Hi, all! A round of recent video viewing on YouTube has freshened by childhood enthusiasm for the pre-Amtrak days of passenger trains (yes, I'm that old) and reminded me that I've always had a question about multiple-loco consists (if that's the correct term) and I've decided to sign up here and ask you folks about it. My question doesn't really depend on whether the lashup is all diesel-electric or a combo of diesel and steam (which I've only seen in rare videos) or whether the locos are all at the front or one is at the rear...it just seems to me that the throttles (or rheostats?) for each motor would have to be *perfectly* matched otherwise you'd have a situation where one loco is over-driving the other one...right? IOW, without some sort of connection to match up the revs, it seem to me inevitable that one loco would be either dragging or pushing the other...even if only slightly. Let's go back to the days of steam: two locos would have had no connection other than the physical hitch and there would have been crews in both cabs, correct? Would those engineers have to have constantly modulated the throttles so that the two were pulling in perfect tandem and not creating the situation I'm describing? Today, with diesel-electric (and lots of system connections) how is this managed? Thanks very much, everyone!
The short answer is NO. Each loco is its own power unit that moves freely on its own and it doesn't care what the others are doing. Think of a load that you want to drag across the ground. Add a long rope. One muscle builder guy can't pull the load. Add a normal guy and they still can't pull the load. Add a teenager and the load starts to move, slowly. Add a child and the load moves even better, etc.
I'm sure that others will respond with a much more technical explanation, however, it is not anywhere near as hard or complicated as it may seem.
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aboard sir! We have fun here and sure learn from each other!
Getting to your question, and mind you I'm not an electrical or mechanical engineer, or a professional railroader, and make no pretense at being any of the above, it sounds like what your referring to is called "multiple-unit control."
This is fairly old technology, it goes back to the turn of the 20th Century when Frank Sprague developed it for control of multiple unit lashups of trolley cars. One throttle or controller would exercise control of all powered units.
It's now applied to multiple diesel unit lashups, although I'm sure it's a lot more sophisticated now than it was back in Spragues time.
The only difficulty encountered now, from what I've learned from various topics here in the Forum, is when units of different manufacture such as GE or EMD are part of the lashup. I've read that the GE units don't "load", that is power up and move, as fast as the EMD units do. There's a bit of a lag. Once the GE's are up to power there's no problem.
Anyway, multiple unit control capability is one of the things that attracted railroads to diesel power. Instead of multiple steam engines with multiple crews any amount of diesels could be coupled together with one engineer controlling the lot. It works, and works well.
If anyone wants to correct me on this go right ahead, I'm not proud.
First: DPool !
Many of the Steam engines that power the Railfan Train trips; seem to have noted at some point in their narrative history's the fact that in the cab of the [steam] locomotive the owing entity has provided a 'control stand' to allow operation of a diesel engine accompanying the steam power. [ MU connections for throttle and other critical features provided to make that steam/diesel combination system work?].
I am not exactly sure how the 'hybrid mechanics' of such a system functions; but they seem to be able to operate very well ? Note: the UPRR's steam heritage fleet operates successfully, many miles, with accompanying diesel power. As do many of the other heavier steam engines,(nee: SP4449, for one other operating example.
In the case of supplemental diesels with steam excursions, a standard hostler control is mounted in the steam locomotive to control the diesels. This control has no linkage of any sort with the throttle of the steam locomotive.
Thanks for the prompt reply! And please don't think I'm arguing with you...I'm simply revealing my ignorance. I think I'm following what you've said but here's where the comparison breaks down for me: in your "folks pulling on a rope" scenario, all of those feet have a tremendous about of variety in how they are interacting with the ground -- some have traction, some are doing the stutter-step, some are maybe even scuffing and slipping. With multiple locos, the analogous contact points are the wheels against the rails. If I understand your example, then there would need to be some "give" at the wheel/rail interface...and the ones that were producing slightly less force than the strongest ones would be able to slip there. Let me reduce my mental image to the basics: if one electric motor, tied to an axle sitting on a rail, is running at 100 rpm and another electric motor, tied to another axle sitting on the same rails (and all joined together at the coupling) is running at 99 rpm....somewhere, sumthin's gotta give. So what am I still missing?
DPoolotherwise you'd have a situation where one loco is over-driving the other one...right?
If you have three units on the front of the train, and one has been shut down and is being pulled/pushed by the other two, then it's not helping, but no problem aside from that.
Or, if two of the three units are 4000 hp each and the third unit is 2000 hp, then each will pull as hard as it's supposed to, and the unequal pulls are no problem.
