hard to get 3rd rail should it if you can have DC/AC and AC/diesel combos?

In theory yes. The problem is that ( I think!) the 221's etc have AC/3 Phase motors which means, in order for them to work of a DC supply, They would have to carry around a very heavy AC to DC transformer.

 

[jopsuk]

The AGC take DC from overhead (1500V). They also, thanks to loading gauge, have more space- the middle of each carriage is "low floor " (double decker style) with a lot of equipment above- including, I think, the diesel engines.

 

[rail-britain]

Hence why the Class 22x units are classified as DEMU
They use a diesel source to power electric motors, the same as diesel-electric locos
The advantage is the ability to haul heavy loads, it would appear this was lost when creating the Class 22x units!

Hence, why they are so innefficient
However, I believe this was the only way to reach speeds over 110mph with reliability

 

[MCR247]

222s are quite quick off the mark though

 

[HST-DAVE]

The new intercity 125 replacements (part of the DfT programme) look likely to be electro-diesel too. The HST class 43 being withdrawn as the replacements come on-line.

 

[coral reef]

Has it ever been tried, or is such a thing even possible or practical?

Possible, easily yes, practical, rarely. There is a tendency towards rationalisation for electrified routes, meaning the number of trains that would be required for leaving an electrified area is likely to be low and/or widely distributed.

The result is that a diesel powered EMU design would be of small production runs (therefore overly costly) or widely distributed across the network which would then beg the question would it be cheaper to electrify more routes and build more EMUs?

A diesel EMU would necessarily be more complex and therefore expensive, and would cost more to maintain.

It has been looked at as an idea, but I think historically each case has been resolved with eventual route electrification (Bournemouth - Weymouth) or opting for DMUs as the easy option. Whether that was short sighted or sensible I'm not sure.