timz wrote: MichaelSol wrote:It looks as though below 2.5 mph the [TE-vs-speed] curve begins to change radically, and the numbers do indeed become incomprehensible ... the relationship breaks down ... has no definition at all.Sounds like a problem, all right. So there's no way even to guess what the engine's tonnage rating will be at 1 mph?The curve isn't doing anything radical or incomprehensible, of course. We typically start by assuming constant horsepower, which implies TE that increases without limit as speed decreases. In reality TE can't increase without limit, even if adhesion permitted-- so we can't expect a constant-HP curve to predict TE at low speeds. So how do we predict TE at 1 mph? That's the $64 question, all right. How about it, Jay? Did GE or CSX tell you what an AC's ultimate TE would be if they geared the wheels to the rail and dropped all software limitations?
MichaelSol wrote:It looks as though below 2.5 mph the [TE-vs-speed] curve begins to change radically, and the numbers do indeed become incomprehensible ... the relationship breaks down ... has no definition at all.
The curve isn't doing anything radical or incomprehensible, of course. We typically start by assuming constant horsepower, which implies TE that increases without limit as speed decreases. In reality TE can't increase without limit, even if adhesion permitted-- so we can't expect a constant-HP curve to predict TE at low speeds. So how do we predict TE at 1 mph? That's the $64 question, all right. How about it, Jay? Did GE or CSX tell you what an AC's ultimate TE would be if they geared the wheels to the rail and dropped all software limitations?
The graph clearly shows that we don't assume that -- the relationship breaks loose between speed and TE at somewhere between 2-3 mph. As Cordon pointed out, the creator of the graphs understood that and intended not to show a relationship there -- and you had missed that when you demanded a calculation at 0.1 mph. Unless someone wants to operate at 1 mph all the time, there's really no point in "guessing" what the tonnage limit might be -- it's probably defined by the drawbar -- but it won't be any less than at 5, 10, 20, or 50, and will in fact be some margin greater since the tonnage constraint -- resistance -- is the least at the slowest speeds, all other things being equal. I mean, what's the point, particularly if the curve goes flat because of software restrictions?
Interesting thread. Needs to lighten up a bit.
I would leave electric locomotives out, though. They have unlimited power. Instead of a 16-cylinder diesel engine amid ships, they are motivated by some turbine the size of a schoolbus with 4 or 6 boilers behind it.
With the right DC motors and drawing on power from the Hoover dam, a single electric with (really) good traction could pull Mt. Ranier into Oregon. Actually that might be a good idea, as Oregon needs a good ski area.
MichaelSol wrote:The AC GEVO locomotives are limited to ... 180,000 lbs TE?Are the GEVO's more limited by their software than the earlier 4400 hp AC models?Do AC locomotives use sand?
The AC GEVO locomotives are limited to ... 180,000 lbs TE?
Are the GEVO's more limited by their software than the earlier 4400 hp AC models?
Do AC locomotives use sand?
A standard ES44AC is limited to 180K TE; however GE offers "optional equipment" that will allow a maximum of 198K TE. I presume that refers to the HTE software. CSXT's ES44ACs will have a maximum TE of 200K, the same as its AC4400CWs with HTE software. So no, the ES44ACs are no more software-limited than their AC4400CW predecessors. Yes, sand is used on AC-traction units.
timz wrote:So how do we predict TE at 1 mph? That's the $64 question, all right. How about it, Jay? Did GE or CSX tell you what an AC's ultimate TE would be if they geared the wheels to the rail and dropped all software limitations?
I really don't know the theoretical answer to the "ultimate TE" question. However GE's and CSXT's practical answer is that, at least for the time being, the ultimate TE is 200K. That's because a TE level in excess of that would risk damaging GE's traction motors and CSXT's track structure and, when produced by two-unit consists, would break couplers.
So a prediction of AC4400CW TE at 1 mph for a CSXT unit with HTE software is 200K, rail conditions permitting; and a counterpart prediction for other AC4400CWs is 180K, rail conditions permitting.
MichaelSol wrote:Why was an old-fashioned DC traction motor, without any of the benefits of modern electronic control technology, able in that instance to apply more TE than a modern AC traction motor? In that instance, 2 DC straight-electric locomotives did the job (generating 716,000 lbs TE) that would have required four GE AC 4400 hp locomotives (720,000 lbs TE).
I wouldn't even venture a guess to that question. If, as I understand, the train didn't wait for a helper, then both electrics were apparently on the head end of the train. If so, their combined 716K TE would have all been applied at the same point in the train. What I don't understand is how a train that required that level of TE to move could be moved, without breaking couplers or derailing the train in a curve, with all of that TE applied to one end of the train, as opposed to being distributed to multiple points.
JayPotter wrote: What I don't understand is how a train that required that level of TE to move could be moved, without breaking couplers or derailing the train in a curve, with all of that TE applied to one end of the train, as opposed to being distributed to multiple points.
What I don't understand is how a train that required that level of TE to move could be moved, without breaking couplers or derailing the train in a curve, with all of that TE applied to one end of the train, as opposed to being distributed to multiple points.
