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To: wrench
You are aware that trains are electric, right? They run diesel generators and use electric drive axles

I knew that, just never understood it, not being an electrician or an engineer or physicist. I always thought that when you convert energy from one form to another that there is a net loss and therefore it would make more sense to run them directly with diesel engines. Is it just not practical for some reason?

I'm dead serious and would appreciate any explanation offered.

21 posted on 11/27/2012 8:50:59 PM PST by Graybeard58 (What G.O.P.e. candidate is in store for us in 2016?)
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To: Graybeard58

I think there is some energy savings in not having all the drive shafts, etc that would be necessary to drive the multiple wheels on a train locomotive. Also high torque at low speed is helpful on a train. And I assume there is no need for a transmission. But, I would never characterize the engine as ‘electric’. Its power comes from a massive internal combustion engine.


27 posted on 11/27/2012 9:05:56 PM PST by lacrew (Mr. Soetoro, we regret to inform you that your race card is over the credit limit.)
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To: Graybeard58

electric motors produce a hell of a lot of torque. They make this torque at all speeds and do not need to shift.

Diesel trucks have 10-18 speeds for a reason. You need to keep the engine in the power band to keep things moving.


31 posted on 11/27/2012 9:31:05 PM PST by cableguymn (The founding fathers would be shooting by now..)
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To: Graybeard58
Simplistic explanation: You need a lot of power/torque to get something large and heavy like a train moving. Once it is moving, the amount of power needed to keep it moving is a lot less. A diesel engine uses little fuel at idle or low power, which is all that is needed to keep the electric generator going and the train moving at normal speeds. Dynamic braking is also advantage of the diesel electric. A straight diesel locomotive requires a large heavy complicated mechanical transmission which reduces the efficiency.

Read the wiki article if you want more in depth info.
http://en.wikipedia.org/wiki/Diesel_locomotive#Diesel-electric

32 posted on 11/27/2012 9:32:33 PM PST by smokingfrog ( sleep with one eye open (<o> ---)
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To: Graybeard58

Diesels, more than gas engines, have an optimum speed and >load< for best economy and perhaps a different speed for max power. But even gas engines have an innate speed at which they operate best. Also, using electric motors for train propulsion offers both incredible torque for starting, because they can handle brief overloads and also have the ability to use the electric motors as dynamic brakes...turning them into generators, the generators power giant resistors, generating heat, and serious braking power results. This means that the locos don’t have to have such giant brake shoes...useful to help stop the whole train. Plus the traction motors can be reversed pretty easily without having to have a geared transmission on the diesel which of course would be massive given the torque and power involved. Furthermore, it is relatively easy to synchronize multiple motors to rotate at the same speed allowing multiple engines to be used in tandem, which makes it easier to match the no of locos to the particular load and the type and amount of uphill pulling the loco must perform. And finally, the ability to control everything electrically meant that fewer people were required to run the train, esp with multiple engines. This was all developed during the early-through-middle part of the last century to a fairly fine art, and it was universally determined by all mfrs and freight/PAX lines that the diesel-electric combo was the best, cheapest, and most flexible. So while you are correct that changing one kind of energy to another involves some losses, these things have been studied and tested to exhaustion and the diesel-electric came to be the chosen mode. Motors, at this point (and even 50 years ago) got to be pretty darn efficient in converting electrical energy into axle torque. Of course, some electric trains have overhead or third rail electrical supply and those don’t need the diesel combustion engine.


35 posted on 11/27/2012 9:45:17 PM PST by Attention Surplus Disorder (This stuff we're going through now, this is nothing compared to the middle ages.)
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To: Graybeard58
I always thought that when you convert energy from one form to another that there is a net loss and therefore it would make more sense to run them directly with diesel engines. Is it just not practical for some reason?

The reason is that it is much more cost effective to generate electricity and use electric motors to drive the wheels than to build a transmission and clutch to handle the power.

Diesel locomotives are typically 2000 to 6000 horsepower, but have a limited RPM range. They would need a lot more gears than a car which has a transmission with 3 to six gear ratios. And, the gears would need to be much larger than automotive gears to handle the loads. Transmissions also have internal losses. These are typically low, but even 1% loss of a few thousand horsepower is a lot of heat, which is where the power loss shows up.

48 posted on 11/27/2012 11:02:01 PM PST by CurlyDave
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