Thanks for the video Paul.
What’s kinda sad is that those particular SD40-2’s are also rather familiar to me…saw them many many times.
How fast the time flies.
John
All this talk about comparing steam locomotives to diesels is all very interesting but all of you seem to have forgotten the most important aspect of all, the respective power curves of both power types. A diesel electric developes maximum horsepower and tractive effort starting out and both rapidly diminish as speed increases due to the fact that the traction motors (all electric motors for that matter) act as generators. As the speed of the traction motors increases so does the amount of back voltage and it takes more and more horse power to counteract this force. That’s why it takes so many diesels to make a train go fast.
A steam locomotive on the other hand has a far different horse power and tractive effort curve. As the steam locomotive starts to pull at low speed it develops its lowest tractive effort and horsepower, that’s why a steam locomotive’s starting tractive effort number is so important. It determines the tonnage of the train it can pull without needing helpers to get going. As the speed increases so does the tractive effort and horse power. At what speed these level out and then fall off is determined by driver diameter, boiler pressure, rate of steam production,cylinder stroke, valve gear timing and the weight of the locomotive.
That’s why a Challenger putting out 5000 hp at 40mph is the equivalent of four or five high horse power diesels, because those diesels have only about 5000 effective hp and similar tractive effort at that speed. Some calculations even suggest that the Challeger uses less btus than than diesels to get the job done but that’s another subject entirely.
It all comes down to something very simple: A steam locomotive can pull a train at speed that it can’t start on it’s own, a diesel locomotive can start a train on it’s own that it can’t pull at speed. Hope this helps.
P.S. That wa
Do you really expect us to believe that “4 or 5 high horsepower” diesels, such as an SD70ACe , AC4400, or an AC6000, have only 5000 HP at 40 mph? If my math is correct, that would only be 1000 HP or 9400 lbs TE per unit @ 40 mph.
Perhaps you would be so kind as to show us EMD or GE tractive effort curves for the above mentioned units at 40 MPH to confirm your statement.
I would love to see those calculations.
Maybe where he went wrong was reading this
and concluding the 4-6+6-4 could equal five GEs. Hopefully CAZEPHYR would disavow any such implication.
We could clarify the situation if somebody had clocked the five GEs up the west side of Sherman Hill with the stack train. Given the train’s tonnage, it would be easy to calculate that if they were really only good for 1000 hp per unit at 40 mph they couldn’t possibly make the speed they actually did make up the 0.82%.
Sorry guys, didn’t mean to ruffle any feathers. When I referred to “high horse power” diesels I was thinking of SD 40-2’s, (They’re putting these things in museums already!) shows how old I’m getting. The Challenger at speed is probably the equivalent of 1 1/2 or maybe 2 at best of today’s most modern power. At start the king of tractive effort and horsepower is still the gear reduced electric motor, be it straight electric or diesel electric.
The fact remains that the more you try to push an electric motor the more it pushes back. Effective horse power drops off following a descending logarithmic curve. I have my old college engineering texts with the relevent data but I’m not computer savvy enough to figure out how to get them posted. (Principles of Electric Generation and Use, McMillan Publishing, 4th ed. 1977, page 279-283)
Back EMF (Electro- Mechanical Force) is the major impediment in the usage of electric propulsion. Engineers try to design electric propulsion systems to stay in the “sweet spot” of the electric motor so that the motor isn’t running too far along the bottom of the curve. The sweet spot of diesel electric locomotives tends to be in the range of 5 to 35 mph, over 35 and the back EMF becomes ever greater and horsepower transmission efficiency decreases thereby lowering effective horsepower and tractive effort.
Staight electric locomotives overcome the back EMF problem somewhat by having a relatively large amount of available horsepower to drive the traction motors and can then run at a higher speed. As far as their traction motors are concerned, they “see” the available horsepower as virtually infinite. This principle allows today’s high speed trains to generate the speeds that they do.
Southern Pacific tried to get around the back EMF problem by trialing the Krause-Maffei diesel hydraulic locomotives.
I’m sure a engineering book from 1977 does not delve into microprocessor controlled frequency drive AC motors. Which is to say a lot has changed in diesel-electric locomotive performance since the time of the SD40-2s, or the time of the FTs.
