I realize this may sound totally ignorant or stupid but I am curious as to why modern diesels locomotives use a diesel to generate electricity to power DC/AC motors on trucks instead of using a diesel, transmission and coupling (differentials) to power trucks directly? I believe that the way it is now is perhaps more efficient but I was just wondering . . . . . . . .
Thanks
wb2002
hydraulic was experimentally tried here with limited success, back in the 1960s.
i thin the reason for the electric drive has to do with what was available when the road diesels were first developed. electric traction motors had been extensively used for probably 40-50 years, both in trolleys and in heavy electric locomotives as well. so rather than scompletely reinvent the wheel and adding exponentially to the development costs of a radical new design of locomotives nobody realized were to be the new standard, they took existing tevhnology for the drive, and used the diesel engine as a portable power plant. essentially that's what a diesel locomotive is: an electric which carries its own power plant.
the early designs, especially the emd ft, became successful beyons anybody's wildest dreams. and the builders saw no reason to mess with a successful design by deveolping an unproven technology. after all, by the end of ww2 they had more orders on the books than they could fill.
on a related note, did you know that the monster off road trucks used in the mining industry are also electric drive?
wb-
It isn't just "modern" diesels which generate eletricity to operate
electric motors on the axles; This is the way diesels (more correctly,
diesel electrics) have always worked.
-- D
The SP and Rio Grande experimented with direct drive Kraus-Maffei locomotives. They didn't work out for a number of reasons. The Southern also tried a small Roumanian made diesel locomotivthat was direct drive.
The answer could be torque. Gas and diesel engines have a torque curve. Some engines have low end torque, others mid range and others high end. Torque gets the wheels moving. Electric motors have instant torque and I believe little to no torque curve.If the locos had a drive line like cars used to have, you would need a transmission, drive line and differential for each axle, all adding to the weight of the loco and reducing fuel efficiency. Joe
You need the electric drive system to isolate the engine's rpm from the axle's. Horsepower is a function of torque and rpm. This explains the advantage of a Diesel-electric over a steam engine.
A Diesel-electric can generate maximum horsepower at 0 mph. A steam locomotive's horsepower increases with speed. It is at its weakest when starting. Therefore, it can run at speed with anything it can get moving, IF it can get it moving. A steam locomotive's speed is limited by balance issues and the ability of the boiler to generate steam.
This is why a steam locomotive's horsepower is defined by the speed it is achieved at. A big steam engine might generate 5,000 hp at 70 mph. But it may not be able to start at train that a 1,500 horsepower Diesel can get moving. Once it gets a train in motion, a Diesel is limited by its horsepower to get it up to speed. This is why it required multiples of Diesel locomotives to haul a train one big steam locomotive would have been able to pull, again if the steamer could get it moving in the first place.
The locomotives of the 1960s that were referred to in previous posts as "direct drive" really weren't. They were equipped with torque converters between the engines and the axles, and were "Diesel-hydraulics". This fluid drive allowed the engines to turn independently of the drive axles. True direct-drive Diesels (with a mechanical clutch) were only used on very small and crude locomotives, as no mechanical clutch could withstand the forces required to start a heavy train.
Les
one other problem could be the smoothness of the drive. remember, in railroading trains are either under tension or compression, with the weights involved, a sudden lurch like a car shifting gears can have disasterous results. i've seen them firsthand during my short railroad career.
in pittsburgh, we have a small fleet of electric hybrid busses, with a traction motor on the drive axle. there is absolutely no comparison between them and a standard bus in terms of ride quality. the old busses lurch from stop to stop, the hybrids glide....
wb-
The diesel/electric scheme also allows the diesel motor to operate at its most efficient
rpms for the load pulled, speed needed and grade. Also, maintenance on traction motors
is less than on transmissions, and replacements are much cheaper if needed.
-- D
and alot easier to sycronize locos togather . there is a loco that uses a big fly wheel and a drive whell rides 90 degrees from that which is transmissioned to the wheels. the plymouth switcher youtube it it was kind of neat dont remember exactly where its the same concept that ariens uses on there snowblowers
Dave
I appreciate all the responses and all are very interesting. I also know that large seagoing vessels, and monster trucks also use this technology (diesel-generator-motor). I would like to hear from an engineer (NOT train - Mechanical) regarding the efficiencies using this method. It is like using a source of heat to generate electricity and using that electricity to again generate heat. And, if this method is efficient and preferred, why not in automobiles and semi rigs? As someone also mentioned, many construction vehicles uses a diesel to drive a hydraulic pump and uses this fluid under pressure for locomotion and many other functions. I imagine there has to be some thought as the best way to transfer power from one source to another to perform the required task we desire.
