Diesel Engines

[coaldust2026]

I'm sure this has been addressed  but would someone explain what is gained by having the diesel engines in a locomotive power electric generators rather than power the locomotive directly? Thanks.

[Train-O]

Less diesel fuel consumption.

Ralph

[MJChittick]

Simply put, the electric generator-motor set functions as the locomotive's transmission.  Although there have been numerous attempts, no effective mechanical transmission has been developed and proven effective for a locomotive application.

[cx500]

Some industrial-type diesels do have a mechanical transmission but they are typically very low horsepower, perhaps 250hp, and only two axles.  Trying to transmit 2,000 hp to a swiveling truck, with each axle individually sprung, by mechanical means would be a design and maintenance nightmare. I haven't even touched on the need for a gear box and clutch, and how you might change gears as speed increases.

Europe sometimes uses a hydraulic transmission instead of electric but when that option was tried in North American freight service it did not win any converts.  The handful of locomotives ran long enough that obviously it could work, but presumably there was no compelling reason found to encourage repeat orders.

John

[daveklepper]

The real reason is the high-horsepower mechanical transmissions have much, much higher maintencance costs.  Secondary reasons are less adaptability to dynamic braking and less efficiency because of a relatively small number of finite gear ratios.

[BaltACD]

Mechanical control of wheel slip situations would be a absolute nightmare. 

 It is sufficiently difficult with the electric traction motor where control only involves reducing or removing the level of electrical power that is being fed to the slipping wheel set.

[coaldust2026]

Thanks everyone for the replies.  I now begin to understand.

[baberuth73]

I'm sure glad this question was asked, but what I can't seem to grasp is this: back in the sixties and seventies when the "horsepower war" between the locomotive manufacturers was at its' height, it seemed that every year or so EMD would increase horsepower on a particular model and shortly thereafter Alco or GE would follow suit. Unless the generator was made larger or somehow better than what was previously available, what real benefit was realized by relatively small increases in prime mover  horsepower? Did it turn the generator at a higher speed and thereby increase the power sent to the traction motor or was it simply a sales gimic or am I completely clueless about the whole thing?

[AltonFan]

The use of electric traction motors is probably as close to a perfect locomotion system for railroad use as we will ever come without abandoning the flanged wheel on a steel rail.  The benefits include dynamic braking, the ability to operate multiple units with a single crew, elimination of dynamic augment, smaller, more agile, locomotives, and better weight distribution.  All this became apparent very early on.

But the problem with straight electric systems is that the power transmission system, either via overhead wires or outside third rail, is very expensive to build and maintain, and presents a number of other problems (e.g. safety).  The only places that electrical transmission are feasible are places where money/profitability is no object, where steam locomotive exhaust creates problems (e.g. long tunnels, subways, pollution), or  where traffic density makes the system economically feasible.

The diesel-electric locomotive engine provides the best compromise to allow the benefits of electric traction without the problem of stringing wires or building outside third rails with their associated infrastructure.

[AltonFan]

baberuth73

I'm sure glad this question was asked, but what I can't seem to grasp is this: back in the sixties and seventies when the "horsepower war" between the locomotive manufacturers was at its' height, it seemed that every year or so EMD would increase horsepower on a particular model and shortly thereafter Alco or GE would follow suit. Unless the generator was made larger or somehow better than what was previously available, what real benefit was realized by relatively small increases in prime mover  horsepower? Did it turn the generator at a higher speed and thereby increase the power sent to the traction motor or was it simply a sales gimic or am I completely clueless about the whole thing?

I'm not an expert, but IIRC, many of these horsepower boosts were made by increasing the efficiency of the diesel engines by turbocharging and tweaks to the prime mover's operation.  Sometimes they just built a bigger engine, and then had to work out the bugs.

Perhaps someone with a better grasp of the technology involved could go into greater depth.

[wjstix]

Back in 1908 (IIRC)  the new Minneapolis St.Paul Rochester and Dubuque Electric Traction Co. - better known as the "Dan Patch Electric Line" was looking to build a freight / passenger railroad using overhead wire. When looking at available boxcab electrics, it was noted that GE had a mobile oil-powered electric generator that would fit inside the engine, so that they could use the engines while building the line, and then convert them to straight electric. They ordered some engines like this, and ended up never putting up the overhead wire.

Although technically these pre-WW1 engines were "oil-electrics" and not diesel-electrics (since the generators didn't burn diesel fuel but oil), they're still often cited as the first step towards the eventual transition to diesel power.

 

http://www.mtmuseum.org/jsr/roster/dpl100.php

[CSSHEGEWISCH]

The distinction between "oil-electric" and "diesel-electric" was not based on the type of fuel used but rather on the type of injection system used.  Both types are compression-ignition without sparkplugs.

[train18393]

In glosing over these responsed I did not see the fact that the electric motor produces its most torque at stall speed, the least torque at top speed, or nearly so. The internal combustion engine is just the opposite. Now when you are trying to start a train you need maximum torque, which an internal combustion engin will not provide, but an electric motor will. The other things mentioned are also important. An increase in horsepower will increase the amount of torque that an electric motor will produce at stall speed. You must also take into consideration that an electric motor at stall or slow speed will heat up rapidly, so you need a large volume of air moving over the motors to keep them cool, which is why there electric are blower motors running fans to keep the traction motors cool. a big problem with early diesel-electric engines was keeping the traction motors cool. Overheated motors equals melted motors, and engines that don't work.

Paul

Dayton and Mad River (Model) Railroad

[wjstix]

The upside of a diesel-electric is you (the railroad) save a lot of up-front money not having to build overhead wire to power the trains. The bad part is that it means you're committed to burning petroleum products to run trains. In areas where overhead wires are already in place - like the extensive systems in Europe - any power source that can create electricity (coal, oil, water power, solar power, nuclear power) can power the trains thru the wires.

[selector]

Also not an expert, but if you want more hp to the traction system, you must generate it.  That takes more hp in the prime mover.  The prime mover, in turn, would be turning larger (more powerful) generator fields which would supply more electrical power (amperage) to the traction motors.  As well, traction mores and the trucks that house them also saw improvements which provided more direct traction control.

Crandell

[AltonFan]

wjstix

The bad part is that it means you're committed to burning petroleum products to run trains.

I've written elsewhere, that any change in locomotive fuel, whether a return to coal, or the adoption of hydrogen cells, those energy sources are going to be used to drive a generator or alternator to provide electric current for traction motors.

[Paul of Covington]

   There was a series of articles in LOCOMOTIVE & RAILWAY PRESERVATION magazine about 15 to 20 years ago with an extensive coverage of the development of the diesel locomotive.   I'm pretty sure I still have them, but I haven't been able to find them.    The responses to this question on this forum pretty well cover the subject, but one other reason I remember from the articles is that with mechanical drive, the rotational reaction to the torque from the engine would tend to tilt the locomotive, such as you see on a dragster when the driver hits the accelerator.