Balancing Locomotive Power

Question

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?

Answer 1

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.

Answer 2

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.