Diesel-Electric v Diesel-Mechanical MU

[dieselbashdave]

Hi,

This is my first post as a new member. I have searched for this question before making this new thread and haven't found anything, so if this is the hundredth billionth time that someone's asked this question my apologies.

I'm wondering if my definition of a diesel-electric multiple unit (or loco) is this: it's essentially a diesel generator on board that produces electricity to drive a traction motor and there is no mechanical linkage to a gearbox etc as there would be in a say a car or bus.

Also, what is the performance (top speed, fuel consumption, power available for climbing Beattock on the WCML for example) compared to a diesel mechanical unit?

I'm just curious as to what the performance difference is (if any) and whether some companies will go for DEMUs over DMUs depending on the area.

Sorry for trying to ask two questions in one post incidentally, makes it slightly untidy I know!

Thanks

 

[WatcherZero]

Length plays into it a lot, for a single rail car or two car unit both with powered traction then a mechanical transmission is simple and meets the needs. If you have a 10 car train for example where not all the coachs have motors or generators then a mechanical transmission becomes impractical compared to electrical motors.

 

[dieselbashdave]

Hi WatcherZero,

Thanks for the response. So this is why 5 car Super Voyagers on Arriva XC/Virgin Trains are electrically driven (DEMU) and the 2-car Sprinter sets (150, 156, 158 etc) are mechanical/gearboxed.

In the case of say an HST or other diesel-electric loco, is the fact that it is a diesel-electric got anything to do with the maximum load a gearbox can take (e.g. 10 full coal-hoppers behind a Cl47 could overstress the gearbox or give very very slow performance) compared to an electric traction motor?

I'm pretty hopeless when it comes to electronics and electrical things, incidentally (just started learning about electrical components and stuff) so I may be talking out of my hat!

Incidentally I've only been "mechanically minded" for about 6 months after I had a very long Christmas break and spent most of it with my nose in a Haynes handbook for my Honda as well as an old encyclopedia!

 

[asylumxl]

While I believe most people think DMUs are more efficient as they are lighter, it's worth noting that a traction motor is capable of producing the majority of it's torque from very low RPMs (often noted as 0, which isn't quite true).

Whether or not this torque can be applied without slip is another problem entirely .

 

[dieselbashdave]

So the diesel-electrics have a lot more low-speed shove than a diesel-mechanical? This should theoretically give good acceleration (correct me if I'm wrong!) as well as a good power range to shove the train over something like Beattock summit?

 

[junglejames]

So the diesel-electrics have a lot more low-speed shove than a diesel-mechanical? This should theoretically give good acceleration (correct me if I'm wrong!) as well as a good power range to shove the train over something like Beattock summit?

Well the 22x's certainly have plenty of go in them. But thats probably down to the fact they have plenty of horses in the engines as well!

 

[asylumxl]

So the diesel-electrics have a lot more low-speed shove than a diesel-mechanical? This should theoretically give good acceleration (correct me if I'm wrong!) as well as a good power range to shove the train over something like Beattock summit?

In theory yes. In practice it's not quite so simple, as metal wheels on metal rails means low rolling resistance, but as a result, low adhesion too.

In higher power applications DMU are just not reliable. Very few transmission systems can handle the amount of power that would be put through them. While there are some high speed DMUs (e.g. 180), the amount of transmission problems (and fires for that matter) prove my point.

 

[dieselbashdave]

In higher power applications DMU are just not reliable. Very few transmission systems can handle the amount of power that would be put through them. While there are some high speed DMUs (e.g. 180), the amount of transmission problems (and fires for that matter) prove my point.

That's answered all my questions, thanks

I'm glad that I've developed a better understanding of "generators on wheels" and why traction motors are used instead of gearboxes in many cases.

Incidentally, is the 185 "Diesel-Dessie" mechanical or electric as I know that it spends most of it's life at 100 mph (only ever been on the FTP Desiros 4 times from Edinburgh to Manchester OR and back again but quite like them; pity about the hard seats though!).

