Continuous Tractive Effort defined

[oltmannd]

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.

[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.

 

[Erik_Mag]

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.

[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Actually, what's a puzzle to me is why anybody would bring up the performance of am outdated locomotive such as a GP38 or SD40 in 2009.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!  

[oltmannd]

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.

 

[oltmannd]

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!  

[Overmod]

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!

 

I was going to welcome GP40-2 back to the forums, as he was a 'known name' in the expert community... but holy zombie thread, Batman!

 

No discussion of CTE makes full sense without reference to both time and speed, and to the hourly and instantaneous ratings (or equivalent) of the transmission.  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.

 

The point of GP40-2 vs SD40-2 is, as Don notes, that it provides as direct a comparison between four-motor and six-motor CTE as we can get.  You could theoretically do this with reference to other locomotive families, say, U36B vs C, but the Forties are familiar and robust.  It would be nice to make the comparison with modern engines but most have been obligate six-axle since the move to 4400hp.

 

Can someone provide an effective drawbar TE curve for the 'three regions' of a GP40-2 and an SD40-2?  That might establish a framework for discussion of the finer points here in a much more unambiguous context. 

[oltmannd]

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.)

 

[timz]

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.

[CSSHEGEWISCH]

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.

[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?

[oltmannd]

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.

[oltmannd]

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

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

 

[Overmod]

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.

[oltmannd]

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.