Well, I do have to go back and point out that the TE curve is, as you also mentioned, a maximum, and includes everything under the curve. Calculating the max doesn't mean it happened, since everything under the curve includes all of the accompanying conditions. That's probably a basic problem in attempting any calculation without all the data ....
I found this report, which discusses HP, adhesion, weight, and TE. It also covers maximum drawbar pull for couplers. Unfortunately, it doesn't get into comparisons of AC and DC motors.
There are lots of numbers here.
MichaelSol wrote:Calculating the max doesn't mean it happened, since everything under the curve includes all of the accompanying conditions. That's probably a basic problem in attempting any calculation without all the data ....
I'm sorry; but I guess that I didn't understand the example. I thought that we were dealing with a situation that actually did happen (i.e. "2 DC straight electric locomotives did the job [generating 716,000 lbs TE]") and that the issue was why this job, which was done by only two DC electrics, "would have required four GE AC 4400 hp locomotives" (720,000 lbs TE)".
In other words, I didn't think that we were "calculating the max" of anything. I thought that we were comparing what two DC electrics actually did against what four AC4400CWs might have been able to do if rail conditions were ideal.
Since I know about zero regarding DC electrics, the only thing that I can add to the discussion is that if four AC4400CWs have to move a train that might require each of the units to produce TE levels anywhere near the 180K maximum, I would expect that the four locomotives would be split into at least two consists and positioned at different points in the train to minimize the risk of generating excessive in-train forces.
Since I've been having some difficulty distinguishing scenarios that involve hypothetical calculations from scenarios that involve actual performance, I decided to offer a for-whatever-it-may-be-worth example that deals entirely with actual performance. I think that it illustrates some of the concepts that we've been discussing.
The train involved was a loaded coal train in excess of 15,000 tons powered by three AC4400CWs with HTE software that allowed each of them to produce 200K TE, instead of the standard 180K. Two of the units were on the head end; and the third unit was shoving on the rear.
The train was ascending a grade at a speed in excess of 10 mph when the lead unit stopped loading. Its TE dropped from something above 100K to zero; the train began to decelerate; and the TE of each of the other two units began to increase to compensate for the reduction in the lead unit's TE. The trailing unit in the head-end consist attained 169K TE. I presume -- but don't actually know -- that this was because rail conditions would only allow approximately 39% adhesion. However the helper (which was shoving on rail that had been conditioned by the passage of two locomotives and 110 cars) was able to attain approximately 46% adhesion and produced its full 200K TE. The deceleration stopped; and the train crested the grade at 3 mph.
Since I got lost somewhere between the curve and the graph - finally something I can understand! Thanx Jay!
She who has no signature! cinscocom-tmw
Mookie wrote:Since I got lost somewhere between the curve and the graph - finally something I can understand! Thanx Jay!
A few points I haven't seen mentioned in this tread:
1. Adhesion limit vs. thermal limit on DC locomotives. On most DC locomotives, the practical, day-in-day out adhesion limit is very nearly the same as the continuous thermal limit of the motors. For example, an SD40's adhesion limit at 18% adhesion is around 12 mph. The thermal limit of the motors is around 13 mph. The improvements in adhesion control were pretty well matched with improvments in the traction motors and TM cooling system. One improvement was no good without the other. (Note that these improvments also came with HP increases, by and large, keeping the balance of HP and max dependable TE about the same)
2. Sand, wheelslip control, wheel creep control. Before EMD Super Series and GE Sentry wheelslip controls systems, wheel slip was measured and controlled so that the wheels didn't slip at all. Sand was the first response to wheelslip detection. Super Series and Sentry changed all that. It was found that greater adhesion is possible if the wheel is allowed to creep slightly faster than ground speed. Sand is the enemy of wheel creep, so is a last resort and much less is use. The control system works to maximize the traction motor current rather than minimize slip. (The radar on EMDs and TM speed sensors on GEs are only used to detect slip beyond the allowable creep limits.)
3. Adhesion on DC vs AC: You control wheelslip on a DC unit by regulating the excitation on the main generator. It's a big inductive machine, so fast changes are physically impossible. On an AC unit, you regulate the frequency out of the invertors, which you can change almost instantaneously. So, you can get much higher adhesion on an AC on a day in and day out basis. Single axle wheelsip control on DC units never caught on, I think, because once you've outfitted a unit with all the electronics, you're half way to having inverters, so might as well plop in those rugged squirrel cage motors to make it a full AC unit.
4. Grade C knuckles are only good for 250-300,000#, Grade E - 350-400,000#. These are above the yield strength of the pieces, so you're looking a finite fatigue life. Two AC units at 180,000# TE each on the head end of a regular old box car train makes me wince!
-Don (Random stuff, mostly about trains - what else? http://blerfblog.blogspot.com/)
oltmannd wrote: 4. Grade C knuckles are only good for 250-300,000#, Grade E - 350-400,000#. These are above the yield strength of the pieces, so you're looking a finite fatigue life. Two AC units at 180,000# TE each on the head end of a regular old box car train makes me wince!
