For a given electric motor and given gearing, smaller wheels do mean higher theoretical tractive effort. But the smaller wheel also means lower factor of adhesion because of a smaller contact patch.
A given motor can have the same theoretical tractive effort regardless of wheel size, simply by changing the gear ratio in proprotion to the wheel size.
A given motor can be used to optimize speed or tractive effort regardless of wheel size (within practical limits) simply by choosing the correct gearing.
This is why high speed steam locomotives needed large diameter drivers and high tractive effort freight locmotives needed smaller drivers (in general, although many other considerations including cylinder bore and stroke, etc apply).
But with diesel-electrics and straight electrics this rule is out the window because there is always a choice of gearing. (Wheel motors are the exception, but that is a whole new story.) Indeed larger drivers for higher effective tractive effort makes sense ---to provide a large contact patch.
could TE be measured on a supplier dynamotor just as they do automobiles?
AnthonyVHow is the TE measured? Or, is it back-calculated
As for drawbar pull-- when you're measuring it with a dynamometer car you have to keep track of the grade and acceleration so you can correct the measured drawbar pull to what it would have been at constant speed on the level. (And I guess you'd have to keep track of the locomotive's fuel level. if you wanted to correct for acceleration really correctly.)
timz You'll see people using the term that way, but far as I'm concerned "drawbar pull" (which is measured at the coupler) isn't the same as "tractive effort". The latter should be measured at the wheelrim/rail. In other words, drawbar pull = TE minus the force needed to move the engine itself. In any case, yes, TE is a linear force, not a torque.
You'll see people using the term that way, but far as I'm concerned "drawbar pull" (which is measured at the coupler) isn't the same as "tractive effort". The latter should be measured at the wheelrim/rail. In other words, drawbar pull = TE minus the force needed to move the engine itself.
In any case, yes, TE is a linear force, not a torque.
If that is the case then:
Drawbar pull = TE - Drag Force - Grade Force - Force to Accelerate Loco (any other forces acting on the locomotive?)
Drawbar pull = TE - Drag Force - Grade Force - Force to Accelerate Loco
(any other forces acting on the locomotive?)
On level track at constant speed:
Drawbar pull = TE - Drag Force
The drag force becomes smaller at lower and lower speeds. At low speeds only
Drawbar Pull = TE (approximately)
How is the TE measured? Or, is it back-calculated based on the drawbar pull measured with a dynomometer car? Or, is it measured using some gigantic dyno?
Anthony V.
fredswainPeople need to think of tractive effort for what it is which is available torque at the wheels. It has nothing to do with traction. That is called adhesion. You can have all the TE in the world but if your wheels slip at half of that, it's worthless which is where the factor of adhesion comes into play. TE looks higher on modern engines over steam engines for a number of reasons but a very often overlooked one is wheel diameter. Let's say we have any generic steam engine. Let's also say that the only thing we changed was wheel diameter, assuming of course we could. If the wheels were made smaller, we'd have more TE. If the wheels were larger we'd have less TE. This applies with no other changes whatsoever including boiler pressure. In the case of a diesel, it would apply with no change in generated electrical horsepower.
People need to think of tractive effort for what it is which is available torque at the wheels. It has nothing to do with traction. That is called adhesion. You can have all the TE in the world but if your wheels slip at half of that, it's worthless which is where the factor of adhesion comes into play.
TE looks higher on modern engines over steam engines for a number of reasons but a very often overlooked one is wheel diameter. Let's say we have any generic steam engine. Let's also say that the only thing we changed was wheel diameter, assuming of course we could. If the wheels were made smaller, we'd have more TE. If the wheels were larger we'd have less TE. This applies with no other changes whatsoever including boiler pressure. In the case of a diesel, it would apply with no change in generated electrical horsepower.
Hogwash.This is wrong in the case of a steam locomotive, and it is wrong in the case of Diesel-electric locomotives too. A larger wheel increases leverage, and it also increases the size of the wheels contact patch which increases the potential force that can be applied without slipping. In the case of steam locomotives you may have noticed that all later "Super-power" steam locomotives had comparatively larger diameter drive wheels than the locomotives that they replaced. The larger diameter drivers on the NKP S class Berks (69" versus 63") on the Mikados they replaced allowed a longer stroke (34" versus 32"), in turn the crank pin was lengthened creating greater mechanical leverage, crankpin is further from the center of the wheel. In every late generation similar changes were made N&W Y6 compared to early Y3 and Y4, C&O H-8 Allegheny versus the predecessor H-7, DM&IR M3 Yellowstone versus the preceding MS rebuilds, in all cases the driver diameter was increased, and the reason wasn't to run faster.
With modern Diesel locomotives, driver size has increased from the 40" typical of the '40s through '70s era, to 44" or 45" found on today's SD70ACe and ES44AC locomotives. It is no coincidence that the EMD locomotives with the smallest drive wheel diameter were the E-units used in passenger service.
