DIY Electric Car Forums banner
41 - 44 of 44 Posts

·
Administrator
Joined
·
1,584 Posts
Right, but they are still not directly comparable.
Yes, they are. They are as comparable as a foot is to a meter. They differ only in absolutely fixed scale, and nothing else.

A horse lifting a weight is not the same measurement of an engine's horsepower.
A horse lifting a weight has very little to do with an actual "Horsepower". Did you think that's what people were talking about? Literal, actual horses?

An actual horse has about as much in common with horsepower as a person's foot does to the measurement of "a foot". What about ponies? What about Clydesdales? What about midgets and basketball players?

Probably less, as James Watt (where the term "watt" comes from), the person who also invented the measurement of "horsepower" deliberately lied about it to mislead people. It includes things like "What if the horse is sleeping? A steam engine can run all night long" into the conversion.

An actual horse can put out 16hp.

Let's cease with this senseless semantics that change based on convenience to one's argument.

... A horsepower is 746 watts. They are directly comparable. You can measure an EV in horsepower, you can measure an ICE in watts (in Europe, they have for decades). It doesn't change depending on what you're measuring.

If you want to talk generalities, about torque and power bands on engines vs. electric motors, there are some generalities that can be cast of typical machines, sure. Loosely, but, sure, some valid. Especially if you're talking about "feel" vs. measurements. Mostly to do with the first few seconds of acceleration that it would take an engine to spin up to a higher speed. Yes, sure, perhaps most electric vehicles feel like they accelerate roughly twice as fast as a comparable horsepower number in an ICE vehicle for the first few seconds. It's typically overblown and overcompared. This has absolutely nothing to do with unit conversions and it's straight up factually false to say otherwise.
 

·
Registered
Joined
·
1,685 Posts
Right, but they are still not directly comparable.

A horse lifting a weight is not the same measurement of an engine's horsepower. An electric Watt is not comparable to an engine spinning up on the dyno.

For instance let's say an electric motor can pull 100KW at 1 RPM from the battery. That is 100 KW but in order for that same motor to make the equivalent 134HP at 1 RPM, it would have to produce over 703,000 ft-lbs. Clearly not possible.
Your example motor would be a massive machine, larger than a locomotive traction motor or an oilwell drill motor, and the dynamometer needed to provide the test load for such a motor would also be massive.

Using a 50V battery pack this motor would draw 2000 Amps in your instance. The motor windings would be designed with a torque constant of about 360 ft-lbs/Amp and a back-emf constant of about 488 V-sec to accomplish this task. A motor designer could take these motor constants to design the big-ass motor, it would likely have windings and rotor on the order of size used in a hydro-electric dam generator.

So your example motor was kinda far fetched, and certainly not practical for an EV or many applications, but why do you think not possible?

The units of power in Watts or HP are basic fundamental physics, long established, accepted and directly converted and compared,

i'm trying to understand why you take issue with the units of power and the conversion and equivalence of units? Is it the concept or the math that you don't feel comfortable?
 

·
Registered
1996 Toyota Land Cruiser
Joined
·
552 Posts
@remy_martian @MattsAwesomeStuff @kennybobby Obviously causing a stir :LOL: Remy absolutely right of course. I guess my point is that you can't compare peak power directly. And you all 3 confirmed it. Peak power by itself is useless, you need to know other things like the RPM range etc to make an informed opinion.

@kennybobby it's not at all a far fetched example, at low RPMs the current pulled from the battery can spike disproportional to the torque output. Using 50v is outdated. That power is only 250amps from a typical 400v EV battery. It's all beside the point anyway.

Using peak power by itself is the only thing I take issue with. A 100hp LEAF motor has about twice the "area under the curve" as a 100hp 4-cylinder engine. But, both are rated at 100hp peak. Can't compare directly without more information.
 

·
Administrator
Joined
·
1,584 Posts
I guess my point is that you can't compare peak power directly.
If you point is, "You can't directly compare a rated max HP of a gasoline engine to a rated max HP of an electric motor", then yes, that makes sense. There's a lot of nuance in there to account for.

Also, just to get right down to it, the "rated" power of an engine or motor comes with a whole pile of nuance in itself. Take a racecar engine and put it into a truck that had an engine of the same HP and even RPM and such. What happens? Racecar engine blows up. The context of how the motor is rated for it's "max" HP matters. A truck needs to be able to pull a trailer uphill all day. A racecar needs to survive a race.

It's hard to even compare what "max" means. Forklift motors even some with S1 S2 and S3 power ratings, which are the various standard duty cycles that motor is rated to be able survive on that level of power. And then that depends on temperature, and airflow, and safety margin, etc etc etc.

...

That's a completely separate discussion from whether HP and Watts can be converted. They can, exactly the same, every time, by definition.
 
41 - 44 of 44 Posts
Top