Continuous Tractive Effort defined

[BerkshireSteam]

Well I've got to the point where I understand about Tractive Effort, and the I get Continuous Tractive Effort thrown in my face. Looking at data for some popular models GP38-2's and GP40-2's are rated at 54,700# CTE at 11.1mph. From what I understand this means that at a minimum sustainable speed of 11.1mph there is 54,700 lbs of tractive force available. But what about higher speeds? 11mph would take over 8 hours to travel 100 miles. Is there a way to figure out the CTE at different speeds for a locomotive? Or maybe even charts? I got my data from thedieselshop.

[markpierce]

My understanding is that tractive effort is the tonnage that can be pulled disregarding speed, which is largely the function of weight on the driving wheels.  However, traction motors on locomotives will burn up if operated at maximum power below a certain speed.  That "certain speed" is the continuous tractive effort (maximum load at the minimum, undamaging speed).

Mark

[oltmannd]

Yes. How about an equation? In general, HP = TE * speed/308 (TE in pounds force, Speed in mph) The limit on the low end is usually the thermal capacity of the traction motors. Try to push more amps through them on a continuous basis and the insulation in the windings go away. The limit on the high end is the maximum voltage output of the main generator. This is usually somewhere above 60 mph. There are lots of exceptions, caveats and asterisks here, and they have be hashed and rehashed ad nauseum elsewhere on these forums. But, this general equation will get you close. If you plug in 54,700# TE and 11.1 mph, you get 1970 HP - right on the money for your GP38. 2000 HP at 40 mph would give you 15,400# TE. (A GP40 that shows 11.1 mph for the continuous TE speed has a special feature that derates the HP as the locomotive slows down below about 20 mph. This feature allow it to run behind a GP38 or SD40 without having to worry about damaging the traction motors on the GP40. As long as the SD40's load meter is in the green, the one in the trailing GP40 will be, too. Without it, 3000 = 54,700 * speed/308, solve for speed = 16.7 mph. So, if you were in notch 8 at 12 mph, the SD40 would be OK and the GP40 would be in trouble.)

[timz]

Say a GP38-2 is pulling a 2000-ton train on level track at 20 mph. They move onto a 1% upgrade and speed naturally drops; on paper it shouldn't drop below 11 mph, required TE will be less than 50000 lb, required current thru each traction motor will be less than 1050 amps, and the blowers can cope for however long it takes-- the motor heats up, but the blower can keep it from heating beyond its maximum allowed temperature.

But if they have 3000 tons, speed will drop below 10 mph, TE will continue to climb, amps will climb to 1500 (if adhesion allows) and the traction motor blower can no longer keep the motor cool enough. The motor can stand 1500 amps for a few minutes, but then the engineer needs to shut off. In reality, speed may drop to zero before they even use up the allowed few minutes-- 3000 tons is way too much for a GP on 1%.

[Paul_D_North_Jr]

Minor clarification:  In Don's formula above, the '308' has been reduced a little bit - down to about 82 % - from the theoretical number of '375' to allow for friction and other mechanical losses and non-traction uses in the locomotive's machinery.  If you derive the formula from scratch, it's as follows: 

1 HP = 550 ft.-lbs./ second = 550 lbs. x 1 ft./ sec.

Since 1 MPH = 5,280 ft. / (60 mins. x 60 secs.) = 1.47 ft. /sec.,

Divide by the 1.47 ft. / sec. per 1 MPH = 550/ 1.47 = 374.15 - call it 375.

Thus, the theoretical formula is:  HP x 375 = TE (in lbs.) X Speed (in MPH).

And 308 / 374.15 = 82.3 %.

Saying with Don's more realistic '308' value, the equation can be rearranged to solve for TE and Speed if the other 2 values are known, as follows: 

TE (in lbs.) = HP x 308 / Speed (in MPH).

Speed (in MPH) = HP x 308 / TE (in lbs.)

Note that for a constant HP, Speed and TE are inverses of each other - as one rises, the other drops commensurately.  That's a pretty important principle and characteristic of diesel locomotives to know and understand when discussing these matters.

- Paul North.

 

[GP40-2]

Also note that the 308 factor needs to be changed to 345 when discussing modern AC motored locomotives. This accounts for the improvements in mechanical and electrical efficiency of today's locomotives compared to the old GP38s and SD40s which were first designed nearly 50 years ago.

