Diesel v Electric bogies

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Domeyhead

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I don't follow this thread regularly so apologies if this has come up before but can any traction engineers explain why diesel electrics (say) above 2000hp are often co-co's, especially freight locos, whereas electric locos are often bo-bo's even though the power can often be well over double this amount. I have just seen a mock up of the new Stadler class 93 freight bi mode which is well over 5000hp but still a bo-bo. I have always assumed that although the method of creating the power differs between them, the actual traction motors in the bogies are basically the same, so the more wheels, the more motors and the more motors the more power at the railhead?
 
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edwin_m

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I guess it's because the diesel engine is heavier so it needs more wheels to support it within the maximum allowed axle load.
 

DelW

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The extra axles being needed for weight carrying rather than power or adhesion is demonstrated by some diesel classes having unpowered axles in their bogies (e.g. classes 31 and 40).
 

43096

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The extra axles being needed for weight carrying rather than power or adhesion is demonstrated by some diesel classes having unpowered axles in their bogies (e.g. classes 31 and 40).
Wabtec (GE as was) in the US builds new A1A-A1A locos (model ET44C4).
 

coppercapped

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I don't follow this thread regularly so apologies if this has come up before but can any traction engineers explain why diesel electrics (say) above 2000hp are often co-co's, especially freight locos, whereas electric locos are often bo-bo's even though the power can often be well over double this amount. I have just seen a mock up of the new Stadler class 93 freight bi mode which is well over 5000hp but still a bo-bo. I have always assumed that although the method of creating the power differs between them, the actual traction motors in the bogies are basically the same, so the more wheels, the more motors and the more motors the more power at the railhead?
Nothing to do with power, but all to do with axle load.
 

Domeyhead

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Exactly Diesel will traditional have had a minimum 15+tonnes extra
Thanks for that. Couple of things though - the extra powered wheelsets in a co-co would give more tractive effort at slow speed which would be ideal in a freight loco, and in a Class 93 there is a large battery pack which can deliver up to 1000hp and which must have quite a weight perhaps even comparable to a diesel power plant. Will be interesting to see the weights of the new bi-modes compared to (say) a Class 66.
 

hexagon789

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Thanks for that. Couple of things though - the extra powered wheelsets in a co-co would give more tractive effort at slow speed which would be ideal in a freight loco, and in a Class 93 there is a large battery pack which can deliver up to 1000hp and which must have quite a weight perhaps even comparable to a diesel power plant. Will be interesting to see the weights of the new bi-modes compared to (say) a Class 66.
The 93 will apparently weigh in at 90 tonnes (which is about 5 tonnes heavier than a 68, and 4 tonnes more than the 88s), for comparison a 66 weighs 130 tonnes.
 

mrgreen

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Spreading the weight over more wheels won't help low speed performance..... you increase the metal-on-metal contact area, but the pressure on the contact area goes down in proportion, so slipping remains the same.
 

DB

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The 93 will apparently weigh in at 90 tonnes (which is about 5 tonnes heavier than a 68, and 4 tonnes more than the 88s), for comparison a 66 weighs 130 tonnes.

As an aside, I believe that the 68 and 88 have identical bogies and traction motors. Wonder whether the 93 will too?
 

hexagon789

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As an aside, I believe that the 68 and 88 have identical bogies and traction motors. Wonder whether the 93 will too?
Certainly the 68 and 88 apparently share the same motors (not sure how the higher output of the 88 is managed on the same motors though).

Not sure about the 93, but it's electrical output is apparently only 550kW greater than that of the 88, so it may well be so
 

R Martin

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In general the main difference between the electric and diesel locomotives is that when starting away the tractive efforts are similar, but as the locomotive accelerates the power required goes up by the square so that you soon run into the diesel engines maximum power output, whereas the electric engine can provided much more power, via its transformer. That means that he diesel locomotives tractive effort drops away quite steeply. In the case of the electric engine the tractive effort stays relatively constant, to a much higher speed. The electric locomotive also has the added advantage that a limited overload (ie. Class 86/87tc) Can be tolerated for a specified short time.
Come on you engineers help me with a clearer explanation, I did my applied mechanics 60 odd years ago!
 

apk55

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With modern traction motors the size is determined by two factors - required tractive effort and maximum speed. Theoretically a motor could provide maximum tractive effort at maximum speed but this would require an massive power source of over 2MW for a single class 68/88 motor. Therefore this is never normally done and at higher speeds the tractive effort is reduced because of the power source limitations.

