Continuous Tractive Effort defined

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Posted by oltmannd on Thursday, August 29, 2019 8:31 AM

The EMD chart isn't the result of specific testing, if I remember right. It was compiled from real world operations.  So, restarting on the grade or curve would be factored in as it occurred in the real world.   

If you could afford the occasional stall, you might be able to dispatch more tonnage by adhesion rating.

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Posted by Overmod on Wednesday, August 28, 2019 8:15 AM

Keep in mind that neither of us is being argumentative or adversarial about 27 to 28% being "near" the 25% that csshegewisch mentioned.  The 28% translates into a FA of 3.57 or so, which is comparable to the nominal 3.60 given for the N&W J at 300psi by Dave Stephenson (288,000WoD/80k nominal TE).  One difference is that the number for the diesel is reliable day-in, day-out whereas I suspect even with relatively low drivers the J would be on the ragged edge of controllability delivering that actual TE at starting, provided that some combination of fixed cutoff and judicious bleed through the cylinder cocks permitted sufficiently high effective MEP in the first place.

Don, can you provide a couple of missing assumptions that are critical to understanding that EMD diagram?  Specifically what the load is, and what the minimum speed 'over the hill' is expected to be?  I would have the further assumption of a stop and restart with the train on the 'defining' worst part of the ruling grade, but I doubt from the magnitude of the numbers implied in those graph curves that this is a criterion.

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Posted by oltmannd on Wednesday, August 28, 2019 7:59 AM

25 year old chart from EMD provides a way to think about adhesion.

https://photos.app.goo.gl/TSNTgh89kqPw8bpE7

 

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Posted by oltmannd on Wednesday, August 28, 2019 7:49 AM

Overmod

 

 
CSSHEGEWISCH
the rule of thumb that I've considered is that formula TE cannot be greater than 25% of weight on drivers for DC-motored units, more or less.

 

Hasn't this hoary old criterion applicable to FA on reciprocating steam locomotives been long obsolete for DC locomotives with reasonable slip or creep control?

 

Yes.   The "all weather" rating is 27-28% and can be higher under favorable conditions.

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Posted by Overmod on Wednesday, August 21, 2019 11:21 AM

CSSHEGEWISCH
the rule of thumb that I've considered is that formula TE cannot be greater than 25% of weight on drivers for DC-motored units, more or less.

Hasn't this hoary old criterion applicable to FA on reciprocating steam locomotives been long obsolete for DC locomotives with reasonable slip or creep control?

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Posted by CSSHEGEWISCH on Wednesday, August 21, 2019 6:54 AM

The formula obviously only works down to a certain point, the rule of thumb that I've considered is that formula TE cannot be greater than 25% of weight on drivers for DC-motored units, more or less.  This explains the advantage of six-motor power and slugs in low-speed service.

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Posted by timz on Tuesday, August 20, 2019 2:20 PM

caldreamer
I think the formulas in this discussion are incorrect as they do not take into account the weight on rails.

They're incomplete, you mean. But they are correct.

caldreamer
Using the formula TE = HP*308/Speed a GP40-2's TE is as follows: 3000*308/11.1 = 83243

That's what it would be, if the GP40-2 were producing 3000 hp at 11.1 mph.

The PF21 (?) module on the GP40-2 started reducing its power at 23 mph (62:15 gear ratio). The TE curve from 23 down to 11 mph was pretty much a straight line; horsepower for traction at 11 mph was around 2000.

Try the formula assuming 3000 hp at 1 mph -- TE comes out 924000 pounds. Which is correct, if the engine is actually producing 3000 hp for traction at that speed.

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Posted by oltmannd on Tuesday, August 20, 2019 9:43 AM

Overmod
 It's of course possible to calculate a 'maximum' curve (that follows the nominal horsepower curve along the constant-horsepower region) but you'd need a good observed measure of actual traction power, not advertised or 'crankshaft' horsepower from the diesel engine.  I believe loadboxing would provide this.

Yes.  In fact the "rated" HP of a locomotive is shaft HP it the main generator (alternator) that's headed for the traction motors.  (load box HP/gen eff.).  The generator efficiency is baked into the "308" (or whatever overall transmission efficiecny you care to use).  

Typically, the builders will provide BHP, THP and NTHP (this is the "Load box" HP) for each notch under standard (AAR) conditions along with fuel consumption as part of their specs for their locomotive models (along with all the auxiliary HP items - blower, fans, AC, etc.)

