Why were most past DMUs designed as DMUs rather than DEMUs?

[AHBD]

Given that overall BR came to the conclusion that diesel-electric was the way to go (as opposed to diesel-transmission) for locomotives*, why were most past underfloor engined DMUs designed as DMUs rather than DEMUs? I had thought electric generators would have scaled down Ok?



*excluding the Hydraulics which didnt exactly disprove them.

 

[Snow1964]

Back in mid 1950s it was cheaper and easier to have the gearbox on DMUs. There wasn’t a high power direct drive gearbox suitable for locomotives.

Hydraulic transmission was less reliable and/or (depending on your view) needed more maintenance than other types.

The Southern region DEMUs were initially ordered on the assumption that they could be converted to straight EMUs if a rolling programme of third rail electrification restarted (remember at the time, it was only 15-20 years after the rolling programme had been interrupted by WW2).

 

[JonathanH]

Given that overall BR came to the conclusion that diesel-electric was the way to go (as opposed to diesel-transmission) for locomotives*, why were most past underfloor engined DMUs designed as DMUs rather than DEMUs? I had thought electric generators would have scaled down Ok?

It is rather inefficient to give up half a coach to an engine when a mechanical engine fits underneath the carriage. Fitting a diesel electric engine under a carriage was too difficult.

 

[hexagon789]

Given that overall BR came to the conclusion that diesel-electric was the way to go (as opposed to diesel-transmission) for locomotives*, why were most past underfloor engined DMUs designed as DMUs rather than DEMUs? I had thought electric generators would have scaled down Ok?



*excluding the Hydraulics which didnt exactly disprove them.

DEMUs were more expensive I believe.

The Southern Region only got them because it was able to argue that having a large fleet of EMUs, DEMUs made sense as a lot of equipment (traction motors, brake equipment etc) could be standardised.

The Thumpers were basically EMUs but with a diesel engine replacing the 3rd rail supply.

The only other DEMUs were the Blue Pullmans, but they were designed as high-speed luxury trains, so cost came second to prestige.

 

[StephenHunter]

It is rather inefficient to give up half a coach to an engine when a mechanical engine fits underneath the carriage. Fitting a diesel electric engine under a carriage was too difficult.

And sticking one to one side with a corridor caused weight distribution issues.

 

[JonathanH]

So to take the OP's question one stage further, why couldn't a diesel electric engine be fitted under the carriage? Dissipation of heat must be one issue, as must power output.

 

[StephenHunter]

So to take the OP's question one stage further, why couldn't a diesel electric engine be fitted under the carriage? Dissipation of heat must be one issue, as must power output.

A lot of the components were too big at the time in a pre-microchip world? They managed to resolve the issue by the time the Meridian/Voyagers came along.

 

[Ken H]

The bus chassis manufacturers were moving from half cabs to underfloor engines. Stuff like the AEC Reliance and the Bristol MW. Automatic or semi automatic gearboxes were also becoming main-stream. And higher powered front entrance double deckers were also starting to be developed, like the Leyland Atlantean.
So plenty of cheap bus technology to use in DMUs

 

[30907]

You have to remember that
(1) mechanical transmission had been established as viable (by the GWR in particular, and I think also in Ireland and mainland Europe) pre WW2, so it wasn't really a BR decision.

(2) DMU engines were typically 150hp under BR, with 2 to a power car - the nearest equivalent diesel-electric in power was the 350hp shunter, which wasn't exactly small. So while in theory the components could have been miniaturised, the technology had developed in a different way. BR used mechanical transmission for smaller shunters BTW.

 

[AHBD]

Snow1964,JonathonH:

My original post was supposed to convey that I meant: why no underfloor diesel- electric units under multiple carriages in a set, in same fashion as a dmu, and like modern 769s or 230s.

I meant to exclude things like nir class 80s or Br class 207(thumpers?)s with half a diesel-electric locomotive in one carriage.

 

[Bevan Price]

The snag with 1st generation mechanical transmission is that it was not considered sufficiently robust / reliable to operate at speeds above 70 mph, so for over 20 years, there was little prospect of introducing faster schedules on many routes operated by dmus.

 

[hoagy_ytfc]

I was going to ask about the higher power Transpennine DMUs, but when I looked it up it turns out they weren't much higher power after all!

British Rail Class 124 - Wikipedia

en.wikipedia.org

 

[GusB]

I think that we need to clarify that there was no such thing as a "mechanical engine" and a "diesel-electric" engine. A diesel engine is just that - an engine that runs on diesel, and it's "mechanical" by design. It's coupled to the wheels by the transmission, and this is what can be electric, mechanical or hydraulic.

 

[Ken H]

I was going to ask about the higher power Transpennine DMUs, but when I looked it up it turns out they weren't much higher power after all!

British Rail Class 124 - Wikipedia

en.wikipedia.org

Class 110 were higher powered but still 70mph. The power was for gradients on the Calder Valley route, not speed.

