• Interesting Article on HP, TE, etc.

  • General discussion about locomotives, rolling stock, and equipment
General discussion about locomotives, rolling stock, and equipment

Moderator: John_Perkowski

  by Denver Dude
Sorry if this guy's stuff is common knowledge here - I haven't seen it before. He gets into some really technical locomotive information on his site. I think it's great.

I was surprised at how high hp locomotives don't seem to be that effective at lugging heavy trains at low speeds.

Any thoughts?

  by Allen Hazen
I just skimmed it this time, but what I saw looks good... and I recall reading some of Mr Krug's essays on railroad technical matters a few years ago and finding them very well-written (= understandable) and informative. So read this one carefully: my guess is that what he says is right. ... It's probably a few years old-- locomotive types mentioned aren't current production, and the only recent four-(powered)-axle units on the Santa Fe main lines are the six-axle/four motor units introduced in the past few years by GE (ES44C4) and that other outfit. But the principles involved are permanent.

The principles aren't even specific to diesels. Consider two steam locomotives from the WW I era: the USRA heavy 4-8-2 and the USRA light 2-10-2. I think (I may have misremembered here) that they had very similar boilers: so about the same potential for horsepower. Difference was that the 4-8-2 turned the horsepower into tractive effort with four powered axles, with a total adhesion weight of around 240,000 pounds, and the 2-10-2 had five, for a total adhesion weight of 275,000 pounds. Result: the 4-8-2 could use its boiler's power at a reasonably high speed-- it was best applied as a fast-freight engine, or passenger engine for heavy service in hilly country. The 2-10-2 wasn't as good at high speeds, but had a higher tractive effort at low speeds: could use full power at a lower speed (and so put out more t.e.) without slipping its drivers and spinning its wheels. Definitely the better engine if you wanted to drag freight up hills. The New York Central had the country's largest fleet of 4-8-2 locomotives -- most a bit bigger than the USRA engine, but many in the same general ballpark -- which it used in freight service and all but the fasted passenger service on the "Water Level Route." They also had a small batch of USRA 2-10-2 locomotives, which were assigned to the system's one mountainous district, the Boston and Albany.
  by Denver Dude
Allen, that's a good response. Thanks. I have to work out the reasons why the higher hp machines don't work as well in the hills and at low speeds. I would think that would be where they'd excel. I have to go back to that article and read it a few times.
  by timz
Yeah, Krug's articles are likely the best you can find online.

When you talk about "working it out", sounds like you're thinking it's more complicated than it is. Just make sure you understand what power is-- you'll be ahead of most fans, and you'll be well on your way to understanding the rest.

Probably no one said high horsepower units were always weak at low speed. An SD38 will likely outpull a GP60 at 3 mph, but an SD60 will sure outpull a GP38 at any speed.
  by Allen Hazen
(This is probably TOO simple-minded for most people here, but when I first tried to understand what locomotives do I had to reduce things to very, VERY, simple terms to get my head around it. So, on the off chance that there is a rank beginner here that it might help....)

The basic high-school physics of it is that ... Tractive Force is a FORCE, and horsepower is a POWER.
For tractive force: imagine the locomotive connected to the train by a spring balance. The "weight" shown on the spring balance is the force exerted by the locomotive in pulling the train. How much force can a locomotive exert? It is limited by the power of the engine, but at low speeds you start spinning the wheels (experiencing wheel-slip) long before you get to the other limit. This is why six-axle diesels (with all axles powered: CC diesels) can exert more tractive force than four-axle. And why, at low speeds-- dragging a coal train up a mountain, say-- a more powerful locomotive doesn't always do better than a less powerful: the tractive effort is limited by the spinning-the-wheels limit, and much of the engine's power can't be used.

Force times distance = ENERGY. Think about lifting a weight: you need a certain force (to wit: something more than the force of gravity holding it down) to lift it, and then the "potential energy" you give the weight is a matter of how high you lift it: how much (vertical) distance you have to exert the force in the process of lifting it. (Or, equivalently, how much (vertical) distance the force of gravity would be working on it if the weight now fell and its potential energy was converted into kinetic energy.)

POWER, finally, is how much energy is put out in a given amount of time. (A diesel engine, of course, doesn't CREATE energy: it converts the chemical energy in the fuel burned into mechanical energy. So, assuming equivalent efficiency, the power of the engine should be proportional to its fuel consumption at full power. Pick two locomotives with engines of equivalent technology: say, for a late 1970s GE example, a B23-7 of 2250hp and a C30-7 of 3000hp. The first has a 12-cylinder engine, the second an engine with 16 of the same kind of cylinders. So, running at full power, the B23-7 will burn about 3/4 as much fuel as the C30-7 in a given period of time.)