[erikem]

 Paul,

The 375 figure is exact.

Original definition of  1 HP = 33,000 ft-lbs/minute (equals 550 ft-lb/sec)

1 MPH = 5280 ft per mile/ 60 minutes per hour = 88 feet per minute

33,000 / 88 = 375

- Erik

[Paul_D_North_Jr]

Erik -

Thanks !    I thought it should be, and was still mildly curious as to why it wasn't.  In my math, I was using the short-cut of 1 MPH = 1.47 ft./ sec., and so that's why the slight discrepancy.  Moral:  When in doubt, go back to the original derivation. 

Altogether, an enlightening little diversion.  Thanks again.

- Paul North.

[timz]

GP40-2

Also note that the 308 factor needs to be changed to 345 when discussing modern AC motored locomotives.

As always, it's a puzzle just what percentage of their rated power tocomotives actually produce at the wheelrim (as distinct from what they're supposed to produce). An additional puzzle is how slow the engine can be moving and still maintain its hoped-for percentage. How often do ES44s actually produce 101,200 lb of TE at 15 mph? The only way we fans can hope to find out is to go out and clock them, hoping that we know the train's actual tonnage.

If a 7250-ton eastward train with four ES44s could climb Donner without dropping below 15 mph, that would be a good indication they were living up to the 345 factor.

[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.

[selector]

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.

Who peed in your cornflakes?

Okay, okay....you admit to being puzzled.  Why not just enjoy the show unfold while they discuss a diesel about the same age as your own namesake?

-Crandell

[JayPotter]

selector

Why not just enjoy the show unfold while they discuss a diesel about the same age as your own namesake?

This discussion began with an inquiry about the definition of continuous tractive effort; and the point that I took from "GP40-2's" posting is that the concept of CTE has changed significantly over time.

Locomotive manfacturers still cite "continuous" TE and "continuous" speed figures; however because of thermal protection circuitry in DC-traction units and the fact that AC-traction motors are much more heat-resistant than DC-traction motors, the "continuous" concept is applied differently today than it was three or more decades ago.  The most extreme example of this is AC-traction technology.  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.

So I think that the issue here is whether the original question related to the concept of CTE as it existed 30-some years ago or to the concept as it exists today.  "GP40-2" seemed to assume that the question related to the latter; and that assumption seems reasonable to me.

 

[Railway Man]

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

RWM

[erikem]

timz

As always, it's a puzzle just what percentage of their rated power tocomotives actually produce at the wheelrim (as distinct from what they're supposed to produce). An additional puzzle is how slow the engine can be moving and still maintain its hoped-for percentage. How often do ES44s actually produce 101,200 lb of TE at 15 mph? The only way we fans can hope to find out is to go out and clock them, hoping that we know the train's actual tonnage.

 

Good question and I don't have a quantitative answer. I've seen plenty of data on motor efficiency versus speed and tractive effort for  DC electric locomotives (e.g. from CERA's Electrification by GE), but nothing for recent AC inverter driven traction motors. There have been a few comments on this forum asserting that the transmission efficiency of an AC diesel-electric locomotive is on the order of 94%. What was not said was what ranges of speed and tractive effort for which the 94% figure was valid. The figure is certainly not valid for max tractive effort at speeds much less than 10 MPH due to slip inherent with induction motors (will be different with synchronous motors).

I have seen the efficiency map for the UQM permanent magnet motor intended for electric vehicles and it implies that the efficiency for a synchronous traction motor will fall off significantly with speed due to windage, hysteresis and eddy current losses. An induction motor will probably show a slower fall-off with increasing speed and decreasing load as the magnetic field can be reduced (lowering hysteresis losses) in proportion to to tractive effort.

- Erik

[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...

[Murphy Siding]

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...

    Reality check:  The railroads have to work with the euquipment they have, not the equipment they may one day have.  I'd say that any railroader who didn't concern himself with the equipment he has now, won't have a job when the the new stuff arrives.

[Railway Man]

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

[timz]

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?

[JayPotter]

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.

 

[Paul_D_North_Jr]

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 . . .

- Paul North.

[timz]

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?

[timz]

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?

[JayPotter]

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.

 

[timz]

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.

[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.

[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.

[Last Chance]

 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.

[n012944]

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. 

[oltmannd]

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.

[oltmannd]

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.