WHY ?

[wb2002]

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

[jward]

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?

[Doneldon]

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

[Woody Elmore]

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.

[bapguy]

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

[Desertdweller]

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

[jward]

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....

[Doneldon]

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

[utdave]

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

[wb2002]

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

[Balrog21]

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!   

[jward]

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.....

[Desertdweller]

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

[jward]

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.

[WoundedBear]

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

Sid