Now the relation between tractive effort and horsepower can be reduced to arithmetic. Suppose you have two locomotives, and have them each exert the same tractive effort: assume this is the maximum tractive FORCE they can put out without wheel-slip. (This is somewhat dependent on speed, I assume, but over a reasonable range of speeds we can, as a first approximation, take the maximum tractive effort as a constant: roughly approximately 1/4 more or less of the weight on the powered axles.) So the amount of ENERGY they put into train-pulling is proportional to the distance traveled. Now suppose one locomotive is more powerful than the other: that means it can put out more energy in a given period of time. But we just said that energy is proportional to distance traveled, so the only way it can do this, exerting the same tractive effort, is to travel a greater distance: in other words, go faster.

The main determinant of the maximum tractive effort a locomotive can put out is weight on its driving axles. (Second determinant is how sophisticated its wheel-slip prevention technology is. The sensitive, microprocessor-based, adhesion control systems introduced around 1980 have made a real improvement. Older diesels could be depended on to achieve tractive efforts a bit under 1/4 their weight, whereas modern ones-- when things are working right-- can do a bit more than 1/4 of their weight.) The difference POWER makes is not how much tractive effort you can get, but at what speed.
  by Jtgshu
Those are very interesting articles!

Being an engineer, I have an interest in these kind of things, but its not always easy to convert whats in words to what you do every day out on the railroad, but its interesting to me, because I understand whats going on, but I dont' always know the reasons or the words or laws of physics :) Im not the sharpest spike in the tie........

There is one factor that is often overlooked with regard to train/locomotive performance.

The Engineer.

That can be a HUGE factor. Just like a professional race car driver could make your Ford Taurus do things you would NEVER imagine, and drive it faster and harder than you ever imagined or ever could, the same applies to Engineers. Everyone drives their automobiles differently, and engineers run differently. And im sure it was even more of a factor back in steam days.

Some engineers can simply get more out of a loco than someone else. I once took a train of loaded stone hoppers over a stretch of track with one locomotive that the engineer who got the job the night before stalled with 3 locos (the closest my commuter operation will come to running a "real train" as some freight guys like to say....). It took a lot of work to get moving, but I was able to get it to move and made it on our way. The Conductor was the same as the night before with the other engineer and he was shocked. Its not anything special, but sometimes you just can't give it gas and go. It takes a little effort to get to move. Why could I get the train to move with a 3000 HP pax geared Geep, while he couldnt' do the same thing with 9000 HP 3 passenger geared Geeps....I honestly don't have any idea how he managed to stall it with that much power, but if you knew this guy......but I bet it had something to do with just throttling out (or not checking the other units to see if they were MU'ed properly...) and hes probably lucky he didnt' rip out a drawbar....the hoppers we use are about 60 years old..haha. BTW, a passenger geared Geep (meaning 103mph I dont' remember the gear ratio off hand) cannot pull for anything.....a single freight geared GP40 would be fine, as it has the power down low, but the passenger gearing means these things are just terrible at anything but passenger trains.

Im not saying im the best guy out there, or the worst guy, just giving a example of how by all measures, 3 Locos should have been more than adequate, and it wasn't for one guy, while I was able to do it with 1.

But at the same time, never underestimate the power of a quit :) "We don't have time to wait for another loco to come....this is fine!"
  by Denver Dude
Some great responses here. Thanks!

From his article it seems that tractive effort is mostly about the weight/adhesion ratio. But I would think that traction motor torque is also a big part of it.

It seems that wheel slip is a huge factor, also due to the weight limit of 35,000 lbs. per wheel. It would be interesting to see the pulling power if the powered axles we geared into the trucks!

Edited by a moderator.
  by timz
My posts can be edited. You sure yours can't be?

Gearing the wheels to the track would increase TE, but without other mods to the locomotive it wouldn't increase it hugely. More TE would still take more amps thru the motors, which would require more amps out of the main generator, which the control system would have to provide enough generator excitation for.
  by Denver Dude
It seems that sometimes I can edit posts - usually just after I post them. After a while I don't have that option.

Hey, does all of this stuff about power and tractive effort mean that those mountain coal trains with the 4400 hp locomotives in run 8 have the hp dialed back because of wheel slip - and they aren't really running at full power when it seems that they are?

I remember seeing them in Tennessee Pass and thinking how cool it was when they were grinding up the mountain, engines roaring, and moving at only 12 MPH or so. Now it seems that they weren't really in full power, after all...
  by Allen Hazen
(Everybody: I'm an AMATEUR! Please, PLEASE, check this and let me know if I'm wrong!)

