CSX would have some great “real world” data, but conservatively, you can assume 35% adhesion for a 420,000# locomotive from start up to where this crosses the HP curve, then follow the HP curve all the way up to 75 mph. The HP curve can conservatively assumed to close to HP = TE x mph/315.
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 As I told, my data may be outdated, and , if I am not completely out of whack, 33.000lbf of pulling force in the high 60ties for an AC6000.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!
Kind Regards
lars
-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.
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.
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.
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.
Very good assumption. The 6000’s “x-factor” ranges from 329 on a hard, slow speed pull to 348 at higher speeds.
Lars,
Here is a video of a single CSX AC6000 pulling 92 coal cars at a good clip.
They usually assumed cylinder MEP equal to 85% of boiler pressure. (That’s how they got 135,375 lb for UP’s 4-8+8-4.)
They operated closer to 85% than 100%.
Might be right or might be wrong, but no reason it would be meaningless.
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?
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
lars
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.
If an AC6000 produced 5400 dbhp at 1 mph that would be 2,000,000 lb.
So 179,000 lb TE at 11 mph. Too bad more railroads don’t have them-- it would be interesting to see how often they live up to that.
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
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…
Roughly, but the bottom end of the speed range for which this is true is bounded by the adhesion limit (and/or the max tractive effort allowed by the locomotive’s control system)
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.
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.
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 y
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 thought tractive effort was measured at the coupler.
Anthony V.