Are electric horses really bigger?

Hi Again,

I was gearing down to an output rpm of 800, not 1200. Not sure that makes much difference though as it would apply to both.

Thanks again

Dave

 

when I see the dyno sheet for an electric motor created to some standard then I might believe it all and do effective comparisons

 

Ok,

So I've found a torque graph for the M92B showing it's torque curve.

Not a great graph but it's all I can find.
https://www.frenchmarine.com/product/Perkins-M92B-Marine-diesel-engine-86-hp-1432-1

I also saw on the Curtis motor graph that it's peak torque drops off at 4900rpm not 5000. So updated the Curtis figures as well.

Recalculating:

Curtis Motor Output
HP 82.58
Torque - ft lb 88.35
Motor RPM 4900

Transmission Output
Output RPM 800
Output Torque 541
Less 10% 54.1
Final Torque 487 ft lb


Cummins M92B @ 2400rpm
260Nm@2400rpm
Torque - ft lb 191.7
Engine RPM 2400

Transmission Output
Output RPM 800
Output Torque 575
Less 10% 57.5
Final Torque 517.59 ft lb @2400rpm (Engine RPM)

So it appears there is little in it.

 

Sorry mate. The above premise is completely wrong, and hence makes pointless any discussion based on it.

 

no horsepower, no rpm, no movement

 

"no horsepower, no rpm, no movement"

Yet steam has the highest torque at 0 RPM, why it was used for a century by railroads before the Air Police.

 

only after the piston moves does it do any work

 

There seems to be some miss understandings in this thread.

Electric motor rating plates show two things, the HP rating, this is shaft output, then an electrical rating to produce that output, this will be higher, indicating the efficiency of a motor.

For example a bog standard single phase 1HP motor is rated 120V @ 11.4A (no power factor was indicated...) that gives you one shaft HP, or 745W. Barely 50% efficient. Shitty little induction motor.

Then as other people have rightly stated, power is power, J/s, doesn't matter where it comes from.

But there is a big difference in how the power is delivered.

Electric has been compared to steam, and its actually a bang on analogy.

Put simply, torque is generated by current, winding's can only take so much current before they melt. This is the only thing that limits motor torque production. This is exactly analogous to steam head in steam engines. Fixed steam pressure is a fixed force on the piston area, there is no more available above this. This is translated to a continuous static torque at the shaft (via the crank lever arm) and until it moves does not deliver power. With a motor drive (inverters etc) you control the winding current, which gives you static or "locked rotor" torque. Any one who is a car guy should be salivating at the possibilities electric cars provide.

And just like steam if you can maintain that current flow with enough steam volume (voltage) then an electric motor will make constant torque with increasing rpm until it flies apart mechanically. The problem of course voltage is always the limit. So as motor back emf or voltage rises, because a spinning motor is a generator at the same time, you have to put a higher voltage on its terminals than its generating if you want it to make torque (still limited by winding current). The same principle applies AC or DC, just a little more math with AC.

Now on a boat, the prop rpm vs torque required is an exponential relationship. Electric motors with an inverter drive makes a flat torque up to the voltage knee speed. So you have an exponent load and a flat line that intersects at the full load point. Ie piles of excess torque available anywhere below that full load rpm. An electric motor has zero trouble changing prop speed to any rpm below maximum regardless of boat motion. Since all internal combustion engines at some point in their rpm quickly reduce their torque output and then produce none, that means the propeller load has to be more carefully matched to the engine. And engine rpm will change as the boat moves into waves changing its torque output and therefore ability to produce thrust, ie bogging down.

Due to the reasonably flat efficiency curve of an electric motor, means you loose nothing really from an economy stand point. Off course the issue of where the electricity comes from is a whole other ball game (I vote nuclear, modern nuclear not 1970`s Chernobyl tech lol).

From an economy standpoint diesel electric is a very interesting possibility. Keep the diesel at its optimal efficiency rpm (L/hp/rpm), generate power, use that power at what ever speed you want. A good motor+drive can hit 90% efficiency, and keep the diesel at its optimal point should give best fuel economy est for boats that change speed a lot.

Sorry for the novel lol.

 

still waiting for an electric motor to go on a dyno so we can compare to a SI/CI engine

 

Even then there were known technical tricks frequently NOT used, especially in America, that would have resulted in improved economy and reduced pollution.

The real cause for moving to diesel-electric was the ability to gang up locomotives and increase the size of the train.

 

Google speed torque curve. Since we are in the 21st century its not like no one has ever measured the performance of an electric machine on a dyno.

 

Why cant steam engines be ganged up?

 

sure but show me a electric motor being sold with the dyno sheet that we can compare to a typical boat engine?

 

Can you provide an example power or torque vs engine speed (rpm) of an IC engine with the torque output quickly reducing and going to zero. I've seen one do so other than at a fraction of full throttle. All the IC engine curves I've seen have the power and torque is above zero for the entire operating range if the throttle is sufficiently open.

 

When it stops turning. Electric motors and steam engines don`t do that, well power goes to zero but torque doesn't.