Mixing diesel power in the real world question

I recall a comment Rich Melvin made here years ago regarding mixing horsepower and it has stayed with me. It is no matter to mix engines of different HP ratings as they all contribute to tractive effort.

 

the only real impediment to putting together dissimilar first generation diesel motive power is that some manufacturers use differing methods for MU control. ie: electronic vs. pnuematic.

Actually it really is THAT SIMPLE. Once the standard 27 pin MU Jumper cable/MU receptacle plugs came into nation wide use after WWII, everything pretty much MUs with anything else.

 

Different wheel diameters don't mater at all.

I know that you and other experts explained this a year or two ago.  It just seems counter intuitive with locos having differen diameters, and gearing etc.  But I liken it to having several beefy Charley Atlases paired up with 97 pound weaklings on a tug of war team that altogether contribute to the overall tractive effort in a pulling contest.  :-)

The various throttle positions on a diesel are not SPEED settings, they are HORSEPOWER settings. When you have engines of different horsepower and gearing in a multiple unit consist, each one contributes its horsepower to the overall effort.

 

Many of you get hung up on the fact that the speeds won't match and one unit will pull or push the other. That happens somewhat with models because the various voltages (throttle positions) ARE speed settings in the model. In the real world, that little 1,000 HP unit MU'd with a 4,400 HP GE is going to add 1,000 HP to the overall effort and do its share of the pulling.

 

Where different gearing between units WILL come into play is the Minimum Continuous Speed (MCS). When a diesel locomotive is working wide open on a grade, as the speed comes down, traction motor current goes up. At some speed you reach a point where the traction motor blowers can just barely manage to carry away the heat being generated due to the increased current. That is the Minimum Continuous Speed for that locomotive. Locomotives with different gearing have different MCS. Switchers may have an MCS of 9 mph, while an E-Unit with 90 mph passenger gearing might be up around 18-20 mph. The highest MCS in the consist governs the overall MCS for that consist.

 

If the grade drags the speed slower than MCS, then you get into "Short Time Ratings." A diesel can be operated below its MCS for a short time, before the motors heat up too much. Then the train must stop to allow the motors to cool.

Thanks Rich, now "I get it" once its explained in this way.

 

Do diesel computers (if so equipped) monitor the air heat and currents such that the unit goes off line to save itself and rings the alarm bell?  Or is it up to the engineer to know whose the "weakest link" so as to not exceed its ratings?

 

Are DC motored engines more sensitive to this rating than AC motored engines (the AC inverters would of course generated heat as more current is needed but I assume the computers monitor these parameters and save themselves.)?  I read "somewhere" that you could stall AC locos on a grade without damage whereas DC get into trouble. 

Thanks for the great info.  I have a much better understanding of how it works and that is a good thing.  Always wonderful to learn something new.

I like to think of lashed power as a Father/young son working together to pull a log.

 

In trucking we have had to occasionally combine whatever horsepower there is against a large object and we simply communicate. Usually one will have a lower set and have to run faster to stay with while the other would simply pick a good gear and pull or push.

 

Whatever it takes to move something, the result is usually the same everything gets moving when sufficient horses/torque overcomes the inertia.

 

A long time ago I found myself with a 140,000 pound load versus 240 horses. It was not very fast but with adequate time and attention to the engine, we got it done.

I saw a consist with four different locomotives, looked very strange, but they didn't seem to be having any problem.  It was pulling around 100 cars, I got tired of counting after a spell.

Excellent explanations. My layout started just because I love running my model trains. I don't often get the chance to see the real thing but lately have been reading as much as I can. This forum is a huge help for me making the connections in my head between real trains and model trains. Thanks a ton for putting this info out there. 

AC motored diesels do not have short time ratings. They can literally be worked down to a stall and not damage the motors. In fact, they have an "electric brake" that uses current to lock the motors in position. The operation of a synchronous AC motor is entirely different than that of a DC motor.

 

The more recent computer-controlled DC locomotives will automatically de-rate themselves when operated at high throttle positions at low speeds. But the vast majority of diesels rely on the skilled hand of an engineer to know when to throttle back - or even stop - to cool the motors.

Actually it really is THAT SIMPLE. Once the standard 27 pin MU Jumper cable/MU receptacle plugs came into nation wide use after WWII, everything pretty much MUs with anything else.

Correct for Diesels from the 1960's to current. I do believe that BLW units could NOT MU with others due to their different MU configuration... using air to MU instead of current.

It never mattered to me how many flat or tapered dynamic brakes were in the consist. The bottom line was how much braking could be attained over all. Today they stress not using the independent brake before using the Dynamic brake. Back in our day it was the correct way to bunch the train with the independent brake first in order to prevent a severe run in if one or more of the dynamic brakes surged upon initial setup. This was caused mostly by not letting the amperage dissipate from motoring before setting up the dynamic brake but not always.
At low speeds there was potential for sliding the wheels when mismatched. The locked wheel indicator warned you of that.

quote:

It never mattered to me how many flat or tapered dynamic brakes were in the consist. The bottom line was how much braking could be attained over all.

Exactly. The seat of your pants will tell you what you need.

Not all air brake equipment was compatible. With a road like the C&NW, with units purchased by different roads, things could get interesting fast. 6BL, 6SL, 6BLC, 24RL, 26L one pipe, 26L 2 pipe or universal. 

Years ago I was watching a Pentrex video featuring at one point, one of Amtrak's western long-distance trains, pulled by two or three Genesis units. It was in the summer, and climbing a mountain grade, the units signaled that they were overheating and the train was brought to a halt for a few minutes to let the traction motors cool.

 

A friend of mine whose knowledge of prototype diesels leans heavily toward first (and maybe second)-generation equipment insisted that the older-generation equipment (less individual HP, but more units in-consist) would never have needed to stop like that. At the time I thought "well, with those units you just wouldn't get an indication they were cooking themselves", but I didn't have any hard knowledge to rebut his claim. So, were the older units less sensitive, or did they simply lack a sophisticated method of telling you they were being worked too hard?

 

---PCJ

The newest AC traction units, i.e. AC traction motors, no longer are electrical current limited/sensitive. The AC traction motors are three phase induction motors, thus "stalling" them does NOT cause them to "overheat" as was the case with the older DC series wound motors that where seriously affected by high current (think stall burns on the commutators, if no rotation was involved).

 

With the AC traction units of today, you can actually "hold" a 19,000 ton coal train on a grade, with 3 SD70MAC units, by simply placing the reverser in forward, and the throttle in #5 or #6. Then release your train brakes, while holding the independent brakes fully on. When the train is fully released, advance the throttle to #7 or #8, and slowly ease back on the independent, and the three units will SLOWLY begin moving the train forward. Stalling an AC traction motor does NOT cause any current heating problems, in fact the cables running from the inverter to the traction motors, are about 1/3 the size of the massive, high current, cables previously required for DC traction motors.

 

The key to motor speed control and torque is a function voltage AND frequency! NOT current.

 

Hope this helps a bit