Eastern Railroad Discussion > How can 13200 HP = 18000 HP?

Last Friday evening while attending a NRHS Chapter meeting at the ex SOU Buechel Depot adjacent to the NS in the eastern outskirts of Louisville KY, an eastbound unit coal train passed with a 4400HP unit on the front end and two 4400 HP units pushing for a total of 13200 HP. Forty years ago, the Southern operated unit coal trains with three 3000 HP units on the front end and three 3000 HP units mid train for a total of 18000 HP.
I must be missing something. How can 13200 HP = 18000 HP?
ALLEN

HP aside, the real question might be how they can move that train with 18 powered axles vs. 36. Were the three 4400's all AC's?

Horsepower is only part of the story... adhesion level/traction control and therefore the ability to translate the power into usable tractive effort are what is important. Also modern AC traction motors can generate more tractive effort than older DC motors without being damaged.

Tractive effort of 6 SD40-2s is 6 x 115000 = 690,000lbf.
Tractive effort of 3 AC4400CWs is 3 x 180000 = 540,000lbf. 
(Tractive effort of 3 ES44ACs (ballasted version) = 3 x 200,000 = 600,000lbf).

But at low speeds those SD40s are in danger of cooking their traction motors so one unit was probably there to relieve that strain on the other 5. Whereas the 3 modern AC units are close to indestructible even when down on their knees grinding it out.

Basically modern units allow you to move more with less horsepower due to advanced traction control software and robust AC motors. We're obviously assuming that the weight of the old and new coal trains are about the same in this comparison...



Edited 3 time(s). Last edit at 08/08/18 13:42 by exhaustED.

CPR_4000 Wrote:
-------------------------------------------------------
> HP aside, the real question might be how they can
> move that train with 18 powered axles vs. 36. Were
> the three 4400's all AC's?

Affirmative.
ALLEN

exhaustED Wrote:
-------------------------------------------------------
> Horsepower is only part of the story... adhesion
> level/traction control and therefore the ability
> to translate the power into usable tractive effort
> are what is important. Also modern AC traction
> motors can generate more tractive effort than
> older DC motors without being damaged.

13200 HP = 73% of 18000 HP.  How much more tractive effort can AC traction motors produce versus DC traction motors?  How much more efficient are the newer prime movers?   Exactly how is this difference reconciled with numbers?
ALLEN       

I've added some numbers to my answer above... :-)

Horsepower equals speed. I'm guessing that 40 years ago the trains went over the hill in the 15 to 18 mph range. Now railroads allow heavy trains to get down to under 10 mph climbing the biggest hills. Never mind what it does to capacity and network fluidity, we'll just lengthen the trains to make up for it.

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Darren Boes
Lethbridge, AB
Southern Alberta Railfan

Weight of train now vs then?

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exhaustED Wrote:
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> I've added some numbers to my answer above... :-)

Thanks for the additional info;

The two UP units could have been AC4400CWs, ES44ACs, or a combination (not that good at spotting). The BNSF unit was almost certainly an ES44AC.

The TE goes a way to explaining the difference between present use of three 4400 HP units versus six SD40-2s that amounts to a 4800 HP reduction.

Following the six SD40-2s, the Southern used four SD60s, two on the head end and two mid train. This was 15200 HP for the SD 60s versus 18000 HP for the SD40-2s, or a reduction of 2800 HP.

Presently, the use of three 4400 HP units as compared to four SD60 units resulted in a further reduction of 2000HP.

So, I’m still not quite sure how this fits together.

ALLEN

Also, back in the Southern Railway days you were probably looking at 100-ton cars, now could be up to 125 tons, I don't know what NS is running. If higher capacity cars, they might be moving the same total amount of coal with fewer cars than before. Or, they might be running heavier trains using the same number of cars.

Did you get  a car count?

Even 35 years ago, the coal trains I saw on an N&W line were not all 100-ton cars. In the 1970's, I doubt they all were.

Good question about the cars.

In the six SD40-2 and four SD60 unit coal train eras, the railroad was using 3600 cuft 100 Ton carbon steel hopper cars. So the type cars do not account for the 2800 HP decrease from the former to latter eras.

In the present three 4400 HP unit era, the railroad is using aluminum hopper cars. So the type of cars could account for the 2000 HP decrease from the four SD60 era to the present era.