 

[hexagon789]

I was going to ask about the higher power Transpennine DMUs, but when I looked it up it turns out they weren't much higher power after all!

British Rail Class 124 - Wikipedia

en.wikipedia.org

230bhp vs the usual 150 (albeit downrated to 200bhp after a few years to improve reliability). What perhaps was more noteable was the use of non-driving motor cars, giving four power cars in a six-car set - 1,920bhp.

 

[Magdalia]

A lot of the components were too big at the time in a pre-microchip world?

In the 1950s generators and traction motors were also heavy.

The snag with 1st generation mechanical transmission is that it was not considered sufficiently robust / reliable to operate at speeds above 70 mph, so for over 20 years, there was little prospect of introducing faster schedules on many routes operated by dmus.

But that didn't matter because most DMUs operated on track where there was no opportunity to go above 70mph anyway.

I was going to ask about the higher power Transpennine DMUs, but when I looked it up it turns out they weren't much higher power after all!

Another example was the 3 car suburban dmus built for the Lea Valley commuter trains. These had 238hp Rolls Royce power units and hydraulic transmission, designed for 70mph running and quick acceleration. A 6 car train had 1904hp.

 

[AHBD]

Even if, as suggested above, in the 1950s generators and motors were heavier than mechanical gearboxes, by the time of the 2nd generation dmus like sprinters and 'super?'sprinters would underfloor diesel electric power units not have been feasable?

 

[Ted633]

Even if, as suggested above, in the 1950s generators and motors were heavier than mechanical gearboxes, by the time of the 2nd generation dmus like sprinters and 'super?'sprinters would underfloor diesel electric power units not have been feasable?

The Class 210 prototypes (1982) were built with above floor power units, so I'd assume not!

 

[ac6000cw]

Even if, as suggested above, in the 1950s generators and motors were heavier than mechanical gearboxes, by the time of the 2nd generation dmus like sprinters and 'super?'sprinters would underfloor diesel electric power units not have been feasable?

Feasible - almost certainly yes, more expensive - probably also yes.

If they come from a supplier that knows what they are doing, mechanical/hydro-mechanical transmissions are reliable and well proven at the power and torque levels needed for a DMU. As evidenced by the recent new build DMUs in the UK, which largely have mechanical transmissions, they are probably also still the cheapest option.

At locomotive levels of power, torque (tractive effort) and wheelslip control requirements, the general consensus has been for many years that electrical transmission is the overall better method (for various reasons).

 

[Gloster]

If the question is why were the DMUs were built as loose vehicles, rather than semi-permanently coupled sets, I would guess that there were a number of reasons. One would be that it was still normal practice to adjust train lengths for different times of the day or flows. A second may have been that it was felt that the equipment was still not very reliable and being able to switch vehicles around meant that you could reduce the number of unpowered vehicles.

Others may be better informed.

 

[AHBD]

So is it modern power electronics that makes recent underfloor diesel electric D(e)MUs feasable?

 

[apk55]

In the 50s electrical equipment was much more costly and bulky than the mechanical equivalent. It also would be very difficult to design a DC generator that could fit underfloor due to the commutator size etc hence the reason for above floor engines. Nowadays with alternators and 3 phase motors there is probably not much cost difference when compared with a decent multi speed gearbox.

 

[61653 HTAFC]

It is rather inefficient to give up half a coach to an engine when a mechanical engine fits underneath the carriage. Fitting a diesel electric engine under a carriage was too difficult.

Is it that much of a problem though? Maybe on routes where formations need to be the maximum length (ironically routes like London to Hastings, which did use DEMUs), but it wouldn't be a problem elsewhere- particularly compared with the tank engine + 2-3 <20m coaches (of which half of one coach would be a guard's van) that the 1950s DMUs were replacing. Cost would be more of an issue, and remained that way into the 1980s- hence 150s being ordered rather than 210-derivatives.

I do wonder if the 769 project might have fared better with a larger engine designed specifically for railway (or marine) use, fitted behind one of the cabs. Even with one third of one vehicle given over to the engine it would still be a significant capacity increase over a 150.

 

[StephenHunter]

Is it that much of a problem though? Maybe on routes where formations need to be the maximum length (ironically routes like London to Hastings, which did use DEMUs), but it wouldn't be a problem elsewhere- particularly compared with the tank engine + 2-3 <20m coaches (of which half of one coach would be a guard's van) that the 1950s DMUs were replacing. Cost would be more of an issue, and remained that way into the 1980s- hence 150s being ordered rather than 210-derivatives.

I do wonder if the 769 project might have fared better with a larger engine designed specifically for railway (or marine) use, fitted behind one of the cabs. Even with one third of one vehicle given over to the engine it would still be a significant capacity increase over a 150.

No walk through capacity then. Problematic in an emergency.

 

[AHBD]

So is it modern power electronics that makes recent underfloor diesel electric D(e)MUs feasable?