With the traditional DC commutator motor the commutator can provide an extra power limitation, apply too many volts at high current to the motor and the commutator will flash over.
 

AM9

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With modern traction motors the size is determined by two factors - required tractive effort and maximum speed. Theoretically a motor could provide maximum tractive effort at maximum speed but this would require an massive power source of over 2MW for a single class 68/88 motor. Therefore this is never normally done and at higher speeds the tractive effort is reduced because of the power source limitations.

With the traditional DC commutator motor the commutator can provide an extra power limitation, apply too many volts at high current to the motor and the commutator will flash over.
Tractive effort isn't usually an issue at high speeds when strong acceleration wouldn't normally be needed.
Standard DC series wound traction motors need field weakening at high rtotational speeds to reduce the back EMF and allow more rotor current to be drawn without increasing the overall applied voltage.
 

apk55

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Starting tractive effort is often a big issue particularity for freight as it can determine the maximum load a loco can haul. If insufficient there is a risk a train can stall on an upgrade. Passenger trains normally have a good surplus of tractive effort for good acceleration so rarely have problems (although with the HST power cars I think they have cut this a bit fine as they can have difficulties if one power car fails)

What I do find interesting is that in America some railways use Slugs. This is where they have taken the diesel engine out of an old locomotive (and replaced it with ballast) and arranged another coupled locomotive to power the locomotives motors via power cables. What this does is double the starting tractive effort so twice the load can be hauled, although at any speed the tractive effort will be no more than a single locomotive. Ideal for shunting or working slow speed industrial lines.
 

edwin_m

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Starting tractive effort is often a big issue particularity for freight as it can determine the maximum load a loco can haul. If insufficient there is a risk a train can stall on an upgrade. Passenger trains normally have a good surplus of tractive effort for good acceleration so rarely have problems (although with the HST power cars I think they have cut this a bit fine as they can have difficulties if one power car fails)

What I do find interesting is that in America some railways use Slugs. This is where they have taken the diesel engine out of an old locomotive (and replaced it with ballast) and arranged another coupled locomotive to power the locomotives motors via power cables. What this does is double the starting tractive effort so twice the load can be hauled, although at any speed the tractive effort will be no more than a single locomotive. Ideal for shunting or working slow speed industrial lines.
Passenger locomotives tend to have less tractive effort at lower speeds than freight locomotives, because they are lighter and the motors and gearing are designed more for higher speed. The reason this doesn't matter is because the trains they haul are also lighter.

In America there are almost no passenger trains to worry about (and the freight companies spend little time worrying over what passenger traffic there is). For a line with a long mainly flat route but short sections of severe gradients, it may make sense to let the trains slow right down on those gradients as long as they don't actually come to a stop. The alternatives would be to provide extra power which might not really be necessary on the flatter sections, or to stop the train to attach and remove helper locomotives. A slug might be useful on that kind of route, and more recently AC motors have offered better tractive effort at low speeds without making much difference to top speed where the diesel engine limits the power available.

This difference is probably why the class 66 didn't have AC motors, despite these being fairly widespread in America when they were built in the late 90s. Most freight trains in Europe have to share with passenger trains, so it's probably not acceptable for them to plod along at walking pace whenever they hit a gradient. Also of course the trains are mostly shorter than even a single locomotive might haul in America. So low-speed tractive effort is less important.
 

AM9

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... What I do find interesting is that in America some railways use Slugs. This is where they have taken the diesel engine out of an old locomotive (and replaced it with ballast) and arranged another coupled locomotive to power the locomotives motors via power cables. What this does is double the starting tractive effort so twice the load can be hauled, although at any speed the tractive effort will be no more than a single locomotive. Ideal for shunting or working slow speed industrial lines.

That is also a cheap (and flexible) way of increasing the adhesive weight of the train's traction provision, (and the ratio of adhesive weight for the whole train). I would imagine that with highly responsive wheelslip protection, the doubling of adhesive weight would allow more of the torque to be used continuously and with less wear on the track.
 

ac6000cw

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Certainly the 68 and 88 apparently share the same motors (not sure how the higher output of the 88 is managed on the same motors though).