 

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Posted by Overmod on Tuesday, August 20, 2019 9:16 AM

BigJim
 
GP40-2
Actually, what's a puzzle to me is why anybody would bring up the performance of am outdated locomotive such as a GP38 or SD40 in 2009. What's the short time rating of a SD40-2? Who cares anymore. Might as well be discussing the performance of a Model T. AC traction is the future of railroading. Railfans should be spending their time researching the latest motive power technology, not some inefficient 40 year old pile of junk.

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Posted by oltmannd on Tuesday, August 20, 2019 8:52 AM

BigJim

 

 
GP40-2
Actually, what's a puzzle to me is why anybody would bring up the performance of am outdated locomotive such as a GP38 or SD40 in 2009. What's the short time rating of a SD40-2? Who cares anymore. Might as well be discussing the performance of a Model T. AC traction is the future of railroading. Railfans should be spending their time researching the latest motive power technology, not some inefficient 40 year old pile of junk.

 

That "old pile of junk", as you so eloquently put it, was the best diesel locomotive for handling trains that I ever ran!  

 

...and the same rules apply!  

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Posted by oltmannd on Tuesday, August 20, 2019 8:49 AM

caldreamer

I think the formulas in this discussion are incorrect as they do not take into account the weight on rails.   Using the formula TE = HP*308/Speed a GP40-2's TE is as follows:

3000*308/11.1 = 83243.243243243

BUT an SD40 which weighs much more and would have greater weight on the rails is:

3000*308/11= 84000 and since an SD40-2 weights about 100000 lbs more than a GP40-2 the calculations do NOT work.

 

 

The formula works, but there is more to consider.  A GP40-2 does not have a minumum continuous speed of 11 mph - unless it has "power matching" (or performance control - GE uses one name, EMD the other and I don't remember which is which).  A GP40-2 without power matching will have an MCS of roughly 18 mph. (depending on gearing)

This feature de-rates the GP40-2 so it can run with an SD40-2 down to the SD40-2's MCS.

The MCS is based on the thermal limits of the traction motor.  Generally, this will roughly match the adhesion limit of the locomotive such that the locomotive won't be in danger of slipping at full throttle at MCS.  This is only ROUGHLY true.  YMMV. It'll also depend on gear ratio.  Most roads used 62:15 in their EMD power.  Conrail had 70:17 in their GP40-2s in order to get the maximum speed up to 70 mph.

 

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Posted by BigJim on Tuesday, August 20, 2019 8:08 AM

GP40-2
Actually, what's a puzzle to me is why anybody would bring up the performance of am outdated locomotive such as a GP38 or SD40 in 2009. What's the short time rating of a SD40-2? Who cares anymore. Might as well be discussing the performance of a Model T. AC traction is the future of railroading. Railfans should be spending their time researching the latest motive power technology, not some inefficient 40 year old pile of junk.

That "old pile of junk", as you so eloquently put it, was the best diesel locomotive for handling trains that I ever ran!  

.

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Posted by Erik_Mag on Tuesday, August 20, 2019 12:38 AM

For a 100% efficient transmission, the tractive effort woud be: TE=HP*375/MPH and the 308 figure comes out to be 82%. This is probably a good estimate for a DC transmission at the max CTE speed, but probably low for AC units. The SD40 should be good for a higher CTE than a GP40, but the SD40 would be running at a lower speed when generating CTE. With DC motored lcomotives, the CTE is typically thermally limited.

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Posted by caldreamer on Monday, August 19, 2019 9:56 PM

I think the formulas in this discussion are incorrect as they do not take into account the weight on rails.   Using the formula TE = HP*308/Speed a GP40-2's TE is as follows:

3000*308/11.1 = 83243.243243243

BUT an SD40 which weighs much more and would have greater weight on the rails is:

3000*308/11= 84000 and since an SD40-2 weights about 100000 lbs more than a GP40-2 the calculations do NOT work.

 

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Posted by oltmannd on Monday, December 14, 2009 11:00 AM
timz
You remember that GM used to give an actual figure for "continuous TE" for their ACs-- e.g. 137,000 lb? None of us knows where they got that figure, but doesn't it seem unlikely that they thought they had a good enough grasp of adhesion to pin a 95% or 99% figure down that precisely?
The number they quoeted for the SD80MACs was 147,000#, if I remember right. That was in the days before they had enough data to talk about dispatchable adhesion probability. I think there is some electrical limit on the AC propulsion equipment that can't be exceeded that determines the max TE that can be produced (assuming adhesion is available) and that works out to 147,000# for the SD80MAC. That would yield a minimum continuous full HP speed of 11-12 mph for the SD80 MAC.