Given feltonhill’s accurate statement that a Challenger is a 4600 HP locomotive @ 40 mph, that gives it a 500 hp advantage over a 4400 HP AC @ 40 MPH and about 900 HP less than an AC6000 @ 40 MPH.
The “sweet spot” on an AC is actually at higher speeds, which is why CSX has used ACs on intermodal from the get go (a trend that I now see the other Class 1s engaging in).
You will get no argument from me that the late steam designs were indeed powerful (not as powerful as some railfans like to pretend they are), and it wasn’t until the last 15 years or so that D-E technology has progressed to the point of being able to give both high low speed tractive effort and high speed horsepower from a single unit.
Thanks for the feedback GP 40-2, can’t fault your logic. It sucks to admit I’m turning into an antique like the machines I love. 1977 seems like yesterday… At least back then I could stand trackside and watch the occasional F-unit or Alco roar by.
You’ve got the wrong idea about back EMF, which is just nature’s way of keeping us from getting something for nothing. We want to get mechanical work out of a motor, so we have to put electrical work into it-- that is, we have to shove the current past the “back EMF”. An SD40-2 succeeds in doing that at 60-70 mph just as well as it does at 20-30 mph.
Like most railfans, you have basically no idea what speed an SD40-2 would make on a given grade with a given tonnage-- for whatever reason, railfans in this country aren’t much interested in that. (Not interested enough to go out a take a proper look, anyway.) But various railroads have long steady grades with parallel highways, where you can pace the trains in your car and get a good idea of their speed. If by chance you live in California you can pace SFe trains up a steady 1.0% grade eastward from Essex; I fear few? no? freights climb that hill at 60+ mph now, but SD40-2s used to take the 991 train up there at that speed, overcoming the back EMF just as they’re supposed to.
Got the citation for that?
That too.
Just to expand on timz’s statement about nature preventing us from “getting something nothing”, the build up of back pressure applies to mechanical systems such as a steam locomotive too. Once the steam expands in the cylinder and does its work, it becomes an exhaust gas, and must be pushed out. There is a finite limit on how efficiently you can push this gas out the exhaust valves and stack. The faster you try to push it out, more and more back pressure is produced, and additional power from the engine is needed to overcome this pressure. A balance point is reached, and no more power will be produced in the cylinders regardless of how much fuel can be burnt in the firebox.
There is no way to get around the laws of physics, even in railroading.
How 'bout a little help here in understanding the ‘back EMF’ = ‘back ElectroMotive Force’ concept, also known as ‘counter-electromotive force’ ? [And no, it’s not an ailment peculiar to GM’s diesels, or one that GE’s are immune from . . . [swg] ]
I’ve done a little Internet research on this, but it’s hard to find something that applies directly to railroad locomotive motors. I gather that the back EMF - mainly in the form of voltage - results from the motor’s armature spinning in the magnetic field that’s created by the electric current running through the stator coils. It’s also proportional to the motor speed - slow speed, little back EMF; fast speed, lots of back EMF. So it’s also a ‘feedback loop’ or ‘check-and-balance’ kind of arrangement on an unloaded motor just going faster and faster . . . [:-^]
I also understand that the rearrangement of the motor circuits from the series to series-parallel and then to full parallel is a method to boos
[quote user=“Paul_D_North_Jr”]
How 'bout a little help here in understanding the ‘back EMF’ = ‘back ElectroMotive Force’ concept, also known as ‘counter-electromotive force’ ? [And no, it’s not an ailment peculiar to GM’s diesels, or one that GE’s are immune from . . . 
]
I’ve done a little Internet research on this, but it’s hard to find something that applies directly to railroad locomotive motors. I gather that the back EMF - mainly in the form of voltage - results from the motor’s armature spinning in the magnetic field that’s created by the electric current running through the stator coils. It’s also proportional to the motor speed - slow speed, little back EMF; fast speed, lots of back EMF. So it’s also a ‘feedback loop’ or ‘check-and-balance’ kind of arrangement on an unloaded motor just going faster and faster . . . 