Not sure about the 93, but it's electrical output is apparently only 550kW greater than that of the 88, so it may well be so
As far a I know (I'm an electronics engineer rather than a 'heavy' electrical engineer) the losses in motors are basically resistive (proportional to the square of the current through the windings) and 'iron' losses due to eddy currents and hysteresis effects in the rotor and stator materials - again these are related to the the current (which is what generates the magnetic fields in the first place).

So really it's the maximum tractive effort (generated by the motor current) that the loco needs to produce which determines the motor (maximum current) rating at low speeds - the 'power rating' isn't a factor at low speeds.

At low speeds a relatively low power diesel engine can easily push enough current through the motors to reach their current limits - at that point it's a matter of pushing enough cooling air through the motors to stop them overheating. I think the worse case scenario for motor overheating is the heavy freight climbing a long gradient at low speed.
 

hexagon789

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As far a I know (I'm an electronics engineer rather than a 'heavy' electrical engineer) the losses in motors are basically resistive (proportional to the square of the current through the windings) and 'iron' losses due to eddy currents and hysteresis effects in the rotor and stator materials - again these are related to the the current (which is what generates the magnetic fields in the first place).

So really it's the maximum tractive effort (generated by the motor current) that the loco needs to produce which determines the motor (maximum current) rating at low speeds - the 'power rating' isn't a factor at low speeds.

At low speeds a relatively low power diesel engine can easily push enough current through the motors to reach their current limits - at that point it's a matter of pushing enough cooling air through the motors to stop them overheating. I think the worse case scenario for motor overheating is the heavy freight climbing a long gradient at low speed.
Which would explain why Class 73s with their original engines can still shift a decent weight on diesel even if they won't manage much of a speed.
 

ac6000cw

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Which would explain why Class 73s with their original engines can still shift a decent weight on diesel even if they won't manage much of a speed.
Exactly.

Also think about the 'cow and calf' double 08s (class 13) locos they used at Tinsley yard to shove trains over the hump - 70,000 lbs (311 kN) of tractive effort (more than a class 56) spread across six axles but very limited power (700hp total).

I would imagine that with highly responsive wheelslip protection, the doubling of adhesive weight would allow more of the torque to be used continuously and with less wear on the track.
Yes - similar performance at low speeds to a pair of locos, without the maintenance cost of the second diesel engine/cooling system/alternator/air compressor etc.

I think the ones used more for local freight work (versus those used mostly for yard switching/hump pushing) are sometimes a bit more sophisticated so that the 'slug' can be electrically disconnected at higher speeds and/or retains its dynamic braking equipment.

and more recently AC motors have offered better tractive effort at low speeds without making much difference to top speed where the diesel engine limits the power available.
The performance of AC-motored locos can be phenomenal at low speeds - there was an article in Trains magazine some years ago that mentioned a run where a coal train was being taken up Cranberry Grade in West Virginia (a long twisting 2 - 2.5% gradient) in a blizzard. The four GE AC-drive locos (two front, two rear) spent several hours at maximum tractive effort (that the rail conditions allowed) at a speed somewhat less than walking pace...a comment from a railroad person was that a torch pointed at the ground - from the cab side window - was really useful in those conditions for checking that you were actually moving forward !!
 
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AM9

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Which would explain why Class 73s with their original engines can still shift a decent weight on diesel even if they won't manage much of a speed.
It is also why a class 769 will likely not have problems climbing quite steep gradients, albeit at low speed, just as the 319s could start on a 1:27 in the TL core.
 

hexagon789

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Exactly.

Also think about the 'cow and calf' double 08s (class 13) locos they used at Tinsley yard to shove trains over the hump - 70,000 lbs (311 kN) of tractive effort (more than a class 56) spread across six axles but very limited power (700hp total).
700hp is still quite a lot, but the huge tractive effort is of course what really counts in shunting.

It is also why a class 769 will likely not have problems climbing quite steep gradients, albeit at low speed, just as the 319s could start on a 1:27 in the TL core.
Though they aren't too underpowered on diesel iirc, aren't they designed to be still able to do 90?
 

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It is also why a class 769 will likely not have problems climbing quite steep gradients, albeit at low speed, just as the 319s could start on a 1:27 in the TL core.
The 769 may be a bit worse at starting on gradients than a 319 because the diesel engines add to the total weight but not to the weight on powered axles, so the same tractive effort is trying to shift more weight. Either will be worse than a Sprinter, which has half of its axles powered. Whether this theoretical difference has any practical effect is probably a question for one of the 769 threads.
 