This is very interesting . . . .
wb2002
I dont know if the cruise ships will ever adapt but all the navy is getting rid of the diesel engines and moving to gas turbines. I was on the DDG52 a few years back, we had one diesel engine on board and that was used to distilled the water, 4 main power plants, 3 generator sets, all gas turbine, the mains were GE DC10 engines basically, and the generator sets were allison k35's. Very impressive though - moored and then up and going at full speed within one minute. We also had reversable pitch propellars which meant we could be going full speed forward switch the props and then be going backwards...full stop from full foward 450 yards..and you wanted to be holding on to something and the wall of water that came over the back end was monstrous when first iniitated! But I know the railraod tried it and it didn't really work..probably wasn't cost effecient enough for them. At full speed - just 1 of the main engines used 1,000 gallons of gas per nautical mile, but hey, we held 450,000 gallons of full in the belly! ;D
in marine and power plant service you can make good use of a turbine because you havw a near constant load situation. turbines tend to run full speed with very little variation.
a locomotive is different. it operates under constantly changing load. there is a reason railroad engimeering departments have developed track charts, which show every change in grade and curvature on the line, most operating personnel i was acquainted with had these track charts in their posessionand had every feature of the lines they ran memoriozed. cut back throttle here, 5 pound brake reduction there, bail off the brakes at this point, etc......in railroad service, the turbines ran full throttle burning cheap fuel which was little better than sludge. it had to be pre heated to flow into the turbine, and they burned incredible amounts of it. in order to make good use of the turbine, you had to keep the train moving at a relatively high speed. eventually, the development of plastics created a market for the fuel the turbines used and drove up the costs. in the end, the turbine locomotives became too costly to operate.
one thing you will notice about gydraulic drives is that they are used in vehicles with relatively low hross weights. a fully loaded semi os heavy compared to your car, but light compared to a 200 ton mining truck or a 14000 ton train.
regarding the sp experoments with diesel hydraulics, the original krauss maffei units sufffered from a european design adapted to the totally different conditions on american railkroads, here our cars are bigger, the weights much greater, and trains much longer. we run our locomotives hundreds of miles, refuel them adn expect them to be ready for another run. american locomotives are designed with this in mind, unlike in europe where runs are short and maintainance standards high. the proof of the superior reliability comes from the uk, where since privatization emd locomotives have become the standard. nothing the british built could match their reliability.
to get back to the diesel hydraulics, after the initial testing, sp solicited bids from the american builders for hydraulics. emd and ge were not interested, having invested heavily in electric drives. alco, in the unfortunate position of having to buy all its electrical components from competitor ge, built three diesel hydraulic locomotives for sp. they became orphans a few years later when alco stopped building locomotives in 1969. reportedly, they performed well, certainly much better than the km;s did. but in the end, with their builder out of business, there was nobody left to build more.....
Good points, jward.
I've never run a turbine-powered train, but I can understand the challenge it would be to maintain a reasonably constant track speed with the engine running at a constant rpm.
In addition to the problem of the ALCO hydraulics being "orphan" locomotives, another problem would be lack of stores of parts for the hydraulic drives. Also, when the locomotives were used over a large territory, people trained in maintaining them would have to be stationed at several locations.
These last two problems would have had to been overcome on the Union Pacific for their gas turbine loco fleet. Apparently the company was large enough and rich enough to stock the parts and train the people. The locos were successful enough to justify the purchase of two generations of gas turbines, a 4,500hp. series, and later, an 8,500hp. series. The 8,500hp series was then upgraded to 10,000hp.
I think another reason for the demise of the turbines was the adaption of run-through power agreements between railroads. While modern Diesels tend to be pretty much the same no matter who owns them connecting railroads would have been reluctant to have these oddballs on their property (what would happen if they broke down on our railroad?). This factor helped kill off the giant Diesel designs used by the UP, also.
Les
while it is true that modern locomotives are standardized designs, there are enough differences to preclude runthroughs in certain sitations. a case in point: i live along the former pennsy mainline just east of pittsburgh. this is cab signal territory. there are and have been many railroads whose locomotives had cab signals, but most of the systems are incompatable. thus, while a union pacific sd70m may look like a ns one, it can't lead a train here because its cab signals won't work on the former pennsy. in fact, many ns locomotives can't lead here because they lack cab signals as well.
it is my understanding that the up ran their turbines in the same territory as the big boys, and that they kept thempretty much in that one area. railroads tend to do that with their oddballs for the reasons you described . it doesn't make sense to have a small fleet of locomotives significantly differ3ent from the others, roaming the system. concentrate them in a specific area where people will get to know their idiosynchrosies, and they can receive the proper attention.
Very interesting discussion, gents.
I stumbled across this on Youtube and thought it might be relevant to this thread. Grab a coffee....she's a long one, but informative.
http://www.youtube.com/watch?v=5uT_aYfTif4 (http://www.youtube.com/watch?v=5uT_aYfTif4)
Sid
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.
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
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
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
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
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.
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