ALLEN

A big factor is the tractive effort. For the SD40-2s maximum factor of adhesion would be  25% of the locomotive's weight and it would fall slightly as the locomotive accelerates. The SD60s could achieve 25% at starting and it would rise to about 28% at low speeds (less than 10 mph). With modern AC traction equipment and individual axle control the factor of adhesion can be as high as 43%  under ideal conditions, and will always be much higher than locomotives with DC traction motors. Even without individual axle control the actual factor of adhesion will be at least 10% higher than that achieveable by DC traction motor locomotives under similar conditions. All GE AC traction locomotives have individual axle control, some EMD SD70ACe locomotives have IAC, and all EMD SD70ACCe-T4 locomotives have IAC.  

Some have tried to explain this, and did a good job.

What you have to get out of your head is “HP”. Horse power, while still used by some railroads, isn’t an accurate measurement of speed or pulling power.

All the engine is doing (keeping it simple) is spinning the alternator. Whenna load is being placed on that alternator, it’s creating a resistance that the engine needs to overcome.

That raw HP of the engine is overcoming that increasing resistance, on top of Hz needed and other factors.

Of course this is dealing with AC motors, whereas DC (depending on make/model) can have a more direct impact (the HP part of it).

Posters above are correct that the greatest advances (aside from AC motors) is the wheelship, adhesion and software used to put more effort in the wheel.

If anything, UP got it right in abandoning using HP/T. They use TPA or tons per powered axle.

Each model of locomotive is given an EPA - Equilicant powered axle. A “standard axle” is rated at 10,000lbs of tractive effort.

AC4400, ES44 And th ET44 are all rated at 12.1 EPA. But each model has different HP engines (ET44 at 4350 IIRC) (AC4400 at 4390) and I don’t recall the ES44. Grantee very minor HP numbers, but all rated the same to the wheels.

Now, the SD9043 has a 4300HP engine, but rated at 11.5 EPA. The SD70ACe also has a 4300HP engine, but rated at 12.0 axles. The SD70ACeT4 is 4600HP but less is available for traction and is also rated at 12.0.

The BNSF C4’s are at about the same as a SD40-2 if I recall the numbers correctly.

So in short, not all HP is created equal. There are more accurate and meaningful numbers used today than a blanket HP number of 30 years ago.

SOO6617 Wrote:
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> A big factor is the tractive effort. For the
> SD40-2s maximum factor of adhesion would be  25%
> of the locomotive's weight and it would fall
> slightly as the locomotive accelerates. The SD60s
> could achieve 25% at starting and it would rise to
> about 28% at low speeds (less than 10 mph). With
> modern AC traction equipment and individual axle
> control the factor of adhesion can be as high as
> 43%  under ideal conditions, and will always be
> much higher than locomotives with DC traction
> motors. Even without individual axle control the
> actual factor of adhesion will be at least 10%
> higher than that achieveable by DC traction motor
> locomotives under similar conditions. All GE AC
> traction locomotives have individual axle control,
> some EMD SD70ACe locomotives have IAC, and all EMD
> SD70ACCe-T4 locomotives have IAC.  

Good post.

Forget horsepower. Usable tractive effort from higher adhesion is the key. Let's not forget the short time ratings for DC traction motors. With AC, running in the red is not a problem.

The BNSF C4's are very similar to an ES 44 DC. By the numbers.

Eng.Shadetree

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exhaustED Wrote:
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> Tractive effort of 6 SD40-2s is 6 x 115000 =
> 690,000lbf.
> Tractive effort of 3 AC4400CWs is 3 x 180000 =
> 540,000lbf. 
> (Tractive effort of 3 ES44ACs (ballasted version)
> = 3 x 200,000 = 600,000lbf).
>

Can someone explain what "lbf" above is. I took a physics class many years ago, but I don't recall something called "pounds force."

I found this, but it makes my head swim...

https://www.convertunits.com/info/pound-force



Edited 1 time(s). Last edit at 08/09/18 04:57 by donstrack.

Lets no forget the difference between solid(friction) bearings and roller bearings on the cars.

donstrack Wrote:

> Can someone explain what "lbf" above is.

As you'd suspect, he just means a pound -- the force exerted by the standard gravity (9.80665 meters per second squared) on the standard pound of mass (0.45359237 kg exactly).

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