Yeah, I confess to struggling to explain what I mean! If you read one of my later replies to another member, I might have been clearer. But, if not....what I'm thinking (or imagining) isn't about the power from each unit but more about the throttling. If two units are sitting on the same rails and both are running...imagine if one unit is throttled (and I'm using that term even if what we're talking about is the control of the electric motors and not the diesel engine) to provide 100 revs of a wheel per minute (that's just a number) while the other one is controlled in such a way as to drive 105 revs of its wheels per minute. In that case, (it seems to me) you've got one unit driving "harder" and either pushing or dragging the other one that, regardless of engine hp, isn't throttled to the same speed. That's what led me to conclude (and perhaps inaccurately) that multiple units would have to be controlled exactly the same....otherwise something would have to "give" somewhere (and maybe that's with some slippage at the wheel/rail interface).
BigJim ...however, it is not anywhere near as hard or complicated as it may seem.
...however, it is not anywhere near as hard or complicated as it may seem.
The possibility that I might be overthinking all of this has never escaped me, BigJim!
Flintlock76 The only difficulty encountered now, from what I've learned from various topics here in the Forum, is when units of different manufacture such as GE or EMD are part of the lashup. I've read that the GE units don't "load", that is power up and move, as fast as the EMD units do. There's a bit of a lag. Once the GE's are up to power there's no problem.
In spite of the fact that this deals with differences between manufacturers, this is pretty much what I was getting at: for that time that the GE unit is "loading up," it's not yet helping and is having to be pulled along by the momentarily faster/stronger EMDs....or, looked at the other way, it's behaving as a drag on the proceedings. So, just imagine that all of the units are one manufacturer or the other...what I was assuming to be true was that they have to be controlled with some unified precision...otherwise it's inevitable that one would be -- however briefly -- either underloaded or overloaded.
Perhaps the first thing to keep in mind with diesel-electrics that the throttle position controls how much power the diesel engine puts out and the electric transmission adjusts to produce the tractive effort that corresponds to that power setting. That is a 4,000HP locomotive will produce roughly twice the tractive effort of a 2,000HP locomotive at the same throttle setting, as long as the locomotives are moving fast enough to absorb the power without slipping wheels.
Relationship between tractive effort in pounds, speed in mph, engine power in horsepower and transmission efficiency as a fraction (i.e. 85% = .85)
T.E. X Speed = Eff X HP X 375
DPoolimagine if one unit is throttled (and I'm using that term even if what we're talking about is the control of the electric motors and not the diesel engine) to provide 100 revs of a wheel per minute (that's just a number) while the other one is controlled in such a way as to drive 105 revs of its wheels per minute.
If the train is slowing, and the throttle remains in Notch 5, then the locomotive's pull will slowly increase -- the control system increases generator excitation as needed to maintain constant power. Constant power is the aim, not constant wheel speed.
Re: "the electric transmission"
Not sure if this is what you refer to but, in any case, my curiosity had be continuing to search today and I came upon this page with these videos >> https://edisontechcenter.org/Dieseltrains.html In Video 2 at 1:11 the guy points out the load regulator which, on the basis of the way he describes it, probably performs the function that I'm struggling with. Very interesting that it regulates voltage to the traction motors "depending on load and speed" and works off the oil pressure in the engine (though he's not clear if this is a reading or the force of the actual pressure).
See my other reply about the video I found and its reference to the load regulator. Is that a component in what you've described to me? Is there any kind of fluid coupling in this drive system? Or is that another branch of the diesel-electric development tree?
DPoolthe load regulator which ... works off the oil pressure in the engine
DPoolIs there any kind of fluid coupling in this drive system?
DPool:
With the DC generator and DC series traction motor combination, the Lemp control system for the generator roughly provided a constant power output to the traction motors. At low track speeds, the generator would provide high current and low voltage, as speed increased the motors would draw less current but require a higher voltage (I'm neglecting transition with the old locomotives).The load regulator would vary the excitation to the generator to increase loading with increasing throttle position.
On the latest locomotives (AC traction motors), the inverter is set up to provide more or less constant power to the motors, which then provides a constant load on the diesel engine. The inverter controller would be aware of the throttle position and adjust the output voltage and frequency in order to provide the appropriate power to the motors.
Someone with the time and knowledge neeeds to explain the AC traction systems and the difference between truck AC inverters and axel inverters. Wheel diameter needs to be very close on truck specific inverters.
First off, American "AC" diesel electric locomotives use induction motors where the torque is proportional to "slip", that is the difference in rotational speed of the rotor and the synchronous speed. An example of this is an induction motor specified to turn say 1750 RPM when loaded while running off of 60Hz and the synchronous speed for that motor would be 1800 RPM. Free running speed (no load) may be between 1790 and 1800 RPM, so slip at full load would be about 3%.
