acceleration advantage for electric drive?

[BobM3]

A colleague of mine says an electric motor driving a vehicle has an inherent advantage in accelerating the vehicle. I'm trying to figure out why. I'd think the rotational inertia of the electric motor is comparable to the rotational inertia of the internal combustion motor. If you're going from 0 to 60 mph in first gear, who wins?

 

[Javier72]

It is true. If you see the torque curve of an elèctric motor is much more similar to the needed one in a car than any combustion engine. In fact,the gearbox in a vehicle tries to aproximate the torque given to the needed. In some cases, it is not needed to have it for an electric motor.

Javier

 

[GregLocock]

also

Bear in mind that the referred rotational inertia depends on the gear ratio ^2, so an IC engine is quite a significant proportion of the inertia of the vehicle in first gear, which has an overall gear ratio of 10.

A direct coupled electric motor (ie wheel motor) might well come out ahead.

Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[MintJulep]

An electric motor is approximately constant power. Since power is torque x rpm, at low speed electric motors have gobs (that's a technical term) of torque.

 

[GregLocock]

Hmm, must depend on the motor type. Mine were current limited, hence torque was virtually constant.

Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[EdDanzer]

Varying speed in any electric motor will produce poor torque outputs at certain rpms. Most electric motors will go into constant torque mode below a certain speed. After testing the output torque on a machine tool spindle drive, don’t believe what electric motor and drive people tell you.

 

[patprimmer]

I am not an electric motor guy, but going way back to my High School Physics in the early 1960s, I thought the faster a DC motor went, the greater the back EMF. I would think this increased resistance with speed would reduce torque with speed.

Regards

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[WolfHR]

And I think the ideal torque curve would be hyperbolic, with torque inversely proportional to speed, so the DC characteristic isn't neccessarly incompatible with that (I think series wound DC motor would have closest characteristic to the desired one*).

* picture:

http://www.maintenanceworld.com/Articles/reliance/maintenance/mtdfig19c.gif

.

 

[BobM3]

Do we all agree that a car would require more torque as its speed increases (due primarily to wind resistance)? If so, then I don't see the "curve" portion of the electric motor's speed/torque relationship being an advantage. Shunt wound DC motors have a nearly constant torque through their speed range and series wound DC motors have the torque dropping off with speed. An IC engine does have an increasing torque output with speed (up to a point). From that standpoint I would think the "curve" of an IC engine is a better match for the load requirement "curve" of the car. I see the low speed torque of the electric motors being an advantage though. My guess is that, all other things equal, the electric motor car would pull out ahead of an IC engine but the IC engine may pass it up in the end.

But there are certainly a lot of other considerations. I hadn't considered the reflected inertia due to the gear reduction. I think IC engines have a much better power to weight ratio - but then you add the gear box. Somebody out there must have written a paper on this.

My standby response is that if something is clearly better then the car companies would already have adopted it. With electric motors, though, there are so many other issues (environmental, controls, batteries).

Maybe we need a race!

 

[EdDanzer]

Actually the torque requirement is much higher at start than highway speeds. That is why a transmission is needed. Starting a vehicle from stop is one of the difficult portions of drive train engineering.

 

[dcasto]

When drag racing HP is not what you want its Torque. you can can accelerate if the motor has the torque available. Withan electric motor, you get 100% torque at 0 RPM with an engine you have 0 torque at zero rpm, as the rpm increases you start making some torque so by the time you get to 800 RPM, you have enough torque to accelerate the car from 0 to 5 MPH.

Once you are moving and you want to accelerate, you drop to a lower gear, jump up the torque curve of the engine and take off.

 

[BobM3]

dcasto-

Sounds like you're saying that an electric motor would be a better choice for a drag racer. If so, then why aren't electric motors used for powering drag racers?

 

[patprimmer]

dcasto

Why don't you go read the definitions of torque and power.

They are kind of related to each other.

You could also Google search this site to get the previous input of some competent engineers.

Regards

eng-tips, by professional engineers for professional engineers
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[dcasto]

torque times rpm = HP

who holds the world record for somes classes of dragsters?

go to

http://www.nedra.com/

PS I'm competent and I play with rice burners at home, anyone can get 327 HP from a 327 V8, try it with a 152 CI 4 sometime.

 

[EdDanzer]

Actually you need the correct amount of torque relative to rpm, weight and coefficient of friction to accelerate a vehicle. The problem with electric motors to drive a vehicle is to have adequate low speed torque they are large and heavy.

 

[dcasto]

Ed, same is true for IC's too, I assumed that. how about these stats:

0 to 60 in under 4 seconds, 248 HP motor, 2500 lb car Oh- and stock.

Can any IC engine car made in the US do this? My 2zzge toyota in an 1800 lb car is almost there.

take a look at this

http://www.teslamotors.com/index.php?js_enabled=1

 

[EdDanzer]

For less money a Dodge Viper will provide better performance, can be purchased today, and has a known reliability when driven hard. My guess if you ran a Tesla in a race environment the cost to maintain would make IC cars cheep.
If you include storage, speed control and motors, the cost per hp is very high compared to IC.

 

[dcasto]

Ed, I'm not a fan of electric cars, yet anyway. But a startup company without the tooling capabilities of a major making a production car under $100,000 says we need more of this thinking in Detroit. At 248 HP output, I doubt the batteries would last 50 miles, not the 250 claimed. The OP was question torque and 0 to 60 times. Clearly, today a 10,000 HP V8 on nitromethane wins, but an electric drive is just around the corner that will beat it.

I'll trow in 2 cents more, a hybred has more available torque than its non-electric brother, but remember, it also carries a few hundred pounds of baggage in batteries and the motor itsself. That said, driving a hybred across Kansas, the hybred will get lower gas mileage.

 

[BobM3]

The Tesla website was very interesting. The motor is the size of a watermelon and weighs 70 lbs! Never mind the controls or the batteries, it appears (from a purely performance point of view) the motor itself might just be superior to an IC engine and its rquired transmission.