I thought tractive effort was measured at the coupler.
fredswainPeople need to think of tractive effort for what it is which is available torque at the wheels. It has nothing to do with traction. That is called adhesion. You can have all the TE in the world but if your wheels slip at half of that, it's worthless which is where the factor of adhesion comes into play.
I'm afraid that I don't understand the concepts of (1) tractive effort having nothing to do with traction and (2) having a certain amount of tractive effort but having part of that be worthless.
I guess my question is if tractive effort is actually "available torque at the wheels" instead of whatever tractive effort a locomotive is actually producing, how is that actual tractive effort measured?
Thanks.
I think of horsepower as what does the work and torque as the amount of leverage to do it. That's not quite accurate but gets the point across. Our smaller wheeled engine of the same power level has more TE because we have slowed it down. It takes more revolutions of the wheels to go the same distance. Basically the smaller wheels put it in a lower gear. This increases torque (TE) however it does nothing for power. If we have a low factor of adhesion, we'll just spin and all this TE does nothing. We could add more weight to cure this but we add to rail stress. At some point we need to add wheels. You get the idea.
While we generally think of only logging locomotives as geared, the reality is that all steam engines are always stuck in only 1 gear. The faster the engine was designed to move, the larger the drivers which is equal to a higher gear. Of course then you lower your TE but these were designed for lighter faster trains. Some of these trains needed helpers over certain grades just for passenger duty as a result even though their rated horsepower level may have seemed adequate. You try starting your car at slow speeds on a hill in high gear! If you only needed slow speed operation but needed the leverage to move trains, you built an engine with small wheels. These are basically low gear engines. They weren't fast but they could pull well. Give me a lever long enough and I will move the world! A little amount of power can move a lot of weight if given enough leverage and time .
There were many things that affected TE of course but wheel size was definitely important. Don't think that just because 1 engine has a higher TE than another that it is necessarily better. You need to see the big picture. What is the adhesion of each engine? At what speed is TE rated for each?
It is hard to compare a diesel/electric to a steam engine as a result of these differences. An electric motor has max torque (TE) at 0 rpm. However at 0 rpm you aren't doing work as you can't have horsepower at 0 rpm. TE is so high because this is the point of max leverage.
Lars Loco10% increased tractive effort, not bad doing this just adjusting the software...
The 180,000- and 200,000-pound figures are the maximum traction-effort levels that the software allows the units to produce. The extent to which a unit actually produces 200,000 pounds of tractive effort is a function not only of its software, but also of factors such as weight, rail conditions, and truck design.
Lars Locois it safe to assume that modern engines will always deliver ~90% of the prime movers output into drawbar-force over their entire speed-range ?
-Don (Random stuff, mostly about trains - what else? http://blerfblog.blogspot.com/)
JayPotterLars LocoOk, 90% may not apply to extreme conditions (speeds down to zero), as this should result in spinning wheels. The units' standard adhesion-management software prevents any traction motor from producing more than 30,000 pounds of tractive effort. In other words, once speed drops below about 11 mph, the unit will not produce more than 180,000 pounds of tractive effort regardless of how much further the speed drops. Advanced versions of the software set the limit higher, up to a maximum of 36,000 pounds per motor or 200,000 pounds per unit.
Lars LocoOk, 90% may not apply to extreme conditions (speeds down to zero), as this should result in spinning wheels.
The units' standard adhesion-management software prevents any traction motor from producing more than 30,000 pounds of tractive effort. In other words, once speed drops below about 11 mph, the unit will not produce more than 180,000 pounds of tractive effort regardless of how much further the speed drops. Advanced versions of the software set the limit higher, up to a maximum of 36,000 pounds per motor or 200,000 pounds per unit.
10% increased tractive effort, not bad doing this just adjusting the software...
timzLars Loco is it safe to assume that modern engines will always deliver ~90% of the prime movers output into drawbar-force over their entire speed-range ? If an AC6000 produced 5400 dbhp at 1 mph that would be 2,000,000 lb.
Lars Loco is it safe to assume that modern engines will always deliver ~90% of the prime movers output into drawbar-force over their entire speed-range ?
Thanx Tim for the calculations,
Ok, 90% may not apply to extreme conditions (speeds down to zero), as this should result in spinning wheels, how about 10mph (cont. pull.) to 75mph?
For me, It would be interesting to know how much efficient the transmission-system over the speed range works, and how much it was improved over the last 20 years. Of course, still hoping to gain some results from real field tests...is the rail industrie really so shy
Cheers
lars
GP40-2 JayPotterif I were going to apply a tractive-effort formula to an AC6000CW in notch eight at a speed of around 11 or more miles per hour, it would be (TE in pounds) equals (6000) times (factor X) divided by (speed in miles per hour), with factor X being a number somewhere between around 329 and around 337. Again, I'm not sure how useful the result of that calculation would be. Very good assumption. The 6000's "x-factor" ranges from 329 on a hard, slow speed pull to 348 at higher speeds.