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Posted by oltmannd on Monday, December 14, 2009 10:50 AM
JayPotter

EMD's AC-traction CTE ratings are based on adhesion levels.  What I'm assuming is that EMD considers various factors in establishing the adhesion levels.

Perhaps some railroads do conduct their own locomotive road tests but don't take them seriously.  My frame of reference is CSXT; and although it does conduct some road testing without manufacturer involvement, the significant road tests are conducted in conjunction with the manufacturers.  And it takes those tests seriously enough to factor them into its locomotive-acquisition decisions.

I agree. Even when RRs do their own testing, they almost always have the appropriate EMD and/or GE guys involved -at some level. I'd bet a sizable chunk of change that EMD's "all weather" dispatchable adhesion ratings came from their own testing and data collection.

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Posted by oltmannd on Monday, December 14, 2009 10:32 AM
Two points. One, in order to understand the basic physics behind locomotive performance, there is nothing better than comparing GP40-2s, SD40-2s and GP38-2s. There are very few confounding factors to consider doing the comparison. Get this, and you can move on to the finer points. Two, the day of the SD40-2 as a mainstay of the US frt RR mainline fleet are over. But, does this mean they are useless? Of course not! The replacement of SD40-2 with newer power was largely done because of the economics driven by two factors - unit replacement ratio and fuel consumption. In applications where the unit replacement would be one for one and the service demand results in low fuel consumption, there is no economic case to replace an SD40-2 with something newer. Helper service is a perfect example. Local train work and yard work would be another.

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Posted by n012944 on Sunday, December 13, 2009 11:04 AM

GP40-2
Paul_D_North_Jr

...Norfolk Southern has now rebuilt about 20 SD50's to 'SD40-E' specs for helper service on the Altoona-Cresson-Pittsburgh grades. See -

Only 20 conversions. How many hundreds of new GEs has NS purchased in the past few years? How many hundreds of GE ACs has CSX purchased in the past few years? Hope you see a trend here, and it isn't favorable to old EMDs remaining in service in any great numbers.

 But there is.  CSX just turned out 2 SD50-3 rebuilds, which are close to what the NS is doing with the -Es.  From what I have been told by a high ranking member of the powerless department, IF the conversions go as planned without any major problems, you will see at least 150 of these units rebuilt.  Also CSX has on its property one of the KCS's ECO EMD rebuilds and is looking at converting some of its stored SD40-2s into these as well. 

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Posted by Last Chance on Sunday, December 13, 2009 1:26 AM

 Thermal. That is going to determine success or failure of the pull over time. All that juice is gonna have to convert to heat at some point which will need to be removed to keep everything working or it will all burn up.

 

Take 40 tons gross, 22% grade over 2 miles, speed lowest gear, max torque roughly 40 minutes to pull the grade. When I got to the top, everything in the truck, all 50+ gauges were in the yellow or red ranges. Four hours later everything settled down and we did not have any damage from that pull.

 

There are going to be locomotives that can lug and dissipate heat and endure the load long enough to make it happen.

Here is another problem. Your railroad has X traffic that wants to cross the same mountain too. You can sit down and wait for your mega loaded choo choo to fight hours over the mountain while the entire railroad waits on you. Or you can provide helpers and/or lessen the burden to get your crews, engines, train over the hill and gone within your schedule, time limits, fuel cost etc.

Your couplers can only take so much. I think about 350,000 pounds can be put on them max, I will have to go back to my pathetic little collection of random snippets in the closet. Will all your couplers stay together?

 

It might just be cheaper to stick some fast four axle units on there and route the train AROUND the mountain. It will get from A to B the same day without the gigantic effort of lifting tonnage up and over the earth. I rather drive 5000 tons 200 miles in the time it takes me to lift it 14000 feet and back down again over 70 miles.

 What goes up must come down. Your wheels are gonna get hot, you would want to balance tonnage so you can sort of... tug gently at it going downgrade on the other side on Dynos with the slight tap now and again on the brakes.

Weather. You might have it all moving nicely at whatever lugging speed and it gets to rain or snow. One slip of the wheel might be enough to stall you out.