I also understand that the rea
In Paul Kiefer’s book, A Practical Evaluation of Railroad Motive Power, the only reference I saw was either 2-unit or 3-unit E7’s in comparison to a Niagara. IIRC, it took three units to equal the performance of a single Niagara, but two units were close enough, and more economical.
That’s the general idea, except that AFAIK no road diesel ever connected all its motors up in one series string-- no need to do so, I assume. Some C-Cs did go from two strings of three motors, to three pairs, to straight parallel, but none? of the post-1965 models needed that initial two-strings-of-three stage. Since circa 1980? all locomotives use parallel connection of the motors all the time; the reconnection (if any) is done in the alternator/generator instead.
The other possibility was field shunting-- not used on any locomotive since… 1972?
Wasn’t Paul Kiefer the Chief of Motive Power on the New York Central or some high post? I suppose that book is out of print and hard to get and, oh well.
I think that the narrative was that from a HP standpoint, a single Superpower steam locomotive – Northern, Challenger, and so on – matched not just a Diesel but a multi-unit Diesel consist, to within certain bounds. That is, back in the day when an FT was what, 1300 HP and an E7 was 2000 HP.
The (multi-unit) Diesel had the steam locomotive beat hands down on weight on powered axles and probably on lugging ability, but as far as I could tell, you could “flog” a steam locomotive until you lost your steam (by using too much) whereas you could do expensive damage to a Diesel consist by exceeding short-time ratings.
But these days, a 4500 HP six-axle AC-motored microprocessor-wheel-slip-controlled Diesel is probably a match for anything steam in both HP and lugging ability. If they were able to pull that consist with a Challenger, they should be able to manage it with a single modern AC Diesel. That they have at least 2 (or maybe 3) such units on a modern intermodal is that they want some reserve for an on-road unit failure, be able to run through without helpers or locomotive changes for grades on certain divisions, have acceleration to meet schedules.
When it appears on eBay it goes cheap enough, maybe $10. If you haven’t seen it you’re probably imagining it’s more than it is; the diesel-vs-steam acceleration figures are apparently not actual test results but just the same sort of calculations you could do for yourself. The financial data is perhaps of interest, if you have any way of guessing whether he’s including all/only the costs he should.
Close to a match, against most steam in most situations. NY Central did do actual tests too, and the time their 4-8-4 took to accelerate 22-car trains to 75 mph will be hard for an AC44 to match.
As far as I know, EMD ALWAYS used field shunting, as well switching from series to parallel motor connections, on all locomotives that used dc generators. (Possibly some switchers may have been exceptions.) This may have been changed once they started with alternators and rectifiers, and certainly was dropped with any ac motor locomotives.
I think you have it right. The last new loco with field shunting I know of was the SD45. The SD45-2s didn’t need it because the max voltage the diodes could take had crept up over the years allowing higher main generator voltage to be generated.
The first loco with gen transition I can remember was the SD50.
Paul,
An analogy that might help is to think of a water turbine - the “back EMF” of a motor corresponds to the “back pressure” of a water turbine that corresponds to the work transmitted to the output shaft - with electric current being equivalent to water flow. The power available from water under pressure (conversely the power required from a pump to generate water under pressure) is the flow times the pressure drop, 1 cubic foot per second at 1,000 psi has the same potential power as 1,000 cubic feet per second at 1 psi. Similarly, DC power is volts times amps.
Getting on to DC motors, the EMF is proportional to the armature speed times the strength of the magnetic field produced by the field windings. The magnetic field strength is proportional to the field current up until the iron in the motor frame starts saturating. The torque from a motor is proportional to the armature current times the magnetic field generated by the field windings.
The series motor has the field current equal to the armature current, so for a constant armature voltage (assuming no field saturation), as speed increases, the current through the armature needs to decrease to maintain the same EMF. Since diesel locomotives operate in a constant power more for a given prime mover speed, this means that the generator voltage needs to increase with increasing track speed. One problem is that there are limits as to how high the voltage can go before the generator (or motor) starts having troubles (e.g. arcing). One way of getting around
What kind of tonnage did those 22 cars weigh?
From a standing start, I would put my money on the AC44. If they were to start a race where they were drifting at say 25mph, then I might go for the steamer. It would be a half mile down the road before that GE started to load up.