Denver Dude--
I'm going to do the arithmetic in my head, so (a) this is a rough approximation and (b) you'd better check it in case I misplace a decimal point or something.
The claim for the newest AC-motored units is that on a good day (clean, dry, rail, and the wheelslip-control system working right) they can get something like 35% adhesion: can exert a tractive effort of 35% of their weight. So a CSX "heavy" ES44AC weighing 432,000 pounds can put out something like 150,000 pounds of t.e.
8 miles an hour is 40,000+ feet per hour: lets say 660 feet per minute. So the train pulling work done is 99,000,000 foot-pound per minute. One horsepower is 33,000 ft.lbs/min, if I remember right, so that's 3,000 hp. I'm not sure what the transmission efficiency of the AC drive is. (Conventional diesel electric locomotives with DC motors were about 82% efficient: their horsepower at the drawbar was about 82% of the rated horsepower of the diesel engine... in the U.S.(*). I've heard that the AC transmission is somewhat better, but I don't know how much.) Let's guess an 85% transmission efficiency: so a 4400 hp locomotive at full power has 3700+hp for train-pulling.
So, bottom line: yes, probably the engine is throttled down a bit at speeds below 10 m.p.h. (On-board computer monitors adhesion and calculates just how much of a power-cut is needed.) Sooner or later the hill will be crested, though, and even a coal train will use the full locomotive horsepower!

(Oops. Just re-read your post. You said ***Run 8*** and ***12 mph***, not 8 miles an hour. At 12 mph-- provided it isn't rainy and there aren't too many Autumn leaves and squished caterpillars on the track (seriously: both of those can cause adhesion problems) and the control electronics haven't crashed-- a modern 4400 hp can use full power.)
(*) North American railroads use a different convention for rating locomotives from that in use in much of the rest of the world. A North American EMD SD-40 and an export EMD GT26C would have used the same turbocharged 16-cylinder 645 engine. The SD-40 was billed as a 3000 hp locomotive, the GT26C in many countries as 3300 hp. Think roughly 2500 drawbar hp for either model.
  by Allen Hazen
Oh. Afterthought.
You don't really need an onboard microcomputer to get an automatic horsepower adjustment at low speeds: 1960s electronics technology could do it, and it was a feature of many "high horsepower, second generation" diesels in the 1960s and 1970s: General Electriccalled it "Power Match."

Now for the comparison. A 4400 hp six-axle diesel has almost the same per-axle horsepower as a 3000 hp four-axle unit. So how does adhesion compare as between today's ES44 and, say, a U30B (produced 1967-1975)? I have an old textbook on unit trains with appendices-- based on data provided by the locomotive builders-- on what you could expect from a diesel. The ES44AC, if my calculations in my last post are right, can use its full power (at least on good days) at 12 mph. Apparently GE's standard option package for the U30B automatically reduced horsepower by almost a third (to 2050) at 12 mph.

Which is why we now see coal trains pulled by locomotives with a per-axle horsepower that, 40 years ago, was for fast freight and passenger service only!
  by Denver Dude
Allen, thanks for all of the info.

I was wondering about locomotive weight. I was under the impression that 70,000 lbs. per axle (35,000 per wheel) was the max. I don't see how a 432,000-lb. locomotive would be supported, as that would be 72,000 pounds. Are they using stronger steel in the rails these days?

Also, it seems that TE is just a factor of weight and adhesion. Does hp play into it, or is it only weight and adhesion?

Thanks again.
  by timz
Denver Dude wrote:it seems that TE is just a factor of weight and adhesion.
Well, yes, if you define "adhesion" that way. A certain amount of weight on drivers allows a certain maximum TE. If you have 100000 lb on drivers you can't expect to get 100000 lb of TE. You understand that, and there isn't much more to understand about "adhesion".
Denver Dude wrote:Does hp play into it, or is it only weight and adhesion?
Horsepower at the rail is TE (in pounds) times speed (in miles/hour) divided by 375. So a locomotive that's producing 150000 lb of TE at 1 mph is producing 400 rail hp, by definition. If the locomotive's prime mover can't produce that much horsepower, then the locomotive can't produce 150000 lb TE at 1 mph. But maybe it can produce it at 0.5 mph.
  by Denver Dude
timz wrote:
Denver Dude wrote:it seems that TE is just a factor of weight and adhesion.
Well, yes, if you define "adhesion" that way. A certain amount of weight on drivers allows a certain maximum TE. If you have 100000 lb on drivers you can't expect to get 100000 lb of TE. You understand that, and there isn't much more to understand about "adhesion".
Denver Dude wrote:Does hp play into it, or is it only weight and adhesion?
Horsepower at the rail is TE (in pounds) times speed (in miles/hour) divided by 375. So a locomotive that's producing 150000 lb of TE at 1 mph is producing 400 rail hp, by definition. If the locomotive's prime mover can't produce that much horsepower, then the locomotive can't produce 150000 lb TE at 1 mph. But maybe it can produce it at 0.5 mph.
Doesn't grade factor into it? Also, don't you divide by 550? Hey, I'm just trying to comprehend some of this. :)

Also, how do you know the speed at which wheel slip will occur? Thanks.