HSTEd

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Supposedly slugs are very popular with freight crews in America because there is no engine noise or vibration in the cab.

So shunters go to enormous lengths to ensure that the slug ends up at the front of the formation
 

AM9

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The 769 may be a bit worse at starting on gradients than a 319 because the diesel engines add to the total weight but not to the weight on powered axles, so the same tractive effort is trying to shift more weight. Either will be worse than a Sprinter, which has half of its axles powered. Whether this theoretical difference has any practical effect is probably a question for one of the 769 threads.
My comment refereed to the traction available at the axles. If comparing classes obviously the number of driven vs trailing wheels, and the distribution of the total weight would make a difference. Some of the 1:29 between City Thameslink and Blackfriars is covered and its location was not exactly a tunnel of broadleaf trees so the railhead conditions would probably only be worsened through rain falling on the open part for the gradient. The maximum torque available at the wheel/rail interface at very low speeds might be higher on a DEMU (or EDMU) than on a mechanical or hydraulic transmission.
 

Railperf

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Class 37, 50 and 55 shared the same bogies - but what about traction motors? Were they vastly different? I would assume so? I've never looked at the specs on these.

In regards to HST's - there were plans to put bigger engines in, but would the existing traction system including traction motors have been able to cope with the extra power?
 

randyrippley

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I don't follow this thread regularly so apologies if this has come up before but can any traction engineers explain why diesel electrics (say) above 2000hp are often co-co's, especially freight locos, whereas electric locos are often bo-bo's even though the power can often be well over double this amount. I have just seen a mock up of the new Stadler class 93 freight bi mode which is well over 5000hp but still a bo-bo. I have always assumed that although the method of creating the power differs between them, the actual traction motors in the bogies are basically the same, so the more wheels, the more motors and the more motors the more power at the railhead?

There's something everyone's forgetting
Ignoring the 1960s designs, virtually all the UK CoCo designs post class 47, electric or diesel have been primarily freight locos with a max speed of 80 or less, while the BoBo designs were all passenger or mixed traffic designs with a max speed of 100+. The 89 obviously was an exception. During the 1970s BR developed an antipathy toward high speed CoCo designs, believing they damaged the track, I believe you're still seeing the results of that belief.

Class 37, 50 and 55 shared the same bogies - but what about traction motors? Were they vastly different? I would assume so? I've never looked at the specs on these.

In regards to HST's - there were plans to put bigger engines in, but would the existing traction system including traction motors have been able to cope with the extra power?
37 & 50 were geared differently - one 50 was fitted with 37 bogies to reduce the gearing and try to use it for heavy haulage
 

43096

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In regards to HST's - there were plans to put bigger engines in, but would the existing traction system including traction motors have been able to cope with the extra power?
That had been dropped by the time the production power cars were built: there was space for a V16 Valenta in 41001/2, but not in 43002 upwards. Production power car main alternators are rated at 1,868kW (against 1,678kW engine output), so not enough margin for a 33% hike in engine power, but then by that stage the V16 plan had been dropped, so why specify the extra capability for something you don't need. I've not seen the rating of the original main alternators in 41001/2 quoted anywhere.
 

DB

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Class 37, 50 and 55 shared the same bogies - but what about traction motors? Were they vastly different? I would assume so? I've never looked at the specs on these.

There were two types of bogie frame - fabricated and cast. 37s and Deltics had fabricated originally. Deltics were later fitted with cast, and 50s only ever had cast. When Deltics and 50s were scrapped a number of the cast bogies ended up under 37s (don't know whehter any were made specifically for them). Most of the Deltics sold for preservation were swapped back to fabricated bogies first as BR wanted to keep the cast ones.

The motors were the same on all of them. Not sure about gearing variations.
 

AM9

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Class 37, 50 and 55 shared the same bogies - but what about traction motors? Were they vastly different? I would assume so? I've never looked at the specs on these.
The feted 2460kW of the class 55s was the maximum power at the engine's output shaft/input to the generator. It would be an absolute maximum determined by the injection settings of the engine. There would be a power loss in the generator, auxiliaries and control gear so the power available to the motors might be 20% lower. Similarly, the class 50 power available at the motors would be lower than the headline 2010kW engine maximum figure quoted. Within those sort of power ranges, it is possible that the EE motors might have been the same (for the 50s and 55s) as DC motors are quite basic designs that can be range rated. The difference in physical size/weight would be minimal and they would fit into the same bogie frame mountings.
 
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