Electric Car Debate

[dereckbc]

And Dereck thought we could keep religion and politics out of this...

Well I tried, but I knew pollitics would enter, I even have a hard time because Politics plays a role. However the real driver is pure econimics

 

[LarryFine]

Without HP or rpm, you do not have torque period.

Are you sure torque can't exist at zero RPM?

If you said you can't have HP without torque, I'd agree.

 

[dereckbc]

Are you sure torque can't exist at zero RPM?

If you said you can't have HP without torque, I'd agree.

Plug zero in the equation of:
T=(5252 x HP)/rpm
and see what you come up with.

 

[tallgirl]

Plug zero in the equation of:
T=(5252 x HP)/rpm
and see what you come up with.

And what about an electric motor? Can you have torque in an electric motor at 0 RPM?

You have your equation backwards -- HP is derived from torque. In an electric motor torque is the total of the magnetic attraction (or repulsion) between the parts of the motor. Since that attraction (or repulsion) can exist in an electric motor at 0 RPM, it's possible to have torque in an electric motor at 0 RPM.

In an internal combusion engine, the instanteous torque is the combustion chamber pressure times the lever arm, and the average torque (what gets printed on dyno slips) is the average of all those instanteous torques over 360 or 720 degrees of revolution (depending on if it's a two or four stroke motor). Because it's not possible to have combustion at 0 RPM, it's impossible to have torque or horsepower at 0 RPM.

 

[winnie]

I'd suggest that the discussion of HP is an interesting side topic that anyone discussing electric cars should understand.

1 Horsepower is a unit of power, equal to 550 foot pounds per second, or if you go through the conversion steps, slightly less than 746 watts. (The electric motor industry redefines the horsepower to be 746 watts exactly.)

The number of watts, and therefore the number of horsepower, delivered to the wheels determines how quickly the car will accelerate.

As a rather different use of 'horsepower', electric motors and internal combustion engines are 'rated' in horsepower. This is a single number used to describe the engine. When you use a single number to describe something as complex as the performance characteristic of an engine, you are bound to throw away lots of interesting information.

The fact of the matter is that an internal combustion engine for automotive use with a 200 horsepower rating by automotive standards has vastly different capabilities than a blog standard electric motor with a 200 horsepower rating by electric motor industry standards. By almost all measures of automotive performance, the 200 horsepower electric motor (if the power supply was available) would vastly outperform the 200 horsepower internal combustion engine. The differences in the rating scheme are so vast, that to a rough approximation an electric motor rated at between 25 and 50 horsepower by electric motor standards will be able to replace a 200 horsepower ICE for most driving.

This in itself does not say that electric motors are better or worse than ICE powerplants. It simply means that the rating scheme is totally different.

In general: Electric motors produce constant torque over a very wide speed range, right down to _negative_ RPM. ICE engines would stall. Electric motors operate much more reliably in 'overload'. For short duration applications you can safely operate an electric motor at current levels that would _melt_ it on a continuous basis. This can be done repeatedly as long as sufficient cooling time is provided. Try to do the same thing with an ICE and it will explode in very short order. Electric motors tend to be rated at their continuous power output basis, automotive ICEs tend to be rated at their peak power output. (ICEs for other applications will be rated on a continuous basis, however.)

Hybrid vehicles are a great concept, but like hydrogen, are being mis-sold.

Internal combustion engines are always less efficient when operated throttled back. Using an oversized engine throttled back to move your car down the highway means that the engine fuel efficiency is much lower than it could be. I bet that Julie's 'vette would cruise down the highway at the speed limit with a 20hp engine in it, and get _great_ gas mileage. The car would also be virtually undriveable, because it would take forever to get to the speed limit, she could never actually merge with traffic, could never pass, etc. But it would get great gas mileage.

The electric motor and battery in a hybrid should be thought of as simply part of the drivetrain between ICE and wheels. It is a lower efficiency drive train than a direct mechanical coupling, however it provides energy storage, speed conversion and overload capability. These capabilities permit a smaller ICE to power the car. Since ICEs get more efficient when used near their continuous rating, having this smaller engine will improve the efficiency of the ICE. Pick your components correctly and you improve over-all efficiency.

If most of your driving is start and stop, then a hybrid can save you fuel. If most of your driving is at constant speed on the highway, then the hybrid will cost you efficiency. I hybrid is not necessarily 'more environmental' than a straight ICE car, but it _may_ give better _performance_ at the same environmental cost. IMHO the first market for hybrids really should have been the performance cars for people with more money than sense, where the goal is best 0-200 time, and gas mileage is irrelevant. Develop the technology and mass production capability at the high end, and then let the tech move to the economy side of the market after you have the mass production capability.

