Multiple locomotive operation

[Murphy Siding]

     From the operations point of view,  is there any difference in moving a train with 4 locomotives fo 3,000 hp each, verses 3 locomotives of 4,000 hp each?

[trainfan1221]

I would guess more traction, but I guess someone here who operates them has some good information. It will be interesting to see what they say.

[wabash1]

Murphy Siding

     From the operations point of view,  is there any difference in moving a train with 4 locomotives fo 3,000 hp each, verses 3 locomotives of 4,000 hp each?

Well yes and no, the 4 units at 3000hp will pull to speed faster than the 3 4k units as far as pulling a hill no differance , getting across the road wont be differance it is mostly throttle response. ( at least its what ive notice) and with the response being quicker slightly differant train handling is all.

[cx500]

Murphy Siding

     From the operations point of view,  is there any difference in moving a train with 4 locomotives of 3,000 hp each, versus 3 locomotives of 4,000 hp each?

 

It may do, depending on what type of operation is happening.  The horsepower is the same, so on a waterlevel route there will probably be little difference.  But when struggling up a grade at slow speed, generally more axles (and weight) to harness that horsepower will be better. 

That's why some railroads use yard and/or road slugs to gain extra tractive effort at slow speeds.  They gain the lugging power of two locomotives without the cost of a second diesel motor.  On the other hand, top speed will be limited to that which the single diesel locomotive would attain.

Your 3x4000hp locomotives will have 18 axles.  The 4x3000hp may have something between 16 and 24 axles depending on the models, so that is also something to consider. 

John

[Tugboat Tony]

Braking would be improved with the 4 units vs the 3 units, but you would burn a lot more fuel in that time.

[Paul_D_North_Jr]

As cx500 indicated, the tractive effort = pulling force may be different, depending on the weight of the 3,000 HP units. 

If they are fairly light such as GP-40's, they might weigh only around 280,000 lbs. each.  So, even 4 of them would be only around 1,120,000 lbs., whereas 3 heavily ballasted 4,000 HP 6-axle units at 420,000 lbs. each would be 1,260,000 lbs., a difference of 140,000 lbs.  At the pulling coupler, with a 30 % coefficient of friction between the driving wheels and the rail head, that would amount to a 42,000 lbs. difference in tractive effort.  However, depending on coupler strength and unit configuration, you might start breaking coupler knuckles with either configuration by the time you get to the point where that additional tractive effort is needed enough to matter - at the same 30 %, the 4 GP40's of 1,120,000 lbs. would be pulling at 336,000 lbs., whereas the 3 - 6 axles would be pulling with 378,000 lbs. - unless they are in DPU mode or something else is done to avoid that problem. 

However, if the 4 - 3,000 HP units are like 6-axle SD40's at the same weight of 420,000 lbs., then 4 of them total 1,680,000 lbs., or 420,000 lbs. more than the 3 - 4,000 HP units, of course.  Again, the additional weight of that 4th unit has the potential to produce significantly more total tractive effort - 504,000 lbs. vs. 378,000 lbs. - but now if that additional tractive effort really matters, you're going to be breaking couplers for sure, unless you're in a DPU configuration.

See also Al Krug's discussion of this in the first couple of sections of his excellent "Tractive Effort vs Horsepower" essay in his "Railroad Facts and Figures" website at - http://www.alkrug.vcn.com/rrfacts/hp_te.htm 

- Paul North.

[Kiwigerd]

Gentlemen, could anyone of you who have so much knowledge please answer one question that intrigues me for a long time:

Looking at multiple unit trains in the US it appears to me that the various railroads mix the locos wildly together, say they run 3 SD40/45 as head end power and perhaps even a pair of SD60 or 70 as DP somewhere in the middle of a train. The super trains having sometimes consists of up to 7 or more locos, with AC4400, SD70MAC, and GP60 in the same train.

