Why are electric motors so much more powerful than entrnal combustion engines of the same horsepower rating?
The philosophers have only interpreted the world in various ways; the point, however, is to change it. (Karl Marx, 1845)
Are they? Kinda like asking why a ton of bricks weighs more than a ton of feathers, isn’t it?
You would think so, but seriously, this observation is based on practical experience. I grew up on a farm and we used a lot of quarter to half horse electric motors to power various things. If by some happenstance there was no electric power available and a gasoline engine was required, a engine of a couple of horsepower was required. This phenomenum was common knowledge among the farmers there, but no one could explain to this farm kid why it should be so.
Ursa is correct. Electric motors do have the neat feature of producing torque at zero RPM. They also are more efficient than IC engines.
Could it be that it is hard to find IC engines smaller than a couple of HP. I didn’t grow up on a farm but IIRC most samll engines started at 2.5 HP and went up from there. The smaller (< 2.5 HP) engines were high strung weed eater and chainsaw 2 strokes which produced little torque and required high RPM to extract any power.
Electric motors develop maximum power at zero. Internal combustion engines develope maximum power at cruising speed. Hence, if it’s power you need from a standing start, electic is the way go to. On Diesel locomotives, the maximum diesel power is developed at running RPM, converted to electricity and fed to an electric motor.
If by “at zero” you mean zero RPM, then this is not true. They develop no power at all at zero RPM, since power = torque * RPM * a constant. Power is zero at 0 RPM by definition.
The question asked in the OP might be wanting to ask about torque, rather than power. X HP is X HP, but there are several other issues:
(1) Gearing can change the available torque for a constant power. Just because an X HP electric motor can produce a particular amount of torque does not imply an X HP gas engine can as well - one needs to know the RPM involved in both cases and the way that torque changes as a function of engine RPM.
(2) The shape of the torque vs. rpm graph is quite different for gasoline and electric motors, and as mentioned by another poster above, electric motors are quite happy to produce large amounts of torque at 0 RPM, which is not possible for a gas engine.
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peas on earth
Some other considerations from an old farm boy:
Electric engines can run at rated HP all day long, gas engines are rated at Max HP, but you don’t want to run them that way all the time. Imagine how long your car engine would last if it was floored all day long. Your gas engines were probably running at the same HP as the electrics once the equipment was running.
If the gas engine isn’t running quite right you still want the equipment to work. Electric motors usually don’t have this problem.
As others have said, the torque difference can be critical, depending on the application.
Welcome to another farm boy.
frolix–not true.
Suppose you were using a generator to supply an electic pump. If the pump were pumping 100 gallons of water from 0 psi to 20 psi, lets suppose the load on the generator is X.
Now suppose somebody slighly closes a valve downstream of the pump, and now the discharge pressure of the pump is 40 psi. Assuming a reciprocating pump, you would definitately noticed an increased load on your generator, i.e., the electric motor would be outputing more HP.
One thing people haven’t mentioned is that internal combustion engines require a lot of moving parts. As the machinery gets older, the “gaps” between the piston and cylinder wall may increase due to wear and you would see a corresponding drop in available HP. I think most people are on the right track about being able to produce maximum torque at start up–but that is typically not a good idea for an electric motor. A lot of plants will “cold start” their pumps. In the above example, instead of the pump starting and immediately supplying 20 psi discharge water, a recycle valve will be open at start up. The water just circulates back to the inlet fo the pump, so instead of 20 psi, the pump is just moving water around and the discharge pressure is just the pressure drop the water experiencing circulating around back to the inlet. The valve closes, and the discharge pressure slowly rises to 1,5, 10 and finally 20 psi. This extends the life of motors.
*Note–they typically don’t do this for small electric pumps, but will for large reciprocating pipeline pumps and such which are high flow rate, high discharge pressure pumps which would cost a fortune to replace or fix if the motor “burns” out.
- Yes, overall efficiency is the key!
- Very technically speaking, Ursa Major’s answer is correct since horsepower is a measurement of power. Thus, hp is hp regardless of the source!
Steam engines also produce maximum torque at zero r.p.m. There are still some applications for which they’re the best, if raw fuel efficience and waste heat aren’t major concerns.
John W. Kennedy
“Compact is becoming contract; man only earns and pays.”
– Charles Williams
Electric motors are more efficient than internal combustion in terms of the space & weight they take up versus the power they produce. But since they use different fuels how can’t really compare them. And if they both have to carry their fuel around with them (a gas tank/batteries) the electric’s efficiency is severely reduced.
I for one welcome our new insect overlords… - K. Brockman
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- I once read an interview of a US car company CEO on his company’s future plans for electric vehicles, and one thing he said was “Electric cars are easy to build cheaply and their roadgoing-performance is fantastic. You just have to have long enough extension cords.” - MC
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