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Diesel-electric versus diesel

  1. Oct 20, 2012 #1
    Hi all. I was wondering what benefits one gains by using a diesel-electric transmission over a simple diesel transmission (such as in locomotives, which now often use diesel-electric transmission). Is there not a loss in energy as one convert mechanicals power to electric power? Why not use the mechanical power from the diesel engine to directly drive the traction motor?
  2. jcsd
  3. Oct 20, 2012 #2


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    Your post is confusing to say the least. What is a 'simple diesel transmission'? How would a diesel engine directly drive a traction motor? Is english your first language?
  4. Oct 20, 2012 #3
    An engineer doing a new clean sheet design of a diesel locomotive has to design some sort of power transmission system to transmit the power to the drive wheels at the appropriate speed. He will make many trade off’s and compromises to find the best overall system for meeting his design objectives.

    He could use a mechanical or hydrostatic transmission, or he could use an electric one.

    The first thing he runs into is that for the best power to weight ratio of the engine, he needs one that has a rather fast shaft speed. Now he has to figure out how to reduce that speed to the wheels.

    Then he notices that the diesel has very undesirable torque and power curves. So if he uses a mechanical transmission, he will need a transmission with many gear ratios for different drive speeds and loads so that he can operate the engine near the optimum part of his torque and power curves. He has the same trouble with his specific fuel consumption curve. That sweet spot will approximately match those of the torque and power curves.

    Then he notices that he needs basically the same performance in both directions of travel.

    So he considers a conventional transmission like in an over the road truck. Making that work on the train will result in a very huge, heavy, and expensive package. It will also be relatively inefficient.

    So he looks at the hydrostatic drive, and he thinks that would be just perfect, but with the current state of the art it also is very expensive. Since components large enough for this project are relatively new, so he is concerned that he really does not have a handle on long term maintenance costs. He hopes that perhaps by the next time he has to do a similar design the prices come down substantially, and someone publishes better maintenance and reliability data. But he is not hopeful.

    Now he looks at the DC motor drive and its flat torque curve and same performance in both directions. No multiple gears, and with a flat curve he can do wonderful things to optimize the operation of the diesel for maximum fuel efficiency and minimum exhaust emissions. Not only that, the system is small enough to actually fit in the available space and has a reasonable cost. The efficiencies of motors and generators are actually quite good, often better than most other options.

    So he designs yet one more diesel electric system.
  5. Oct 20, 2012 #4
    Great answer pkruse!
  6. Oct 20, 2012 #5
    Not only that, can you imagine a discreet ratio box on a train.
    There would be a tidal wave of tepid coffee at every gear change.
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