JayPotterif I were going to apply a tractive-effort formula to an AC6000CW in notch eight at a speed of around 11 or more miles per hour, it would be (TE in pounds) equals (6000) times (factor X) divided by (speed in miles per hour), with factor X being a number somewhere between around 329 and around 337. Again, I'm not sure how useful the result of that calculation would be.
My understanding of the TE vs speed for a steam locomotive is that it is maximum at 0 speed when the full operating boiler pressure is applied to the cylinder. As steam starts to flow, cylinder pressure drops due to friction of flow thru the throttle, pipes and valves.
AC motors have a starting torque about 50% of their max due to the severe lag between the rotating field and the stationary rotor fields. As the AC motor starts to turn that lag is reduced and the two fields (stator and rotor) align for stronger pull. The variable frequency systems in AC motors reduce the lag at low speeds and allow the AC motor to have higher torque at low speeds.
The electric motor starting advantage is due to the fact we can overload the system for a few minutes (at ratings well beyond that of the generator and motors). We cannot overload the steam engine by doubling the boiler pressure for a short time.
Thank you GP40-2,
trainspotting at youtube is always fun and with a big sub-woofer almost becomes real...
Some years ago, I gained those GE-tables for an AC6000 tonnages vs speed/grade it became clear, that nothing, maybe except the Allegheny at upper speeds, comes close to it.
- a link hopefully for your interest, why not 12-Axle Power?
http://books.google.de/books?id=HtwDAAAAMBAJ&pg=PA81&dq=8500+horsepower+turbine+popular+mechanics&cd=1#v=onepage&q=&f=false
One more thing, beside those horsepower-factors puplished here by Mr. Oltmannd and Jay, is it safe to assume that modern engines will always deliver ~90% of the prime movers output into drawbar-force over their entire speed-range ?
Kind Regards
Valleylineit was my impression that builders and railroads calculated steam locomotive starting tractive effort by arbitrarily using 75% of maximum boiler pressure.
ValleylineUnder normal conditions steam locomotives operated much closer to 100% than 75%.
ValleylineIf this is the case, any attempt to accurately compare diesel starting TE with published steam TE would be somewhat meaningless.
Lars,
Here is a video of a single CSX AC6000 pulling 92 coal cars at a good clip.
JayPotterI'm not at all familiar with how steam locomotives perform; and my familiarity with diesel performance is pretty much limited to the adhesion segment of tractive-effort curves, as opposed to the horsepower segment. So I don't rely on, or even deal with, formulas very much. But if I were going to apply a tractive-effort formula to an AC6000CW in notch eight at a speed of around 11 or more miles per hour, it would be (TE in pounds) equals (6000) times (factor X) divided by (speed in miles per hour), with factor X being a number somewhere between around 329 and around 337. Again, I'm not sure how useful the result of that calculation would be.
I'm not at all familiar with how steam locomotives perform; and my familiarity with diesel performance is pretty much limited to the adhesion segment of tractive-effort curves, as opposed to the horsepower segment. So I don't rely on, or even deal with, formulas very much. But if I were going to apply a tractive-effort formula to an AC6000CW in notch eight at a speed of around 11 or more miles per hour, it would be (TE in pounds) equals (6000) times (factor X) divided by (speed in miles per hour), with factor X being a number somewhere between around 329 and around 337. Again, I'm not sure how useful the result of that calculation would be.
Back in the good old days it was my impression that builders and railroads calculated steam locomotive starting tractive effort by arbitrarily using 75% of maximum boiler pressure. Under normal conditions steam locomotives operated much closer to 100% than 75%. If this is the case, any attempt to accurately compare diesel starting TE with published steam TE would be somewhat meaningless.
Tractive effort is how hard a locomotive can pull. For a diesel electric locomotive this number is maximum at very low speed and is limited only by either the adhesion of the wheels to the rail or by the amp rating of the traction motors. A GP7 at 1500 HP and GP 40 at 3000 HP have very similar tractive effort, about 68,000 lbs at 10 MPH or there abouts. Newer locomotives have some extra electronics in them that can boost this number by about 15% by watching wheel RPM and ground speed.
Railroads are limited by how much weight a wheel can put on a rail. Adhesion between the wheel and rail is a direct function of the weight on the wheel. Thus, having 6 axles allows a locomotive to be heavier than having four axles. The main determiner of tractive effort is the adhesion between the wheel and rail. More weight = more adhesion. Thus an SD 7 at 1500 HP and an SD 40-2 at 3000 HP have about 98,000 lbs maximum sustained tractive effort. Again, AC drives and wheel slip electronics allow this number to be increased by about 15%.
