# Power in / power out for electric motor

frogman
electric motors a far more efficeint than internal combustion. My question, could I not use a smaller IC engine & generator to power an electric motor and make the same amount of power as a larger IC engine. but a little more efficeintly.

## Answers and Replies

Gold Member
No. If you think about every stage of the process, you'll find losses that defeat the purpose.

frogman
ok i'll buy that.

the reason for my question. They run electric motors on trains and lagre ships for propulsion. what is the advantage for them? they still produce the electricity by deisel or jet engine.

Gold Member
It's primarily for the consistency of the power delivery from electric propulsion. It would be very difficult to shift gears on a train or ship, and electrics don't need a transmission. I'm not actually familiar with electric ships that much, though, except for submarines. I thought that most still use constant-geared Diesels.

Science Advisor
Gold Member
Very large internal combustion engines are most efficient when run at or close to a constant speed. By using the engine as a generator, they can run it at constant sped and instead add/remove fuel as needed based on the load placed on it.

Science Advisor
Also, electric motors don't suffer nearly as much harm if the load suddenly veries. The only ships I know of that use motors powered by an ICE are ice-cutters, and that is exeactly the reason. If a big chunk o fice suddenly stops the props, the electric motor will not experience the vibrational forces of impact that a mechanically-linked ICE would.

Mentor
electric motors a far more efficeint than internal combustion. My question, could I not use a smaller IC engine & generator to power an electric motor and make the same amount of power as a larger IC engine. but a little more efficeintly.
If the efficiency is a fixed-fraction of the capacity, you always lose the same fraction to inefficiency.
Example:
Gas motor: 35% efficiency
Generator: 95% efficiency
Electric motor: 95% efficiency

Input: 1 kW
Output: 1*.35*.95*.95=316W

Input: .5 kW
Output: .5*.35*.95*.95=158W

They run electric motors on trains and lagre ships for propulsion. what is the advantage for them? they still produce the electricity by deisel or jet engine.
In addition to what has already been said, the torque curve of an electric motor is pretty flat, but an ICE needs a certain rpm to run at all. So for for a train, which can take minutes to get up to speed, trying to use a clutch and gears to get it moving can be a nightmare. I actually wonder why they don't use diesel-electric for trucks, which have something like 18 gears.

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Science Advisor
Gold Member
I actually wonder why they don't use diesel-electric for trucks, which have something like 18 gears.

You'll start to see diesel-electric drive for off-highway construction equipment (bulldozers) next year.