Run 8 vs. Run 8 Question

[Denver Dude]

Okay, we have two trains with modern locomotives. One train is grinding up a 3% grade section of Cajon Pass in full throttle with a track speed of 15 MPH.
The other train with the same power is sprinting across the Texas panhandle at 74 MPH in the 8th notch.

My thinking is that to the prime mover it's all the same. It's working at the same level in both situations. Full power is full power, no matter what the train is doing.

Is this an incorrect assumption?

[Wayside]

The diesel engines are producing the same hp at the output shaft, yes. The difference is in the hp delivered to the wheels. As the speed of a diesel-electric locomotive increases, the amount of electrical current that is able to power the traction motors decreases due to increasing counter voltage produced by the turning components of the motors. Eventually, it will reach a speed where it can no longer accelerate because of this effect.

[Statkowski]

To the prime mover it may essentially be the same, but beyond that it becomes a case of "yes, but...."

Is the resistance offered by the alternator or generator due to electrical load the same? Is the equipment GE or EMD? Are the prime movers identical or, even if the same manufacturer, variations on a theme?

There were far more variables involved back when one had to mix and match first or second generation diesel electrics, and may indeed be less so today, but Cajon Pass and flatland Texas are two different things and Run 8 on one may not be the same as Run 8 on the other, even as far as the prime mover is concerned.

[Denver Dude]

As I mentioned, the exact same power.
I knew it wouldn't be a simple answer, and I find this interesting.

Your response ties into something else. I believe that the generator or alternator creates more resistance as the demands for power increase, right? In other words, when the train transitions from a relatively flat stretch to a grade, does resistance increase? The prime mover isn't just about increasing RPM, but about dealing with increased (and decreased) resistance due to different power demands and then having to adjust to them. True?

[Statkowski]

True. The prime mover is constantly adjusting its output based on loading. Trailing gross tonnage constantly changes, especially so when uphill and downhill is involved. If part of the train is going uphill while the other part of the train is going downhill or is on level track, the physics change. Throw in some sharp curves and then you've got wheel resistance to deal with. Where I live, back in the days of steam, downhill loaded trains needed pushers to overcome the resistance due to the curves.

Yes, Run 8 is Run 8, except when it isn't. Throttle setting is only part of the equation. As long as tractive effort exceeds train resistance you're ahead of the game.

[Wayside]

The load control system doesn't know the difference between motion resistance coming from tonnage on a grade and acceleration to higher speed on level track. As I referenced above, the engine is producing max hp in an effort to accelerate at high speed. The load amps (power to the motors), however, will decrease as speed increases, due to counter electromotive force. This effect can be observed on the load indicator. The engine will be burning just as much fuel/hr doing that job in Run 8 as it would moving tonnage on a grade.

[timz]

Full power is full power

But Run 8 doesn't always mean full power. If you leave the throttle of an ES44 in Run 8 as the train runs from level onto an upgrade that slows it to, say, 5 mph, "full power" would mean more tractive effort than adhesion will allow-- maybe 200000 lb. So less than full power at the wheel rims, so less than full power at the crankshaft.

But from, say, 15 mph on up, the ES44 in Run 8 is supposed to produce just about constant power at the crankshaft, and at the wheel rims.

[Denver Dude]

Full power is full power

But Run 8 doesn't always mean full power. If you leave the throttle of an ES44 in Run 8 as the train runs from level onto an upgrade that slows it to, say, 5 mph, "full power" would mean more tractive effort than adhesion will allow-- maybe 200000 lb. So less than full power at the wheel rims, so less than full power at the crankshaft.

But from, say, 15 mph on up, the ES44 in Run 8 is supposed to produce just about constant power at the crankshaft, and at the wheel rims.

That makes sense. As I understand it, too much power means wheel slip, and the traction control system then derates the prime mover, correct?

[Wayside]

As I understand it, too much power means wheel slip, and the traction control system then derates the prime mover, correct?

Well, it derates the main alternator output by reducing excitation, which also reduces the load on the diesel a bit. Governor/control thus has to inject less fuel to maintain RPMs for run 8 requirements.

[Denver Dude]

Thanks. This is stuff that 99.9999999% of the population doesn't think about. I find it very interesting.

I appreciate your responses.

[Pj]

All throw this in..

In the DC days, the locomotives effort was more directly tied into the HP of the engine.

With modern locomotives, it’s the overall electrical transmission system, computer control and other magic.

An example that I use with the new fireman..

The SD9043 is a 4300hp engine.
The SD70ACe is a 4300hp engine
The SD70ACT4 is a 4600hp engine.

Both the ACe and T4 produce the same effort (120,000lbs continuous) while the 9043 is at 116,000.

AC4400 is 4390HP, ES44 is 43 something and the ET44 is 4365. All produce 121,000lbs.

DC versions of the same are all over the map.

As speed increases the effort needed will decrease. Tonight I had a train that at 50mph, it produced about 36k of effort in N8. Following a slow train up the hill, in N4 it was around 58k.

Depending on what school of thought you go with, there are many answers. Even the instructors I had at Engineer school had different answers. The best I thought of, was HP is needed to overcome the resistance of the alternator/generator when a load was called for. Based on my former life with electrical stuff, that makes the most sense.

Bigger the alternator or generator to need the demand of the traction motors, grater the engine needed.

Makes sense to me.