I'm asking because I've noticed they seem to take off like they've been strapped to a rocket when leaving Waverley, moreso than the 170 I was on beforehand and they seem to be quiet inside (no transmission whine or "gear change" at 41 mph that I observed like on the 158s and 170s we have up here at Inverness).

 

[asylumxl]

The Class 185s used by FTPE are DMU.

The 185s use the same engines as the Voyager family (including Meridians). I believe they can infact do 125mph (despite the lack of need for such speed). Even if they desired to run them at 125mph, without removing passenger accomodation from atleast the first third of each DM (driving motor) they would not be able to get permission from the relevant authorities to run them as such.

Incidently, the 185 have 2250hp, while 170s only have 1266hp for a three car formation.

 

[dieselbashdave]

Wow there's a huge power difference between them. The Desiro seems to have a lot of power; suppose it's coz it's got to tackle the Pennines.

 

[LE Greys]

Looking at it on loco scale, comparing a Warship and a 40, you got a much higher power-to-weight ratio with the Warship, but more tractive effort with the 40. AIUI, you had a torque-converter and a gearbox with the Mekydro system, and Swindon Warships would change gear as they accelerated (which I suppose is hydro-mechanical rather than hydraulic). Again AIUI, Voith used several torque converters with different ratios, although I'm not sure how they changed from one to another. What to modern DMUs use and how do they change gear?

 

[WatcherZero]

The 185's operate a lot of the time with one of the three engines turned off as they are overpowered, heck going downhill or in depot moves they operate with only one engine.

 

[Nym]

Hence why it wouldn't be much of a problem to order another 51 coaches, unpowered and slot them into each of the TPE units...

 

[matchmaker]

Strictly speaking, most "Diesel Mechanical" units nowadays are "Diesel Hydraulic"!

 

[dieselbashdave]

The 185's operate a lot of the time with one of the three engines turned off as they are overpowered, heck going downhill or in depot moves they operate with only one engine.

IIRC there's something called "Ecomode" which shuts down the engines when they're not needed, like going downhill for example.

I'm not entirely certain what the effect is on the QSK 19 itself as they seem to smoke and cough a lot; there's a few YouTube vids (http://www.youtube.com/watch?v=v3ZQPZpUxw4) and a thread on RF about it (somewhere!)
--- old post above --- --- new post below ---

What to modern DMUs use and how do they change gear?

IIRC it's more or less the same method as you describe with the 40s. Any experts out there, feel free to correct me if I'm wrong though. There is a "changeover" at 40-41 mph (on the Sprinters anyway) that results in the engine decreasing in rpm noticably.

I know the Sprinters use Voith xmissions (it's 158s that are used on my local line).

 

[jopsuk]

Strictly speaking, most "Diesel Mechanical" units nowadays are "Diesel Hydraulic"!

Indeed- though the TOPS categorisation lumps mechanical and hyrdualic together. The new 172s are "proper" mechanical I think?

That is a point- if anyone is unsure, 1xx classes are all Mechanical/Hydraulic, whilst 2xx units are all Electric transmission.

 

[matchmaker]

Indeed- though the TOPS categorisation lumps mechanical and hyrdualic together. The new 172s are "proper" mechanical I think?

That is a point- if anyone is unsure, 1xx classes are all Mechanical/Hydraulic, whilst 2xx units are all Electric transmission.

The 172's are the only mechanical ones that I know of of the current classes. It was different with the first generation units where most were DMMU (Diesel Mechanical Multiple Units). There were DHMU (Diesel Hydraulic Multiple Units), but not many. Interestingly, the DHMU weren't used on the Western Region, which, given that virtually every WR loco was hydraulic, is a bit surprising! The most well known use of DHMU was the Class 127's on the St Pancras-Bedford services.

 

[jopsuk]

The Pacers were delivered as mechanical, but upgraded to hydraulic.