-Jon

 

[tallgirl]

Internal combustion engines are always less efficient when operated throttled back. Using an oversized engine throttled back to move your car down the highway means that the engine fuel efficiency is much lower than it could be. I bet that Julie's 'vette would cruise down the highway at the speed limit with a 20hp engine in it, and get _great_ gas mileage. The car would also be virtually undriveable, because it would take forever to get to the speed limit, she could never actually merge with traffic, could never pass, etc. But it would get great gas mileage.

Probably not 20HP, but you're not off by much. The speed limit for that car is right around 2,000 RPM, and y'all are free to look at the maximum possible horsepower (but you'll have to extrapolate the curve) available from the engine at that point.

The absolute best point of your post (besides said that a 20 or 50 HP electric motor would probably suffice for most applications where a 200 HP gasoline engine is used) is that electric motors produce constant torque. And that's a huge advantage of electric motors over internal combustion.

Finally, since I have a reader who thinks I'm a blow-hard (they were kind enough to send me a PM), I'd like to point out that it was Carroll Shelby who coined the phrase "Horsepower sells cars, torque wins races". I think those of us who are old enough to know who Carroll Shelby is will agree that he's a pretty sharp cookie who builds some pretty nice rides.

 

[winnie]

Julie,

HP is relevant to the discussion. The reason is HP is an integral part of torque equations. You cannot have without the other. It is like saying you can have current without watts, not possible and violates laws of physics. T=(5252 x HP)/rpm. Without HP or rpm, you do not have torque period.

An mechanical device can produce torque at 0 RPM, similar to a device that can produce force at 0 speed. My keester sitting on my chair produces a constant force of about 800 newtons. The mechanical _power_ output is zero. If I were to sit on the end of a diving board, then the torque about the board pivot might be 2400 newton-meters, again with no motion and now mechanical power output.

There is no mechanical _power_ output when a device produces torque at 0 RPM, and therefore the horsepower measurement would be 0.

An 4 pole 60 Hz electric motor rated at 200 hp and 95% efficiency will have a nominal continuous torque rating of about 600 foot pounds. Put this motor on an appropriate drive, and it will produce 600 foot pounds at 0 RPM (power output 0 hp, efficiency 0%).

This is a significant difference between electric motors and internal combustion engines. An electric motor can produce force at zero speed, and ICE cannot. This does not mean power output at zero speed (in fact, there is power input, all of which is converted into heat), and one could say that the magnetic field across the airgap in the motor is providing the same utility as the slipping friction of the clutch or torque converter.

-Jon

 

[dereckbc]

Julie,
I don?t know what else to tell you other than what we are discussing is right out of any electrical, physics, or mechanical engineering 101 book. You cannot have torque without HP and RPM, and you cannot have HP without torque or rpm. The two formulas are one in the same and interchangeable just like Ohm?s law.

T= (5252 x HP)/rpm
HP= (RPM x T)/ 5252

Plug zero in anywhere you want and you always get zero, the equations always have to be equal, or they are not true.

 

[Bob NH]

Horsepower and only horsepower will make the car or any other machine go. Power is work per unit of time.

The maximum acceleration at any speed will be achieved by the engine and drive train that can deliver the most power at that speed. It doesn't make a bit of difference what the engine torque is as long as the drive train ratio allows the engine to operate at its maximum horsepower point.

Greater POWER combined with proper gearing will ALWAYS deliver more torque to the drive axle and more accelerating force to the vehicle.

When they ran turbine engines at Indy, they had very low torque compared to the 4-cylinder Offenhausers, but they won every race until they were excluded because the were too good.

 

[winnie]

You have your equation backwards -- HP is derived from torque. In an electric motor torque is the total of the magnetic attraction (or repulsion) between the parts of the motor. Since that attraction (or repulsion) can exist in an electric motor at 0 RPM, it's possible to have torque in an electric motor at 0 RPM.

HP is not 'derived' from torque. HP, torque, and speed are inter-related in exactly the same way that power, voltage, and current are inter-related for DC circuits.

If you define any two of the terms, you can calculate the third.

If you define voltage as zero and assume constant power, then the equation says that current goes to infinity. In reality something breaks, usually the assumption of constant power.

If you define speed as zero and assume constant power, then the equation says that torque goes to infinity. In reality something breaks.

-Jon

 

[tallgirl]

Julie,
I don?t know what else to tell you other than what we are discussing is right out of any electrical, physics, or mechanical engineering 101 book. You cannot have torque without HP and RPM, and you cannot have HP without torque or rpm. The two formulas are one in the same and interchangeable just like Ohm?s law.

T= (5252 x HP)/rpm
HP= (RPM x T)/ 5252

Plug zero in anywhere you want and you always get zero, the equations always have to be equal, or they are not true.