Now, how on earth does this work out, when the controlling engineer in the head loco just controls the notch position of the power lever. A GP60 in run 8 would deliver different power than a SD40 or any AC loco. Why does it work together nevertheless ? I mean, one loco produces 3000 hp and the other 4300 hp and different tractive efforts as well. The common task they have to do is pulling a train with say 50 mph over the distance or going up 15 mph a steep grade. One should think that this can't work, but obviously it does. Could you please lift the mystery for me ?  Thank you all in advance.   

[BaltACD]

Kiwigerd

Now, how on earth does this work out, when the controlling engineer in the head loco just controls the notch position of the power lever. A GP60 in run 8 would deliver different power than a SD40 or any AC loco. Why does it work together nevertheless ? I mean, one loco produces 3000 hp and the other 4300 hp and different tractive efforts as well. The common task they have to do is pulling a train with say 50 mph over the distance or going up 15 mph a steep grade. One should think that this can't work, but obviously it does. Could you please lift the mystery for me ?  Thank you all in advance.   

In the Diesel-Electric locomotive the diesel engine supplies it's horsepower to the Main Generator (alternator).  The wheels that actually move the locomotive are powered by electric motors that use the generator as their source of electrical power.  When in Run 8 each Prime mover is feeding maximum horsepower to it's generator....be that 2000HP for a GP38, 3000HP for a SD40, 4400HP for a CW44 or 6000HP for a CW60.  The traction motors that move the wheels only draw power from the the generator of the locomotive they are mounted on and are electrically sized to match the generators output ability.  Each individual locomotive has it's own wheel slip management system to keep the power the traction motors driving the wheels from turning the wheels faster that the wheels can move on the rail without slipping or spinning. 

A characteristic of electric motors is that they develop maximum power & torque when the output shaft of the motor is stopped by the maximum load, the faster the output shaft moves the less power and torque the motor puts out.  Needless to say, at maximum power & torque, traction motors develop heat...heat that has the ability to destroy the traction motor, as a consequence traction motors have to be controlled in a manner that won't overheat them.  In normal operation each type of DC locomotive has a manufacturer listed a 'Short Time Rating' that designates how long the traction motors can operate at higher than normal amperage's.  At track speeds from approximately 9 MPH to 12 MPH (dependent up the specific locomotive type), the traction motor can handle the maximum electrical current without any potential damage to the traction motors....this speed is known as the 'Minimum Continuous Speed'.  The traction motors of each locomotive are controlled by the electrical control apparatus for that individual locomotive only,  the prime movers and the 'gross electrical control' (Forward-Reverse - Increase or decrease electrical current) are train lined between the locomotives and each individual locomotive responds to it's 'instructions' to the best of it's ability.

These elements of how multiple engines are controlled and work in concert with each other is a technical discussion that can go on for pages and pages. 

[oltmannd]

Provided they could both get the given train over the ruling grade on the route, they would both make the same running time.

[Kiwigerd]

Thank you very much for this little excurse. I think that I understand the principles, but I'm still amazed that it works so nicely in practice.

But that leaves open the next question to me: why does the same NOT WORK on my model railroad? I have engines that I can pair easily but a good many that can't be made together (I'm operating in analog modus, not DCC). Even similar models of the same manufacturer sometimes won't work together usually leading to derailments or unwanted coupler openings. This happens everytime when the puller engine on the head is slower moving than the helper at rear, whereby a minor difference might be already sufficient for that. So it seems that in reality the anti skid electronics do the work, something models don't have of course.

Once again, thank you.

[WSOR 3801]

Kiwigerd

But that leaves open the next question to me: why does the same NOT WORK on my model railroad?

 

Full size engines freewheel more than the model counterparts.  In 1:1 scale, differing engines with different gear ratios and takeoff speeds (loading) work together better, as each axle can turn independent without binding. Similar to a man and his young son working together to move a cart.  Each has differing speeds and effort they can exert, but they work together to move the load.  Coasting also happens, and brakes are required to stop. 

The models for the most part have on motor in the middle, then a gear train to turn the wheels all together.  Stopping the motor stops everything, including the train behind.