 

[dieselbashdave]

So the mechanical transmissions, do they have an actual "gear-stick" or are they "automatic boxes" to put it into crude layman's terms? Would the driver select a gear or would the train do that all by itself?

 

[notadriver]

The 172s have 6 speed ZF Ecomat transmission as used on buses and coaches. The operation of this is transparent to the driver.

 

[dieselbashdave]

Cool, thanks for telling me.

Just out of interest, are the 172s like the 170 Turbostars (i.e. mostly 3 coaches for express services)? If they are members of the Turbostar family, is there much equipment sharing between the two of them or are they brand-new with a new design altogether?

 

[asylumxl]

Cool, thanks for telling me.

Just out of interest, are the 172s like the 170 Turbostars (i.e. mostly 3 coaches for express services)? If they are members of the Turbostar family, is there much equipment sharing between the two of them or are they brand-new with a new design altogether?

The 172s have lightweight bogies (as used in 220/222s), and interior wise are more similar to Electrostars, but overall they're quite similar. The transmissionss were previously Voith and then put through a ZF final drive, but the 172s apparently now have ZF transmissions and final drives.

 

[LE Greys]

The most well known use of DHMU was the Class 127's on the St Pancras-Bedford services.

Noted for good acceleration. If one ever got loose on the fast lines from King's Cross to New Southgate, it could beat Deltics into a cocked hat. Probably that's one reason why most units use hydraulic these days.

Interesting contrast though 127 / 172
--- old post above --- --- new post below ---

So the mechanical transmissions, do they have an actual "gear-stick" or are they "automatic boxes" to put it into crude layman's terms? Would the driver select a gear or would the train do that all by itself?

Well, some of the boxes on heritage units were made by a company called Self Changing Gears. According to an old Ian Allen book, they are four-speed epicyclic, although I'm not sure how that works. Not sure if there's ever been a unit with a real gear lever, does anyone know?

 

[ChrisCooper]

The first generation DMUs used Self Changing Gears mechanical transmissions. These were used on quite a lot of buses particularly Bristols, and built under licence by Damiler for the Fleetline. They are semi-automatic which means they have no clutch but instead a fluid flywheel (also known as a fluid coupling) which is an oil filled centrifugal clutch (input shaft rotates vanes in the centre, flings oil out onto vanes on the outside which are connected to the output shaft). Almost identical fluid flywheels are also used on the hydralics and the 172s, and all automatic and semi-automatic buses. Won't go into the details but gear selection is by applying brake bands to parts of the gear train which causes them to transmit power at a certain gear ratio. Braking different parts changes the ratio, and the whole thing is locked up for top gear. Little has changed from the early pre-select gearboxes used on things like RTs and the GWR Railcoaches, to the boxes on modern buses and 172s. The SCG boxes on the 1st generation DMUs were not capable of changing up at full throttle, the engine had to be returned to idle before selecting a new gear. Gear selection was by a lever operated by the drivers right hand (left hand throttle). Pacers also used SCG boxes but fully automatic. They could not cope with the higher power and automatic operation, so tended to fail or part company with the pacer and end up on the track. The boxes on the 172s and most modern buses are oviously computer controlled with the computer also controlling the engine, giving smooth changes (not been on a 172 yet but I imagine you can't feel the gear changes, I know you can't on most modern buses).

With hydralic transmission, the "gear change" comes from switching from the torque converter to the fluid coupling. This is done by draining oil from the converter and filling the coupling, and also gives a smooth change. AFAIK 180s and 185s use a 3 step version with two torque converters geared at different ratios, and the fluid coupling. Warships also had multi stage setups, again working in the same way. Westerns were a mix of hydralic and mechanical. They had a torque converter but also a mechanical gearbox. Some bus and truck transmissions are like this (Voith who make the hydralic transmissions for most trains do a 3 speed bus gearbox with a torque converter which was particularly popular on the MCW Metrobus. Again gives nice smooth changes. Contrast to some more modern buses (before computer control) where you could almost get whiplash from the changeups (I imagine Pacers were the same).