Is it that much of a problem though? Maybe on routes where formations need to be the maximum length (ironically routes like London to Hastings, which did use DEMUs), but it wouldn't be a problem elsewhere- particularly compared with the tank engine + 2-3 <20m coaches (of which half of one coach would be a guard's van) that the 1950s DMUs were replacing. Cost would be more of an issue, and remained that way into the 1980s- hence 150s being ordered rather than 210-derivatives.

I do wonder if the 769 project might have fared better with a larger engine designed specifically for railway (or marine) use, fitted behind one of the cabs. Even with one third of one vehicle given over to the engine it would still be a significant capacity increase over a 150.

Northern Irelands 80 class and Castle class both had one diesel electric unit in front half of one end carriage, in the castle class this carriage was noisy and vibrating, and allocated to Smokers..
I can t remember being seated much in an 80 class engine carriage as they had a guards area section for bicycles parcel etc... So the seats in the engine carriage of such Halflocomotive-halfcarriage carriagesare sort of wasted unless you have a productive non passenger use for the space such as a guard's/ luggage van.

The stadler flirt DEMU has a short separate diesel generator carriage in the middle, but but with a passenger walkway, so all seating carriages would be quieter and vibrate less:
In the 50s to 70s would his have been feasable / affordable?...I presume the carriage might need to be longer?

 

[61653 HTAFC]

No walk through capacity then. Problematic in an emergency.

No walk through on 319s/769s anyway other than in an emergency. No walk through on 150/1s, 195s, 323s, 331s, 333s. Why would it be an issue on these hypothetical super-769s?

Northern Irelands 80 class and Castle class both had one diesel electric unit in front half of one end carriage, in the castle class this carriage was noisy and vibrating, and allocated to Smokers..
I can t remember being seated much in an 80 class engine carriage as they had a guards area section for bicycles parcel etc... So the seats in the engine carriage of such Halflocomotive-halfcarriage carriagesare sort of wasted unless you have a productive non passenger use for the space such as a guard's/ luggage van.

The stadler flirt DEMU has a short separate diesel generator carriage in the middle, but but with a passenger walkway, so all seating carriages would be quieter and vibrate less:
In the 50s to 70s would his have been feasable / affordable?...I presume the carriage might need to be longer?

Modern soundproofing and insulation would solve the noise/fumes issue- you wouldn't be replicating 1950s technology. If the noise issue was really insurmountable, then use the adjacent section (between the doors) as a dedicated cycle storage area. People are always complaining about a lack of bike spaces so you'd solve that problem too.

 

[OVS Flatbottom]

The stadler flirt DEMU has a short separate diesel generator carriage in the middle, but but with a passenger walkway, so all seating carriages would be quieter and vibrate less:
In the 50s to 70s would his have been feasable / affordable?...I presume the carriage might need to be longer?

Walker Brothers of Wigan did build some railcars for the Great Northern Railway (Ireland) in the 1930s, and then after WWII others for the Victorian Railways in Australia, that consisted of two passenger vehicles articulated to a central power unit. There are some images of the Australian ones at https://www.victorianrailways.net/motive power/walk280.html and https://auscisionmodels.com.au/280HP Walker Rail Car Page.htm. I don't think they had a passenger walkway through the power unit, although I could be wrong. I can't say either whether attaching more than two passenger coaches to a single power unit would have been possible with the technology of the time.

(Note that the single-unit Walker cars that were more common in Ireland, particularly on the County Donegal, also had the power unit - in this case, with the cab attached to it - articulated to the passenger saloon. Coincidentally, I think Walkers were also involved in the mechanical side of early BR DMUs.)

 

[Bevan Price]

In the 1950s generators and traction motors were also heavy.


But that didn't matter because most DMUs operated on track where there was no opportunity to go above 70mph anyway.

In fact it was the other way round - lines that had formerly had limits of 75 mph (or over) had their limits reduced to 70 mph because there were no (regular) trains permitted to exceed 70 mph.

 

[Strathclyder]

Walker Brothers of Wigan did build some railcars for the Great Northern Railway (Ireland) in the 1930s, and then after WWII others for the Victorian Railways in Australia, that consisted of two passenger vehicles articulated to a central power unit. There are some images of the Australian ones at https://www.victorianrailways.net/motive power/walk280.html and https://auscisionmodels.com.au/280HP Walker Rail Car Page.htm. I don't think they had a passenger walkway through the power unit, although I could be wrong.

This image of the first Walker 280hp power unit seperated from it's passenger vehicles awaiting scrapping (retrieved from your first link) in 1978 shows that were was a offset walkway that allowed passengers to move between the passenger vehicles on the move:

 

[Ken H]

Even if, as suggested above, in the 1950s generators and motors were heavier than mechanical gearboxes, by the time of the 2nd generation dmus like sprinters and 'super?'sprinters would underfloor diesel electric power units not have been feasable?

DC kit is heavier. AC alternators and squirrel cage motors are lighter. And electronics are lighter than banks of contactors and relays. That made the voyagers viable with underfloor engines.
DC kit also has maintenance heavy commutators.