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My first post here. I'm curious about how power is balanced in multiple diesel-electric locomotives. I understand to that two locos can be ganged in terms of controls, to set speed etc, so that one set of driver controls manages the whole pulling group. (sorry, I don't know the correct vernacular) How is it possible to ensure that each is pulling it's weight evenly compared to the others in the group. Or, put another way, how can I avoid one loco pulling at 5000Nm and a second pulling at 6000Nm. Is it possible to read the tow force in the couplings?

Steve Taylor
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  • 20% difference of force between the locs isn't that big of a deal. – ratchet freak May 30 '17 at 12:42
  • @ratchetfreak please provide citations – Carl Witthoft May 30 '17 at 14:58
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    I'm not convinced you're asking the right question. The wheel speeds must match or you'd have horrible cases of breaking traction. If any engine applies too much or too little force, it'll either see the equivalent of back-stall or it'll cause wheel slippage. – Carl Witthoft May 30 '17 at 15:00
  • You don't necessarily *want* to have equal drive forces. If the power units are distributed along the length of the train, you may want to optimize the dynamic effects when deceleration closes up the couplings as the train attempts to overrun, etc. That type of behaviour can cause oscillations along the length of the train that last for the order of minutes, if they are not damped out, and can cause failures and accidents if they increase rather than decrease over time! – alephzero Jun 01 '17 at 00:04
  • @CarlWitthoft, yes, this is my thinking also. The wheel speeds must match but I'd want to be able to manage how much pulling power is being delivered by each locomotive. The question, I guess, is more like how it done now? Do trains employ active couplings that can feed back force/load information to the control system? – Steve Taylor Jun 02 '17 at 01:50
  • @alephzero, yeah, I can see how this could be a requirement. One use case is where I might use a lower powered loco but i'd want each to be running 50% capacity (which could be different power figure for each) or I might want to use the more fuel efficient engine most of the time and have a second just pull enough to carry itself and power on line when negotiating a hill. And, yes, this case where a loco at the rear needs to hold back. I'd want to know how much, right? – Steve Taylor Jun 02 '17 at 01:55

2 Answers2

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Actually, no effort is put into balancing it. Every loco obeys the command from the heading unit, eg "power at 70%" and applies it to the wheels. It doesn't really matter which one does most of the pulling. When units are of same model, then the power will be roughly same. But sometimes completely dissimilar locomotives are used together (eg helper engine at the back), so it's bit like communism: "From each according to his ability". More curious still, most modern locos and MUs have single traction motor per axle, so even with single locomotive your concern is still valid, balancing the power between axles.

The worse that could happen is wheelslip. But it's not related only to the difference of power between units, but also (or rather mostly) to applying correct power for a given speed and the friction between a single wheelset and the rail (adhesion). Eg. the first unit could be on track contaminated with slippery leaves while others still have it clear. And the answer is that each unit (sometimes even each axle) manages slip on it's own.

When certain wheels start slipping, power to them is reduced, until they stop slipping. That's all. (Additionally, sand is also applied to increase grip.) Basically, it's same concept as ABS but applied for acceleration instead of braking.

//edit: See this video about duplex steam locomotive T1. It's basically two 2-axle steam engines sharing one boiler, so most of the muti-engine concerns apply here. At about 1:12 there is a clear example of the front engine slipping and overspeeding - they both are at same power, as T1 had one throttle controlling both engines, yet acceleration causes the front to lift slightly and suddenly it turns out that the front set has too much power while the rear one is perfectly fine. This is visible example that the engines should not be at exactly same power setting. (I've used steam example because connecting rods make observing slip easier)

//another edit: as the previous video is no longer available, here's a video of assorted wheelslips, including diesel multiple-operation. At 0:46 there's another T1 situation, but more relevant to your question are one diesel unit losing all adhesion spectacularly at 6:07, and another diesel unit slipping one bogie/truck out of two at 11:07.

Agent_L
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  • According to an engineer who runs a commercial model railway layout, it is an issue in those layouts to the extent that they can't run 2 locos in tandem without damaging them in short order. – Hobbes Jun 07 '17 at 16:05
  • Thanks @Agent_L. That's a really interesting response. This gets more and more interesting by the day. – Steve Taylor Jun 08 '17 at 01:31
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A characteristic of the electric motors diesel-electrics use is that torque is inversely proportional to lag angle. The wheels aren't synchronous: if one wheel is loaded more it will go slower, if it lags it will get reduced power. If one engine is going faster, the wheels on the slower engine will be overdriven, lag will reduce to zero, and power going to the wheel on the other engine will increase to the maximum.

This only works for small differences in the angle of rotation between wheels: a wheel that is slipping will get maximum power, just like a wheel that is being overdriven by being pulled by another engine.

edit: It is pointed out that the EMD-F7 (the engine so popular that it virtually defined what a 'diesel electric train' should look like) was a DC, electric motor. Consequently it was (a) commonly used in double-heading when more power was required, and (b) it was notoriously difficult to do so, required close matching of all components and tolerances, and frequently exhibited the same wheel slip problems that steam engines had. Where more power was required, they were eventually replaced by AC units like the SD70MAC and the AC4400CW when those became available.

"Matching" electric motors requires tight tolerances on the air gaps. Even so, modern AC traction units can be half the weight for the same tractive effort and braking effort, because the wheels don't slip like on those DC units.

david
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  • You seem to be describing AC diesel-electrics. You might want to clarify that in your answer. I only worken on GM diesel-electrics and these were all DC and your comments on lag angle, etc., wouldn't apply. – Transistor Oct 21 '22 at 07:13
  • @Transistor : I'll take downvotes for being wrong: downvoted wrong answers are informative. Are GM DC diesel-electrics ever run in tandem? I thought they were only for light loads? – david Oct 23 '22 at 05:34
  • Yes, they were run in tandem. See Irish Rail https://www.steamtrainsireland.com/rpsi-collection/15/b141 and especially their 121 Class https://www.hattons.co.uk/directory/versiondetails/article?id=584 which were run nose to nose to avoid having to use a turntable to get a leading cab. – Transistor Oct 23 '22 at 08:01