Steam engines were different. Their tractive effort peak occured at a much higher speed. They were not as good at starting a train as a modern diesel. Thus on the DMIR, two SD9 locomotives with a combined 3500 HP can readily handle the same weight train as a Yellowstone 2-8-8-4 with a theoretical 6000+ horsepower available. They just climb the hills a little slower. For iron ore trains, this is no big deal.
Thus three four axle locomotives have about the same tractive effort as two six axle locomotives. Three GP 38's perform essentially the same as two SD 40's, both with 12 driving axles and 6000 HP.
Tractive effort determines whether or not you can climb the hill at any speed. A farm tractor can climb a very steep hill, but will do it slowly. It has high tractive effort but low horsepower.
The other part of the equation is horsepower. As speed increases, tractive effort is limited more and more by the horsepower the diesel engine can apply to generating electricity for the traction motors. As traction motor speed increases, tractive effort decreases because it is limited by the output of the diesel engine.
This is where horsepower comes in. Given a train weight and track grade, doubling the horsepower will roughly double the speed that the train will have climbing the hill, again keeping things like traction motor heat ratings, drawbar strength and the like in mind.
Horsepower determines how fast you will climb the hill. Going back to the tractor analogy, a Corvette can climb the same steep hill much faster than a farm tractor.
There are some interesting developments in this. GE just delivered some EVO type locomotives to BNSF with A1A trucks. These trucks do not have a traction motor on the center axle.
Track can handle much heavier wheel loadings at low speed than at high speed. Also AC drives and traction motors can handle much more current and deliver much more torque to the wheel than the wheel can normally transmit to the rail. In low speed tough pulling situations, these locomotives actually hydraulically lift most of the weight off the center axle, transmitting it to the outer axles and increasing the tractive effort on those axles.
At higher speeds where tractive effort is limited more by diesel horsepower than by adhesion, the weight is put back on the center axle to keep the axle loadings at speed within a tolerable range. The main advantage of this is two fewer traction motors per locomotive at the expense of some relatively minor suspension complications.
These locomotives are said to have similar performance to a standard GE EVO locomotive with six powered axles.
Clear as mud, but an engineering description.
If Jay Potter is following this thread, he may be able to chime in. Jay has a way of "extracting" CSX inside information. LOL Here is a teaser about how much power at speed the big GE ACs have. Granted, these are only the 44s, but you can see a pair of them have no trouble pulling 130 loaded coal cars at 50 mph. www.youtube.com/watch?v=gOYeLGlzHxE
If Jay Potter is following this thread, he may be able to chime in. Jay has a way of "extracting" CSX inside information. LOL
Here is a teaser about how much power at speed the big GE ACs have. Granted, these are only the 44s, but you can see a pair of them have no trouble pulling 130 loaded coal cars at 50 mph.
Sir, I hope Mr. Potter can join us, as a knowing and informative person he is indeed.
As I read, CSX is consequently not only shifting freight, as well as a lot of bits and bytes while improving their steering-software. As I told, my data may be outdated, and , if I am not completely out of whack, 33.000lbf of pulling force in the high 60ties for an AC6000. The best I can say for BB is 25.000 to 30000lbf @70mph.
Thanx for the link!
-edit-
A single GE AC4400 is probably capable enough, to take 4000tons of load with the speed of a Big Boy up to Wasatch, replacing them one by one. With an AC6000, you may take 300,400, 500tons more.
GP40-2Lars Loco Do you have any actual tractive-forces table for the GE? Kind Regards lars Do I have CSX test data? Yes. Can I discuss specifics? Sorry, No. If Jay Potter is following this thread, he may be able to chime in. Jay has a way of "extracting" CSX inside information. LOL Here is a teaser about how much power at speed the big GE ACs have. Granted, these are only the 44s, but you can see a pair of them have no trouble pulling 130 loaded coal cars at 50 mph. www.youtube.com/watch?v=gOYeLGlzHxE
Lars Loco Do you have any actual tractive-forces table for the GE? Kind Regards lars
Do you have any actual tractive-forces table for the GE?
Can I discuss specifics? Sorry, No.
Sir,
I really would like to join a ride how a AC6000s really pour on power, you can bet.
This table was puplished by GE, though may be outdated: Trailing tons vs speed:
Grade: |10,7 | 15.0 20.0 30.0 40.0 50.0 60.0 70.0 75.0 0.0% |60640 | 55771 39124 22236 14090 9370 6424 4459 3701 tons
Yes, the AC6000 gives the BB a hard time here, but I suppose it could handle those tonnages at those speeds as well.
GP-40,
Sir, thank you for the information.
I did not read that Kratville has stated that before, at least not in his BB-book.
Source for the 6680ihp is K.'s "Challenger"-book, the comparison with the F3-units came from "Motive Power West", I think.
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