Straight electrics were first, such as trolleys and some a bit bigger. The thought was to add a power plant to an electric. It worked, quite well.
The diesel-hydraulics take a lot more maintenance to keep going, The wheels have to be real close to the same size, as they are geared together. The transmissions themselves also required a lot of maintenance.
A diesel-electric with individual traction motors doesn't really have to deal with this. If the wheels are different sizes, it doesn't matter as much, as each wheel and motor will go the speed needed.
Mike WSOR engineer | HO scale since 1988 | Visit our club www.WCGandyDancers.com
The RTL Turboliners, as well as the French RTG's. had gas turbine hydraulic drive. The Voith 411 bru Transmission was designed specically to make optimum use of the gas turbine's characteristics. In addition to being very reliable, they had the added advantages of simplicity and immunity from the problems of flying snow. The RTL's provided 25 years of good service and were very popular with the riding public and the staff at Rennselaer if not with diesel electric tribe at headquarters.
It could be a "QWERTY" effect (after the first letters on the top row of your keyboard) where the first typewriters had that strange arrangement of keys, supposedly to slow typists down not to jam the typing keys, and people stuck with it since.
For example, hydraulic drive was more popular in Germany, and Rio Grande and Southern Pacific tried out those imported Diesel hydraulic locomotives, and they may have been unsuccessful because they were subject to lugging heavy freights in the US rather than the lighter trains on flatter lines in Germany. I heard there were problems with them that may not have been with the hydraulic drive. Maybe the German maker didn't do their "homework" -- think of the troubled history of GE Diesel electrics in the U.S. and how GE and their railroad customers stuck with the GE locomotive design until after many years they got it right.
For a scientific answer, any transmission that trades off torque for speed as do both the hydraulic and electric drives, that transmission has losses and hence must dissipate heat. For the lugging that U.S. locomotives are called upon to do, the electric drive with its traction motor and generator blowers may dissipate heat more effectively than a hydraulic transmission with its oil cooler.
The other thing is that an electric drive, in its fundamental form, is a piece of wire through which current flows, which in turn develops the tractive force. Electric drives are more forgiving of mechanical wear and precision of alignment. Hydraulic drives depend on pumping hydraulic fluid, which depends on close mechanical tolerances for efficiency, tolerances that are subject to wear if there is the least bit of contamination of the hydraulic fluid. A piece of wire can't suffer from the "wrong kind of electrons" getting mixed into the current.
For example, aircraft have long been an application of hydraulic actuators of varying kinds, to lower and raise the landing wheels, to move control surfaces. Hydraulic systems allow high levels of force and power in compact sizes with light weight. When you look out the window during the approach to landing and see this motor turning a lead screw to crank down the flaps, I am told that this is a hydraulic and not an electric motor.
There is a movement in the aviation industry to replace as much of the hydraulics as they can with electric motors, newer more lightweight and powerful kinds of electric motors. Again, hydraulic systems require seals that can leak, filters that can clog -- they just seem to require more expensive maintenance than electric systems. Even on an automobile, they are replacing hydraulic power steering with electric power steering.
This is not to say that someone could not build a successful hydraulic locomotive for US conditions. It is just as with everything else, there are various advantages and disadvantages, various engineering trades. You would also be competing with the electric drives, where they now have really rugged AC systems.
If GM "killed the electric car", what am I doing standing next to an EV-1, a half a block from the WSOR tracks?
Why was diesel electric transmission chosen over Diesel mechanical where the diesel engine power the traction motors directly or diesel hydraulic where Diesel engine's power is turned in turned into hydraulic motion?
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