My question is about why locomotives first generate electrical current in the diesel motor and than use the current in trucks to move the train. It would seem to make sense to use that power directly through a tranmission or something to power the wheels directly rather than this two-step process which causes a lot of energy loss.
Mitchell,
I found this link on another board and it has an excellent explanation of the answer to your question…
http://www.howstuffworks.com/diesel-locomotive.htm
Short answer, the reason is because this arrangement eliminates a mechanical transmission that would be prohibitively huge and complicated. Hope this helps,
Doug A.
The latest in automobile technology, the “hybrid” cars of Honda and Toyota, use electric motors. Actually, there is far less power loss than there would be through a hydraulic transmission, allowing much greater mileage. Several attempts to harness diesel engines directly to locomotive wheels have been attempted, the latest in the 1960’s, but the results have never been as efficient as the Diesel-electric, especially as far as maintenance was concerned.
Thanks!
What a cool web site.
Thanks!
The diesel engine and generator share the same shaft so the power transfer is quite efficient. Electricity is then sent to motors which are mounted over the locomotive wheels and connected with gears. That too is very efficient. The efficiency is the reason for diesel electric locomotives.
The real principal reasons for diesel-electric success don’t really involve the transmission being ‘most efficient’; as you note, the cumulative efficiency loss is indeed there. In the bad old days of DC machinery the system loss was on the order of about 15% (don’t make me go after cites!!); modern AC drive is supposed to be about 94% efficient.
Initial problem with direct drive is that the diesel engine turns at a minimum speed, or the high required compression will stall it; taking starting torque directly off the shaft damps the periodic excursions of torque in the crankshaft at low rpm (whereas a generator accomodates them adaptively) and you can imagine the shock to the driveline (or the nasty white smoke) if you try slipping a giant clutch into a typical railroad starting load. But far worse is the shock transmitted back up the driveline when the throttle is abruptly closed and the train slack runs in against the drawbar. If you look up the mechanics of the McKeen car, which used mechanical drive from a transverse engine mounted directly on the truck frame, you can read all about problems with vibration and shock…
The Germans made a great push in developing first 'fluid flywheel and then torque-converter drive for locomotives. This started with the desire to have a light engine with high horsepower, and if you look on the Net you will find German and British examples of diesel-hydraulics that use the principle. In the Fifties, Baldwin used a Mekydro drive and Maybach engine for its fast, light train X engine. Then in the early Sixties, as desire for single-unit high horsepower began to stir on American railroads, the Krauss-Maffei company attempted to market locomotives with small, light high-speed diesel motors and hydraulic drive (these were extensively covered in Trains Magazine in the early 1970s and you can find the article when, hint, hint, you have bought your DVD…) If you want good detail pictures of one of these, th
FYI- The reason the original poster is shown as being anonymous, is because that person no longer belongs to the forum. Consequently, you are giving answers to someone who will never read them. The reason, probably has something to do with the fact that the post was started close to 12 years ago.
I enjoy reading the replies
Looks like John was digging for ancient artifacts.
I once read a big dissertation on locomotive development. It said that if, on a locomotive, you had a clutch and big transmission like a truck; when you let the clutch out, it would rotate the locomotive from the counterforce.
This was said to be overcome by driving the axles with motors having their rotors in line with the axles.
That rotating locomotive is something to think about.
I’m no expert on this, but my simplistic understanding is that the flexibility of easily switching the generator and motor windings (i.e., series, parallel, shunt, etc.) to use various combinations of current (amps) and voltage to match the varying torque, RPM, and hence horsepower output of the diesel engine prime mover - and to easily and properly control all that, namely keeping the generator closely in step with the diesel - is what made the electric transmission so well suited for this purpose. There was an excellent series of articles in Trains in the 1960’s and 1970’s by David H. Hamley (I believe) about this, General Electric, Herman Lemp, etc. For example, see the Nov. 1973, Dec. 1973, and Jan. 1974 issues:
“How to control an engine of limited power - How it all began”
by Corley, Raymond F., and Hamley, David H.
from Trains, January 1974 p. 26
- Paul North.
Such movement could be used to dump coal, ore, and other cars carrying dumpable material?[:)]
Try 1961… [:D]
The K-M’s had their torque converters mounted in the carbody and used driveshafts to transfer power from the torque converter box to the trucks. Since the axles of one truck were geared together, the wheels had to be kept to the same diameter (IIRC, tolerance was 1/8"). The torque converters and driveshafts at optimum speeds just matched the efficiency of a DC electric transmission.
- Erik