For a single inverter per truck, all of the motors on that truck will ave the same synchronous speed, so a difference in wheel diameter will mean that the motors will be experiencing differing amounts of slip and thus producing differing amounts of torque, smaller wheels will turn faster and have less slip. When running at the limits of adhesion, a small difference in wheel diameter can lead to the larger wheels can lead to the slightly larger wheels exceeding the adhesion limit, while the smaller wheels are comfortably below it.
The inverter per axle allows individual control of the motors, so the inverter frequency for an individual motor can be set so that it gets its appropriate share of the tractive effort.
The shift from inverter per truck to inverter per axle happened when IGBT's (Insulated Gate Bipolar Transistors) where made large enough for locomotive inverters. This alowed for much simpler drive circuitry, where the gate driver only needs to pump a few amps for a couple of hundred nanoseconds in/out of the gate to switch it, where the GTO (Gate Turn Off) thyristor driver needed to pull several hundred to a couple of thousand amps out of the gate to turn it off. Next change in inverters is likely to be Silicon Carbide MOSFETs.
Incidentally, the IGBT was devloped by the Harris Corporation in the late 1980's (they called it the COnductivity Modulated FET, COMFET). I'm guessing this was by the group of semiconductor experts that Harris got when they bought GE's semiconductor group during the early Welsh era.
- Erik (formerly erikem)
timz (You know that the diesel in an F7 turns a generator that produces DC current for the electric motors that turn the wheels?)
(You know that the diesel in an F7 turns a generator that produces DC current for the electric motors that turn the wheels?)
Yes, I do...but I wasn't sure if there was ever any kind of fluid coupling or torque mulitplier *between* the diesel engine and the generator. However, I can see how my previous comment sounded like I was looking for some sort of transmission between the generator and the traction motors...which I wasn't.
DPoolI wasn't sure if there was ever any kind of fluid coupling or torque multiplier *between* the diesel engine and the generator.
No, and in fact there are some examples where there isn't even a shaft between the two: the generator only has a 'back' bearing, with alignment being assured entirely by the rear main of the engine (!) That was done to save both length and weight, but it complicated a variety of things often significant in practical use.
To my knowledge, no builder implemented a clutch (or field-removable coupling) between the engine and generator, either. In fact, many early EMDs used a (fairly large-diameter) winding on the generator as the 'starter motor' for the engine; there is a periodic complaint on RyPN that EMD specifically disadvised using this to 'bump the engine over' for maintenance, clearing cylinder hydraulics to avoid lock at starting, etc.
There was, at one time, a very specific potential use of a harmonic reducer or gearbox: Baldwin locomotives had very good generators and traction motors by Westinghouse, but these were driven by a diesel engine with only about 2/3 the peak rpm of, say, an EMD 567 or 645. So re-engining these became much more expensive and involved, with the Alco conversion of some Sharknose engines being perhaps the saddest (the result was a good locomotive, said to be 'an RS-18 above the deck in a streamlined carbody ... but still in a streamlined unidirectional carbody with almost incredibly limited visibility to sides and rear). A fairly simple harmonic reducer could likely have been developed at probably somewhat less cost and better long-term reliability than a typical gearbox to match speeds. That this was never done in the long checkered history of trying to get more investment out of Baldwins may be a good indication of the cost-effectiveness of such an approach.
There's no point to building a hydrokinetic transmission for the generator or alternator of a diesel-electric. That would add cost, weight, and additional points of failure to achieve almost nothing of any particular operational value. Since the excitation of either a generator or alternator is inherently variable, technically almost steplessly (the 'notch' system is imposed by digital control of the diesel-engine governor, and locomotives like early Alcos or Baldwins used continuous air throttles that could take advantage of continuous regulation) there is no point in tinkering with the rotational speed vs. the crankshaft speed of the engine if you are going to transvert the alternator output to reasonably-smoothed DC, which as far as I know is done for any contemporary system of AC inverter drive on locomotives.
Now, when cheap HEP via tap or windings on the traction alternator started to become popular for certain commuter districts, the additional fuel burn needed to keep the alternator turning at 'HEP frequency' (usually somewhere in the 900rpm range for 60Hz AC) could be severe; that's the reason so many operators turned to asynchronous genset power instead. But note that an intermediate change-speed device that would allow "900rpm" on the alternator at lower engine speed, determined in large part by actual electrical load on the HEP relative to varying traction demand on a running train, would produce what might be significant fuel savings. Again, I am not surprised for a variety of reasons that this wasn't tried even as a test...
