What's the distinction between different types of multiple units powered by diesel fuel?

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Crossover

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Then what's a DMMU or DHMU? Since these are definate subsets of the DMU Class, but DEMU isn't???
For reasons unbeknown to me, DMMU and DHMU seem to be classed under the umberella DMU whereas the traction system of DEMU appears to be more widely separate
 

yorkie

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Surely any multiple unit that uses diesel fuel is a diesel multiple unit, by definition?

Within that umbrella term, you have further classifications:
  • diesel-mechanical
  • diesel-hydraulic
  • diesel-electric
 

Nym

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Given that the majority of DEMUs don't use the same alternators for traction as they do for auxiliaries anyway, all it is is a different means of power transmission, I don't see how that defines DEMUs as a different super-set to DMUs at all.

If the same alternators where to provide traction and auxiliaries, from my experience of the marine world that would be a different traction type. Where the traction type would be known as IEP, FEP or IFEP (Intergrated Electric Propultion) as apposed to the older systems with shaft, hydraulic, hybrid or electric propulsion systems.
 

big all

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Given that the majority of DEMUs don't use the same alternators for traction as they do for auxiliaries anyway, all it is is a different means of power transmission, I don't see how that defines DEMUs as a different super-set to DMUs at all.

If the same alternators where to provide traction and auxiliaries, from my experience of the marine world that would be a different traction type. Where the traction type would be known as IEP, FEP or IFEP (Intergrated Electric Propultion) as apposed to the older systems with shaft, hydraulic, hybrid or electric propulsion systems.
'Thumper' DEMUs are pretty basic; they have 110v starting batteries and separate lighting batteries,
main generator supplies, traction power and heat

The auxiliary generator supplies field exciting 'battery charging and air compressor.

The compressor won't run unless the engine is running, so suspect it's 440volts, or whatever the main generator gives out.

So the aux gen has a low load, therefore no need for a complicated sub class whereas perhaps on a ship the aux load can be around the same or even more than the propulsion load?
 
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aleggatta

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Whilst I can appreciate that there may have been a convenience for distinction in the past, whether that be a straightforwards tell tale of any additional risks after an accident being understood by the differentiation in unit number allocation (i.e. traction electronics that may still be holding a charge), realistically any self propelled unit, is a self propelled unit. Generally all DMU/DEMU's are understood when simply described as 'DMU'. I think there will be more debate to come however when you encounter other forms of self propelled units such as battery or hydrogen based systems.

The only time you might need to differentiate on these forums is when explaining the distinct differences between specific traction systems, in which it is normally very clear in the individual posts. Honestly the only time there ever seems to be an issue with DMU/DEMU terms on this forum is when an unnecessary level of pedantry exists and leaves a sour note to the generally pleasant discussion at hand.

Apologies if any offence has been caused by this.
 

hexagon789

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Perhaps down to the Southern Region having only DEMUs while the other regions had DMMUs or DHMUs when British Rail began it's dieselisation and that geographical separateness gave rise to the 'separateness' often made between DMUs and DEMUs?

(Though both the WR and LMR had DEMUs in the form of the Blue Pullmans but they were of course rather separate to the rest of the fleet.)
 

AM9

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DEMUs were significantly different to all other diesel multiple unist when the TOPS scheme was first mapped out:
1) they (generally) had bogies that were connected to the prime mover by power cables, i.e. no cardan shafts​
2) as a result of 1) above their maintenance regime was considerably different, involving high power traction electrics, - all other DMUs only had moderate electrical power arrangements for auxiliaries and hotel supplies​
3) potentially, DEMU interoperability with S & T has always required more attention to detail above the normal issues of axle loading and gauging​
In these more enlightened times, there is now even more reason to demark DEMUs from DMUs fitted with either mechanical of hydraulic transmissions as unlike the others, DEMUs are not dead-end technology, - forever relying on burning hydrocarbons. DEMUs have more in common with EMUs and BEMUs in terms of their driving characteristics and by way of bi-mode capabilities, bring the opportunity of reduced or even zero CO2 and pollution in sensitive areas.
 

30907

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BR(SR) distinguished between DMU and DEMU in Working Timetables and other documents. I presume there was a reason - maybe as AM9 has just outlined?
 

PupCuff

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This thread would probably have more context if a moderator could perhaps include the post Nym was challenging as a quote in the first message.

I'm working on the assumption that it was someone suggesting a DEMU is not a DMU. As it's been pointed out, DMUs with a hydraulic transmission are DHMUs, DMUs with a mechanical transmission are DMMUs, so should therefore logically follow that a DEMU is also a subset of a DMU.

Equally, nobody seems to be arguing that it isn't correct to refer to a Class 37 or a Class 66 or whatever as a diesel locomotive and that we should regard diesel-electric locomotives as different from diesel locomotives with a hydraulic transmission eg a Western, or a diesel locomotive with a mechanical transmission, eg an 03.

I'd suggest we refer to Diesel Hydraulic MUs as 'Diesel Multiple Units' yet Diesel Electric locomotives as 'Diesel Locomotives' as they're the "norm" in their areas and the general assumption is if you're referring to anything with a specifically "different" transmission you'd then specify the type of transmission.
 

AM9

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Yes, that is indeed the unknown context of the now 1st post.

I was challenging someone's opinion that DEMUs are not DMUs.
I would agree that as long as the prime mover in a multiple unit is a diesel engine, then it can be called a DMU. Thus any DMU with a mechanical, hydraulic or electric transmission can reasonably be called a DMU. Operationally, as I suggested in post #9, there would be a significant difference between a DMU with an electric transmission and the other two types of transmission, - the torque/speed characteristics would necessitate different driver training, and the inclusion of a large generator, motors and electric traction control gear would require a very different maintenance operation. Thus from an operator's point of view, classifying the DEMUs separately from all other DMUs would be a logical thing to do, which may be why BR did just that from their introduction.
 

Metal_gee_man

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I must assume from a Passenger perspective DEMUs are favourable now due to the lack of high revving, long term revving diesels under your feet, I could also see them being more fuel efficient when used in conjunction with very small battery storage units for regen braking etc...
It's been mentioned that the LNWR class 230s and the 802 IETs have a barely noticeable diesel noise, where mechanical and hydraulic DMUs are loud, droney and slow to accelerate compared to a decent set of electric motors.
The driving style I'd assume would be different too, as the engines would only rev when it needs high levels of electricity and would die down when the storage levels would have topped up, this would be computer controlled, where a regular DMU would be very rudimental "that lever controls the revs" much like your cars accelerator pedal so the training would be different.
So I believe they should and must be classed differently
 

yorkie

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a couple of now deleted comments on here seem to reckon DEMUs are a separate type.
This thread was created after another thread went-off topic; yes someone tried to claim that a DEMU was not a DMU, whereas all sources I can find indicate that there are different categories of DMU depending on the transmission type (i.e. hydraulic, mechanical and electric).
 

jopsuk

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Whilst clearly DEMUs are a subset of DMUs and to argue otherwise is madness, is the reason the eg BR put DEMUs in their own category (in TOPS) to do with interoperability? I know not all 1xx units are interoperable, but there are definitely mechanical transmission units that can work with hydromechanical units- eg in modern use class 172s work in multiple with 168s and 170s. Has there ever, in Great Britain, been any examples of DEMUs that worked with DMMU or DHMU?
 

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