WHY ?

[Burlington Route]

Perhaps I missed it, but don't forget about the regenerative qualities that a diesel electic system has when going downhill under load. Those motor driven electric axels can slow the engine down and top off the batteries...dynamic brakes?
As has been said, a direct drive system would entail too many weak links for coupling, if one happens to burn out or wear out an electric motor/axle it can keep going with the rest of the driven axles to carry the load..a direct coupling system that lost it's primary drive coupling, say to a truck assy, or both, would be dead on the tracks. 

[utdave]

Wounded bear     i live close to ogden utah   they have a turbine in there museam .   i have walked through the inside of this before they locked her up.         and a great show on you tube on those turbines  i saw that several weeks ago  a must see    theres another one that was for trainning  for the UP.   it was also long  but a great one to watch also.
Dave

[Hamish K]

There are three types of diesel locomotives, diesel mechanical, diesel hydraulic and diesel electric. None are really direct drive. Diesel mechanical locomotives usually have a fluid coupling between the diesel engine and a mechanical gear box. The power of a locomotive is generally too much for a mechanical connection to work reliably, although a few were made. Diesel mechanicals are generally confined to small low speed locomotives, e.g. small shunters. Diesel hydraulic locomotives have torque converters (which also use fluid to transmit power) and have automatic gears. (I know this is a gross oversimplification). The advantage of a diesel hydraulic over a diesel electric is mainly lighter weight.  Thus they can have a higher power to weight ration which makes them useful on lightly laid track. Their disadvantages are that they are a little less efficient, and may require greater maintenance. Also they were confined to medium power, unless two engines and thus two transmissions were used. However a hydraulic drive capable for coping with 4000 hp has now been developed . Diesel hydraulics have been quite popular in some European and other countries, in particular for small to medium powered locomotives and railcars. (I live in Australia and we currently have several classes of diesel hydraulic railcar.)

Diesel electrics are probably better suited for american conditions. Track is usually relatively heavy (by world standards) in the USA  multiple locomotives are often used together, this is less common in Europe (length of yards etc).  Thus the higher power to weight ratio of hydraulics is not needed.  Maintenance issues were a problem with the Krauss Maffei locomotives tried in the USA, they were unfamiliar technology. Why use unfamiliar technology when its main advantage is not needed?

Hamish

[wb2002]

After reading the wealth of information in the responses to this question I asked on this topic, I believe the biggest factor in this popular method/configuration is the innovation and performance of the traction motor.  The torque-to-energy consumption ratio makes it very efficient in addition to all the other advantages that are available using this method. I, for one, am amazed at the power of a traction motor regardless of what other equipment that is needed to provide the electricity to make it work - be it alternator, overhead power lines, batteries, or whatever.

wb2002

[Desertdweller]

wb2002,

I think you are correct.  And yet, the traction motor itself is probably the one component that has changed the least over the years.

For all but the very beginning of Diesel production, there were only two makes of traction motor available in this country, EMD or GE.  And these were enough alike that they were interchangeable.

Modern traction motors are now produced in AC or DC variants.  But almost all the development has been in systems to feed and control the motors, rather than the motors themselves.  These motors are a good example of something that was gotten right the first time.

Improvements in traction motors has been limited to their ability to absorb and develop more power.  They are the limiting factor in locomotive horsepower.  It does no good to develop more horsepower than the traction motors can consume.  Consider early high-horsepower locomotives, like Baldwin Centipedes (which I think used Westinghouse motors), 4,500hp gas turbines, and GE U-50's.  All these used extra axles and motors. 500-600hp. per motor was pretty much the limit.

Les

[jward]

one factor not considered here is that the motors are probably capable of delivering more power than practical. the limiting factor has been adhesion rather than the power output of the motor itself. this power has to be controlled at the rail in order for the power to be useful. too much power and the locomotive will spin its wheels, possibly stalling the train. ths we have had ever more sophisticated forms of wheelslip control.

earlier i had alluded to experiencing the awesome power of the traction motors firsthand. i will relate one instance here:

years back i worked the shelocta coal trains in pennsylvania. one evening we were called to go to riker yard in punxsutawney to bring a single locomotive back to conway. an unusual run to say the least as we never ran shelocta trains to conway, they went a different direction. upon arrival at the yard, we found the locomotive, a conrail gp38, had the entire front step area and coupler bent upward a foot or so. it seems the crew before us was pulling the coal train up locust hill, about a 2% grade when the the locomotives on the head end lost their footing. the consist was a typical mixture of 5 axle diesels. some, like the gp50s and gp60s were high adhesion, with radar wheelslip controls which actually measured wheel speed in relation to ground spees. the older ones like the gp38 used a current based wheelslip control based on the theory that once the wheel slips, the amperage of the motor falls drastically. these locomotives instantly cut power until the wheel grips the rail again, but they don't respond as fast as the radar units do.

when the consist lost its grip on the rails, the high adhesion units in the consist recovered right away, and the resultant tug snapped the underframe of the gp38, which was against the train, just ahead of the cab. the whole front end bent upward under the strain until the coupler rode over the knuckle of the first car's coupler. usually, the knuckle or coupler shank would snap, but in this case it was the locomotive frame......

we had to take the locomotive under a 10mph speed restriction to conway. the story has a happy ending. conway shipped the locomotive to altoona where is was repaired, and evenually rebilt into a gp38-2 in the 5600s. not too long afterward, ns sent the sd80macs to us, where they performed very well. being ac drive, they could be loaded down much more than the dc locomotives, and we ended up with far fewer stalled trains and pulled drawbars with these matched consists.

[wb2002]

I can't imagine the magnetic field intensity generated by these traction motors. I am surprised they don't light up when building these gigantic magnetic forces that attract and repel one another. They have to be tremendously strong even if the power is directed through reduction gears before actually driving the wheels. I also imagine that by now, you can sense my fascination over this simple technology.

Other than the wealth of information provided with the original question, I have been blessed reading the story of a true life event - thanks for sharing. I am rather new to this website and forum and do not know if there is a location where stories can be written and shared. That would be a great idea - a location where someone that may have the talent, can entertain us with their true life train stories. I'd better stop before I get too far from the topic of this post  -  just a thought.

Thanks to all

wb2002