One particular advantage with diesel electric for hauling heavy loads is the ability to have much finer control over wheelspin. With mechanical or hydralic you can really only cut the engine power and it's fairly slow to react, plus it effects all wheels driven by that engine. With electrics though you can cut power to individual bogies, individual axles, or even have very fine and precise control over them. The 60s have a very advanced wheelspin prevention system with seperate excitation of each motor giving fine control and doppler radar to measure actual speed vs. wheel speed, allowing creep control which gets the most traction out of the wheels (it allows the wheels to slip a little which actually gives maximum traction).

 

[dieselbashdave]

One particular advantage with diesel electric for hauling heavy loads is the ability to have much finer control over wheelspin. With mechanical or hydralic you can really only cut the engine power and it's fairly slow to react, plus it effects all wheels driven by that engine. With electrics though you can cut power to individual bogies, individual axles, or even have very fine and precise control over them. The 60s have a very advanced wheelspin prevention system with seperate excitation of each motor giving fine control and doppler radar to measure actual speed vs. wheel speed, allowing creep control which gets the most traction out of the wheels (it allows the wheels to slip a little which actually gives maximum traction).

Cool, the WSP is pretty technologically advanced by the sounds of it. Wonder if I could get it stuck onto the 158s up here in the winter; hopefully will stop the unit sliding right past you hissing and spraying sand under it's wheels

Naah, to be fair an over-run at winter-time only happened once to me in 3 years of travelling as a commuter on the trains.

Going off at a tangent, didn't 158s have problems at introduction with not activating track circuits due to the brakes squishing leaves into maple syrup or is that another urban myth on Wikipedia?

 

[ChrisCooper]

Cool, the WSP is pretty technologically advanced by the sounds of it. Wonder if I could get it stuck onto the 158s up here in the winter; hopefully will stop the unit sliding right past you hissing and spraying sand under it's wheels

Naah, to be fair an over-run at winter-time only happened once to me in 3 years of travelling as a commuter on the trains.

Going off at a tangent, didn't 158s have problems at introduction with not activating track circuits due to the brakes squishing leaves into maple syrup or is that another urban myth on Wikipedia?

You're mixing wheel slide under braking and wheel slip under power. Don't think the 60s even have WSP. 158s do but under bad conditions it won't do more than prevent the wheels from getting flat spots. Just locking works best when you want to stop ASAP.

Wheel slip prevention on 158s and other hydralics is very basic, just throttle back the engine.

You are right, 158s had big problem with track circuits when first introduced. It's because they have disk brakes yet are fairly light weight (disk braked EMUs never had problems, presumably due to the heavier motor coaches). Tread brakes keep the wheels clean by scrubbing them so they conduct better, wheras disks don't causing a film to build up on the wheel which prevent conduction. The temporary solution was to form hybrid 156/158 sets (1 car from each) during leaf fall so the 156 would operate the track circuits. Later track circuit actuators were fitted which induce a current to flow through the axle and help conduction of the track circuit. Other Sprinter and Pacer classes were also fitted.

 

[dieselbashdave]

You're mixing wheel slide under braking and wheel slip under power.

Doh! I should have noticed that :roll:. That'll teach me to post on a forum when half asleep.
--- old post above --- --- new post below ---

You are right, 158s had big problem with track circuits when first introduced. It's because they have disk brakes yet are fairly light weight (disk braked EMUs never had problems, presumably due to the heavier motor coaches). Tread brakes keep the wheels clean by scrubbing them so they conduct better, wheras disks don't causing a film to build up on the wheel which prevent conduction. The temporary solution was to form hybrid 156/158 sets (1 car from each) during leaf fall so the 156 would operate the track circuits. Later track circuit actuators were fitted which induce a current to flow through the axle and help conduction of the track circuit. Other Sprinter and Pacer classes were also fitted.