Your Railroad boss gave you a train to pull with a minimum amount of horse, pull and weight to get the job done within the schedule of the day. You should be able to do it without trouble.

It does not matter how many or how old the choo choos are because those are the engines availible that day to your train. Sure a 379 pete might work well for me but I got a Mack today and that is what I have to work with.

 

Cheers.

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Posted by GP40-2 on Sunday, December 13, 2009 12:48 AM
Paul_D_North_Jr

...Norfolk Southern has now rebuilt about 20 SD50's to 'SD40-E' specs for helper service on the Altoona-Cresson-Pittsburgh grades. See -

Only 20 conversions. How many hundreds of new GEs has NS purchased in the past few years? How many hundreds of GE ACs has CSX purchased in the past few years? Hope you see a trend here, and it isn't favorable to old EMDs remaining in service in any great numbers.
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Posted by JayPotter on Saturday, December 12, 2009 9:11 PM

EMD's AC-traction CTE ratings are based on adhesion levels.  What I'm assuming is that EMD considers various factors in establishing the adhesion levels.

Perhaps some railroads do conduct their own locomotive road tests but don't take them seriously.  My frame of reference is CSXT; and although it does conduct some road testing without manufacturer involvement, the significant road tests are conducted in conjunction with the manufacturers.  And it takes those tests seriously enough to factor them into its locomotive-acquisition decisions.

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Posted by timz on Saturday, December 12, 2009 8:50 PM

JayPotter
I don't know what basis EMD has had for any of the tractive-effort ratings that it has assigned to its locomotives. 

Except that you do assume their ratings for ACs are based on their estimate of adhesion, and nothing else?
JayPotter
My only point is that railroads perform road testing of locomotives for a reason.
The reason being, the manufacturer's claims may be worthless-- even tho those claims were based on their tests. So the railroad does their own tests, but no reason to hope they're any better at testing than the manufacturer-- so the RR probably won't take their own tests too seriously either. They'll dutifully print tonnage ratings in the timetables, but unless they're unusually conservative no one will be surprised when 10%? 20%? of trains stall with their "rated" tonnage.

I see at http://www.emdiesels.com/emdweb/products/sd70ace.jsp that EMD says the SD70ACe has 157,000 lb continuous TE, 191,000 lb starting TE; wonder if they claim the same percentage reliability for those two figures.

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Posted by JayPotter on Saturday, December 12, 2009 8:15 PM

I have pretty much the same answer to all of Timz's questions:  I don't know what basis EMD has had for any of the tractive-effort ratings that it has assigned to its locomotives.  And the same applies to GE's counterpart ratings.

That's not to say that I consider those ratings to be necessarily inaccurate; but it is to say that I view them as estimates of actual performance.  And if that view is accurate, then the products of whatever mathematical calculations anyone might make using those ratings would never be anything more than further estimates.  Some of those estimates might be accurate and useful; and others might be just the opposite. 

My only point is that railroads perform road testing of locomotives for a reason.

 

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Posted by timz on Saturday, December 12, 2009 6:38 PM

JayPotter
And I presume that if railroads viewed them as absolutes, there would be no need for the railroads actually to test locomotive models before acquiring them.

When GM says the continuous TE of an SD70ACe is 145,000 lb or whatever, they just mean "That's our best guess, but you'd be crazy to take our word for it"? Was the same true in 1975, when GM said a 62:15 SD40-2 was good for 83,100 lb continuously as long as adhesion permits?

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Posted by timz on Saturday, December 12, 2009 6:32 PM

JayPotter
EMD estimated (calculated, determined, or whatever) that ... an SD70MAC could be relied upon to maintain 33% adhesion 99% of the time and 39% adhesion 95% of the time.

In a year (with the expected variation in conditions during that time) the railroad sends 1000 trains up the hill, each with tonnage that requires 33% adhesion from each SD70MAC (leading and trailing units) despite the curve greasers. EMD claims to think that 990 of them will make the grade?

You remember that GM used to give an actual figure for "continuous TE" for their ACs-- e.g. 137,000 lb? None of us knows where they got that figure, but doesn't it seem unlikely that they thought they had a good enough grasp of adhesion to pin a 95% or 99% figure down that precisely?

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Posted by Paul_D_North_Jr on Saturday, December 12, 2009 5:17 PM

GP40-2
Railway Man

Point me to a Class 1 railroad operating officer with responsibility for moving trains that doesn't need to know about or care about the minimum continuous speed of an SD40-2.