And while I am making a complete fool of myself - while we generally run DPU (usually 1-2 head and 1 rear) I never see DPU on any freight - just 2-3 motors on the headend.
Just my ramblings for today.
cordon wrote: I found this report, which discusses HP, adhesion, weight, and TE. It also covers maximum drawbar pull for couplers. Unfortunately, it doesn't get into comparisons of AC and DC motors.
A useful and interesting summary. Both Al Krugg and Jay Potter above mention "curvature" as an element in resistance calculations and drawbar strain -- defining the TE need -- yet, as timz also points out above ... what about compensated curves/grades? While in the absence of information, I might default to a calculation using uncompensated grades, aren't most mainline grades compensated these days?
MichaelSol wrote:aren't most mainline grades compensated these days?
JayPotter wrote: [Each of the units would have been producing 180,000-or-less pounds of TE. The extent to which those TE levels approached 180,000 pounds would have depended on the levels of adhesion that rail conditions allowed the units to produce.
[Each of the units would have been producing 180,000-or-less pounds of TE. The extent to which those TE levels approached 180,000 pounds would have depended on the levels of adhesion that rail conditions allowed the units to produce.
Jay:
You hit the nail on the head regarding AC locomotive performance at 0.2 mph in Colorado.
According to the November 1998 Trains Magazine titled "AC Rules", page 67:
"The event recorder from the lead unit, Southern Pacific AC4400 202, reveals that the locomotive was producing between 170,000 and 180,000 pounds of tractive effort the entire time."
Also, I am confused as to the definitions of starting tractive effort and maximum continuous tractive effort. Could you clarify them for me?
Thanks.
Anthony V.
Mookie wrote: Since I got lost somewhere between the curve and the graph - finally something I can understand! Thanx Jay!
Way over my head too .... for me it's simple , I don't want to pull trains slower than 2 MPH , I don't want to pay BIG money for AC motors and drives . I don't want to pay the BIG money to repair that expensive motor when it dies. I don't want the shop downtime when you have a failure in the comlicated electronics (yes , although the motors run cool , the electronics don't). I still have a diesel engine to maintain , still have wheels that are going to burn off faster when I'm at the limits of adhesion. I certainly don't want to be recrewing trains just so the locomotives can show off thier slow speed pulling ability.
Randy Stahl wrote: Mookie wrote: Since I got lost somewhere between the curve and the graph - finally something I can understand! Thanx Jay!Way over my head too .... for me it's simple , I don't want to pull trains slower than 2 MPH , I don't want to pay BIG money for AC motors and drives . I don't want to pay the BIG money to repair that expensive motor when it dies. I don't want the shop downtime when you have a failure in the comlicated electronics (yes , although the motors run cool , the electronics don't). I still have a diesel engine to maintain , still have wheels that are going to burn off faster when I'm at the limits of adhesion. I certainly don't want to be recrewing trains just so the locomotives can show off thier slow speed pulling ability.
Never mind about the definitions - I found your previous post in which you explained clearly the definition of maximum continuous TE.
....It is my understanding the grade up around Horseshoe {Altoona}, drops to about 1.45 % while the rest of the climb is around 1.8%....This was constructed and opened in 1854, but I have no idea if it was done with the compensation then, or much later. {The old main line Pennsy}.
Quentin
Joemcs - we will sit over here on the sidlines and visit a minute.
Pulled out my roster of "seens" - and can tell you that we have a lot of different types running around.
Lots of 8's and 9's yet - some two axle 8's - GP's from 7-50 and our switchers are SW10 and SW15. Yard engine is an SD9.
We have had B23-7's go through here and the 60 Power Units. SD40 is still very popular.
And - we only watch on weekends, so hard to tell what we miss during the week that comes and goes.
We have a diesel shop very close to watch site, so we do see some "oddities" from other railroads every so often.
Now if I just had a little more knowledge on each one, besides the sightings. Working on it, slowly!
Mook
cordon wrote: I found this report, which discusses HP, adhesion, weight, and TE. It also covers maximum drawbar pull for couplers. Unfortunately, it doesn't get into comparisons of AC and DC motors. There are lots of numbers here.
A few more numbers:
EMD's "all weather" adhesion rating for their locomotives (99% of the time, conditions will allow at least this adhesion level). They're a bit lower than those Krug cites and closer to the ones Conrail used for their drag tonnage ratings. (Except Conrail used 18% for the SD40-2s because of the Flexicoil trucks)
Mookie wrote:Joemcs - we will sit over here on the sidlines and visit a minute. Pulled out my roster of "seens" - and can tell you that we have a lot of different types running around. Lots of 8's and 9's yet - some two axle 8's - GP's from 7-50 and our switchers are SW10 and SW15. Yard engine is an SD9. We have had B23-7's go through here and the 60 Power Units. SD40 is still very popular. And - we only watch on weekends, so hard to tell what we miss during the week that comes and goes. We have a diesel shop very close to watch site, so we do see some "oddities" from other railroads every so often.Now if I just had a little more knowledge on each one, besides the sightings. Working on it, slowly!Mook
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