That disc thing bolted to the EMD crankshaft is a "coupling disc" and in the owners manual it says never to run the engine if the generator is unbolted as the armature functions as a flywheel. Marine engines that have clutches in the gearbox such as Rientjes, Lufkin, and Haley requir that weight rings be added to coupling disc. Falk gears with airflex clutches between the engine and the drum part is rotating all the time with the engine has enough inertia to function as a flywheel.
On locomotives the generator traction motor is the variable speed transmission. The only application of a fluid coupling is on the voith-schneider propellers used on some Stten Island ferries and at least one Panama Canal tug. Its a requirement by Voith-Shcneider to avoid tortional vibration problems.
"and either pushing or dragging the other one"DPool,I may be wrong about this, but, it seems that your mind is thinking that these units are phyically "geared" to the generator/alternator much like a model train. This is not the case. They roll on their own free will, whether in power or not (isolated). Nothing is getting literally "Dragged" nor "Pushed" like a model train.So, it doesn't matter what each unit is doing. They all do the best that they can to help, just like in pulling on the rope.
DPool,Go to "Al Krug's Home Page" at:
https://web.archive.org/web/20150205123806/http://www.alkrug.vcn.com/home.html
...and search around. You might learn some things there!
While there is mcuh good info in this thread, it is trending toward diesel electric operations. But what about steam. I too have often wondered how two steam engines can run in tandem. I would think one would be constantly banging on the other or dragging/pushing to detrimentl effect. The rope analogy is ok but assumes the rope is taunt. Steam engines don't exactly work that way. Thank you for your replies.
Also consider that the wheels on one engine are usually of different diameters due to uneven wear, axle/motor combo replacements, etc. and therefore do not operate at exactly the same RPM. On a steamer it's a different story; the drivers must all be the same diameter since they are connected by rods.
Dave
Just be glad you don't have to press "2" for English.
http://www.youtube.com/watch?v=zQ_ALEdDUB8
http://www.youtube.com/watch?v=6hqFS1GZL4s
http://s73.photobucket.com/user/steemtrayn/media/MovingcoalontheDCM.mp4.html?sort=3&o=27
I’ve been reading this thread and DPool, I don’t think most of the respondents understand your question. I don’t have the answer, but let me offer a thought experiment to explain your question. Suppose you have two locomotives from different manufacturers and different horsepowers sitting side-by-side on parallel tracks. They are each connected to identical consists, let’s say twenty tank cars full of crude oil. You wave a flag and each engineer puts the throttle in notch 5. What you will see is that one of the locomotives, probably the one with higher horsepower, will move out ahead of the other. Now suppose you repeated the experiment by lashing the two locomotives together in front of a single consist. Put them both in notch 5 and the higher horsepower locomotive would seem to be trying to accelerate at the same rate it did on its own, but the lower horsepower one would be trying to accelerate more slowly. So what is happening? Is the lower horsepower locomotive being dragged along by the higher horsepower locomotive? Now suppose you put the lower horsepower locomotive in notch 8. Would it now be trying to push the higher horsepower locomotive faster than it wants to go?
I think you are overthinking things. Let's go back to the 1910s, when there is no such thing as a diesel or multiple-unit control, etc. Yet there were still trains with multiple locomotives. In a case such as this, there IS kind of a link between the locomotives, even if one is on the front of the train and the other is 40 cars away pushing. The engineers both had to coordinate, and a skilled engineer would be able to tell what the other was doing just by observing the slack, listening to the whistle, and generally knowing what had to happen.
Thanks blw!
I have been wondering the same thing regarding multiple steam engines and have never been able to get any answer. After all the answers above regarding diesel engines, you hit the nail on the steam head! There seems to be a fairly straightforward answer (which you gave) and it makes sense. Thanks.
blw I think you are overthinking things. Let's go back to the 1910s, when there is no such thing as a diesel or multiple-unit control, etc. Yet there were still trains with multiple locomotives. In a case such as this, there IS kind of a link between the locomotives, even if one is on the front of the train and the other is 40 cars away pushing. The engineers both had to coordinate, and a skilled engineer would be able to tell what the other was doing just by observing the slack, listening to the whistle, and generally knowing what had to happen.
Agreed. I recall an article in TRAINS magazine where a steam loco engineer talked about just this situation, multiple steam engines with the train whistle the only means of communication. He described it as " a pride thing." I'll bet it had a lot to do with "feel", for want of a better term. Failing mechanical or electronic synchronization, I don't see how the condition of one or the other locomotives pushing and/or pulling the other(s) could be avoided.
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