Were there ever any accidents or near-misses caused by a 158's failure to show a danger signal when occupying the block or was the problem discovered before a dangerous situation could occur?

 

[LE Greys]

The first generation DMUs used Self Changing Gears mechanical transmissions. These were used on quite a lot of buses particularly Bristols, and built under licence by Damiler for the Fleetline. They are semi-automatic which means they have no clutch but instead a fluid flywheel (also known as a fluid coupling) which is an oil filled centrifugal clutch (input shaft rotates vanes in the centre, flings oil out onto vanes on the outside which are connected to the output shaft). Almost identical fluid flywheels are also used on the hydralics and the 172s, and all automatic and semi-automatic buses. Won't go into the details but gear selection is by applying brake bands to parts of the gear train which causes them to transmit power at a certain gear ratio. Braking different parts changes the ratio, and the whole thing is locked up for top gear. Little has changed from the early pre-select gearboxes used on things like RTs and the GWR Railcoaches, to the boxes on modern buses and 172s. The SCG boxes on the 1st generation DMUs were not capable of changing up at full throttle, the engine had to be returned to idle before selecting a new gear. Gear selection was by a lever operated by the drivers right hand (left hand throttle). Pacers also used SCG boxes but fully automatic. They could not cope with the higher power and automatic operation, so tended to fail or part company with the pacer and end up on the track. The boxes on the 172s and most modern buses are oviously computer controlled with the computer also controlling the engine, giving smooth changes (not been on a 172 yet but I imagine you can't feel the gear changes, I know you can't on most modern buses).

With hydralic transmission, the "gear change" comes from switching from the torque converter to the fluid coupling. This is done by draining oil from the converter and filling the coupling, and also gives a smooth change. AFAIK 180s and 185s use a 3 step version with two torque converters geared at different ratios, and the fluid coupling. Warships also had multi stage setups, again working in the same way. Westerns were a mix of hydralic and mechanical. They had a torque converter but also a mechanical gearbox. Some bus and truck transmissions are like this (Voith who make the hydralic transmissions for most trains do a 3 speed bus gearbox with a torque converter which was particularly popular on the MCW Metrobus. Again gives nice smooth changes. Contrast to some more modern buses (before computer control) where you could almost get whiplash from the changeups (I imagine Pacers were the same).

One particular advantage with diesel electric for hauling heavy loads is the ability to have much finer control over wheelspin. With mechanical or hydralic you can really only cut the engine power and it's fairly slow to react, plus it effects all wheels driven by that engine. With electrics though you can cut power to individual bogies, individual axles, or even have very fine and precise control over them. The 60s have a very advanced wheelspin prevention system with seperate excitation of each motor giving fine control and doppler radar to measure actual speed vs. wheel speed, allowing creep control which gets the most traction out of the wheels (it allows the wheels to slip a little which actually gives maximum traction).

Hmm, fascinating. I have noticed the much smoother upchange on modern units (and buses) and remember how clunky the 101s could be sometimes, but I honestly had no idea that the driver would have to decide when to change gear and control the throttle. They must have been a lot harder to drive, and I take my hat off to them.

 

[ChrisCooper]

Hmm, fascinating. I have noticed the much smoother upchange on modern units (and buses) and remember how clunky the 101s could be sometimes, but I honestly had no idea that the driver would have to decide when to change gear and control the throttle. They must have been a lot harder to drive, and I take my hat off to them.

I presume you don't drive a car? From the gearchange point of view a first generation DMU is easy, no clutch. Basically it's open the throttle, wait until the revs reach a certain point (marked "change up" on the rev counter, to make it nice and simple), close the throttle, wait a few seconds for the revs to die down, select the next gear, wait a second or two, open the throttle and so on and so on. The only skill is to time the changes right to get a smooth change, but it's more about not rushing than anything. To stop just leave it in top gear and then knock it into neutral as it comes to a stop. Simples. The real skill in driving first generation DMUs is the vacume brake.

 

[notadriver]

Are you a train driver Chris?