I think you should read Jon's response. What he's described is exactly (more or less) what I've been trying to explain.

(I'd also point out, that if you plug zero into the first equation -- T = HP x 5252 / RPM -- you don't get "zero".)

 

[roger]

(I'd also point out, that if you plug zero into the first equation -- T = HP x 5252 / RPM -- you don't get "zero".)

Julie, tell us what you get.

Roger

 

[tallgirl]

Julie, tell us what you get.

Roger

Infinite torque

But as Jon wrote earlier, an electric motor can produce torque at 0 RPM. Since HP = Torque * RPM / 5252, what's the horsepower? Again, as Jon wrote there is no horsepower output --

An 4 pole 60 Hz electric motor rated at 200 hp and 95% efficiency will have a nominal continuous torque rating of about 600 foot pounds. Put this motor on an appropriate drive, and it will produce 600 foot pounds at 0 RPM (power output 0 hp, efficiency 0%).

 

[roger]

Infinite torque

But as Jon wrote earlier, an electric motor can produce torque at 0 RPM. Since HP = Torque * RPM / 5252, what's the horsepower? Again, as Jon wrote there is no horsepower output --

And so what are you saying?

Tell us in a numeral form what you get when you perform the equation

Roger

 

[tallgirl]

And so what are you saying?

Tell us in a numeral form what you get when you perform the equation

Roger

Certainly.

I'm not the motor designer that Jon is, so I'll allow him to provide the amount of current required to produce 600 ft-lbs of torque at 0 RPM from his electric motors, but the mechanical power out is calculated thusly --

600 ft-lbs x 0 RPM / 5252 = 0 lb-ft per second = 0 horsepower

All agreed? Good.

Let's see if we get that back out ...

0 lb-ft per second * 5252 / 0 RPM = 0 ft-lbs torque

Where'd my 600 ft-lbs of torque go?!?

Now, plug in some lb-ft per second and we get this --

1 lb-ft per second * 5252 / 0 RPM = infinite ft-lbs

(See http://en.wikipedia.org/wiki/Division_by_zero)

 

[dereckbc]

Now, plug in some lb-ft per second and we get this --

1 lb-ft per second * 5252 / 0 RPM = infinite ft-lbs

Huh? some lb-ft per second indicates some means of time and distance like RPM, therefor your equation is not true as some sort of RPM (even fractional)would have to be defined.
x/o = error, undifined, or a computer freeze in a floating point processor.

 

[tallgirl]

Now, plug in some lb-ft per second and we get this --

1 lb-ft per second * 5252 / 0 RPM = infinite ft-lbs

Huh? some lb-ft per second indicates some means of time and distance like RPM, therefor your equation is not true as some sort of RPM (even fractional)would have to be defined.
x/o = error, undifined, or a computer freeze in a floating point processor.

Of course it's undefined.

But it's still possible to produce torque from an electric motor, or your arm, butt, or a brick on the end of a board, at 0 RPM. There's no power output, because the RPMs are zero, but that doesn't mean it's impossible to have torque at 0 RPMs.

Grab a beam-style torque wrench. Torque a bolt to 100 ft-lbs. Now, apply force on the end of the wrench until it measures 50 ft-lbs. Calculate the amount of horsepower you're producing at 0 RPM with that wrench.

 

[roger]

Certainly.


600 ft-lbs x 0 RPM / 5252 = 0 lb-ft per second = 0 horsepower

That's as far as you need to go

Let's see if we get that back out ...

Why?

0 lb-ft per second * 5252 / 0 RPM = 0 ft-lbs torque

Same answer as above

Where'd my 600 ft-lbs of torque go?!?

It's where it was in your first two equations, it has not moved.

Now, plug in some lb-ft per second and we get this --

1 lb-ft per second * 5252 / 0 RPM = infinite ft-lbs

Yep, and this is the same answer as above, zero.

Roger

 

[winnie]

Okay, now I am confused as to what the discussion actually is.

Is anyone here actually saying that electric motors _cannot_ produce torque at zero speed?

-Jon

 

[tallgirl]

Okay, now I am confused as to what the discussion actually is.

Is anyone here actually saying that electric motors _cannot_ produce torque at zero speed?

-Jon

Yes.

Have snippet from earlier --

Originally Posted by dereckbc
Julie,
I don?t know what else to tell you other than what we are discussing is right out of any electrical, physics, or mechanical engineering 101 book. You cannot have torque without HP and RPM, and you cannot have HP without torque or rpm. The two formulas are one in the same and interchangeable just like Ohm?s law.

T= (5252 x HP)/rpm
HP= (RPM x T)/ 5252

Plug zero in anywhere you want and you always get zero, the equations always have to be equal, or they are not true.