RWM

For how much longer? SD40-2s are being taken out of mainline service about as fast as railroads can speed dial the sales department at GE for new motive power. A trend I see accelerating once we are out of this recession OR the cost of diesel fuel increases, whichever comes first. Like I said, you might as well be discussing the performance of a Model T... 

 

Evidently at least one Class I doesn't see it that way, and is quite happy with the SD40 platform - so much so that they are now essentially building their own. 

Norfolk Southern has now rebuilt about 20 SD50's to 'SD40-E' specs for helper service on the Altoona-Cresson-Pittsburgh grades. See -

Juniata Diesel Conversions
SD50/SD50S to SD40-E

at http://www.altoonaworks.info/rebuilds/ns_sd40e.html 

and especially the details/ scope of the work at the bottom of the page.

I also see them in many lash-ups on the mainline runs, and even taking turns on the local freights.  They all look well-maintained, and have probably outlived several other models of both EMD and GE parentage.  I wouldn't call them Model T's - more like 1960's Chevy pick-up trucks.  Not glamorous and and the most efficient and not state-of-the-art - but easy to understand, repair, and basically dependable power - as close to 'bullet-proof' as we're ever likely to see.  At least they can be fixed with standard wrenches - don't need to re-boot a computer for most functions . . . Wink

- Paul North.

"This Fascinating Railroad Business" (title of 1943 book by Robert Selph Henry of the AAR)
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Posted by JayPotter on Saturday, December 12, 2009 4:42 PM

timz
Seems like a "CTE" figure that depended on adhesion and nothing else would have to be based on probability-- the unit has an X% chance of maintaining Y pounds for an indefinite period of time. Next question: does that percentage assume dry rail? If so, what's the percentage for wet rail? How wet? Does it apply to the lead unit? Or just trailing units? Does it assume the possibility of curve greasers?

Yes, I think that the CTE concept, particularly as applied to AC-traction units, is "based on probability".  And as the term "continuous" implies, variables such as consist position and rail conditions apparently have been factored into whatever the assigned probabilities are.  For example, EMD estimated (calculated, determined, or whatever) that an SD50 could be relied upon to maintain 24% adhesion 99% of the time and 31% adhesion 95% of the time and that an SD70MAC could be relied upon to maintain 33% adhesion 99% of the time and 39% adhesion 95% of the time.

Timz asked where figures like those come from; and my answer is that I really don't know.  And I presume that if railroads viewed them as absolutes, there would be no need for the railroads actually to test locomotive models before acquiring them.

So my personal for-whatever-it's-worth view of the CTE concept is that CTE is a parameter that is more useful for comparing the estimated performance of one locomotive model against another locomotive model than it is for estimating the actual performance of any given locomotive model.

 

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Posted by timz on Saturday, December 12, 2009 3:59 PM

JayPotter
For AC-traction purposes, CTE is defined entirely differently than it is for DC-traction purposes.  It doesn't relate to the maximum amount of tractive effort that a locomotive is capable of producing "continuously" without risk of overheating its traction motors but, rather, to the amount of tractive effort that the locomotive will produce at the level of adhesion that it can be relied upon "continuously" to maintain.

Sounds like you're saying an AC unit can produce, say, 180,000 lb for eight continuous hours, if adhesion is that good for that long. In the past, didn't GM say an SD70ACe was good for 137,000 lb or 145,000 lb (or whatever it was) "continuous tractive effort"? If that depends on adhesion and nothing else, where did that figure come from?

Seems like a "CTE" figure that depended on adhesion and nothing else would have to be based on probability-- the unit has an X% chance of maintaining Y pounds for an indefinite period of time. Next question: does that percentage assume dry rail? If so, what's the percentage for wet rail? How wet? Does it apply to the lead unit? Or just trailing units? Does it assume the possibility of curve greasers?

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Posted by Railway Man on Saturday, December 12, 2009 3:38 PM

GP40-2
Railway Man

Point me to a Class 1 railroad operating officer with responsibility for moving trains that doesn't need to know about or care about the minimum continuous speed of an SD40-2.

RWM

For how much longer? SD40-2s are being taken out of mainline service about as fast as railroads can speed dial the sales department at GE for new motive power. A trend I see accelerating once we are out of this recession OR the cost of diesel fuel increases, whichever comes first. Like I said, you might as well be discussing the performance of a Model T...

 

I genuinely wish you could be on my next 0500 conference call to express your opinions.

RWM

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