Efficiency of Geared vs. Single Speed transmission.

As far as I know, the main reason Tesla uses a single speed reduction gear set, is that an electric motor has so much torque it snapped all the multi-speed transmissions. You could have a multi-speed transmission if the motor was very low power, but then you'd have other problems (like selling the car).

jerry33 said:

As far as I know, the main reason Tesla uses a single speed reduction gear set, is that an electric motor has so much torque it snapped all the multi-speed transmissions. You could have a multi-speed transmission if the motor was very low power, but then you'd have other problems (like selling the car).

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Hmmm.. I've never heard of an electric car with a multispeed gearbox, but that's generally because with the high motor efficiency and wide power band, it's an unnecessary complication, and for what benefit? I suppose it would allow Tesla to keep pace with the high powered ICE engines at higher speeds, but I don't see that providing enough benefit to justify the added cost & complication. The torque may be very high in automotive terms, but certainly not in terms of general geared machinery.

As an aside, the AWD Teslas effectively have a dual speed gearbox. They just run the motor that's in the best location on the efficiency/power curve.

The first Roadsters had a multi-speed transmission. However, the motor's torque kept breaking them and all were converted to single speed.

sandpiper said:

Hmmm.. I've never heard of an electric car with a multispeed gearbox, but that's generally because with the high motor efficiency and wide power band, it's an unnecessary complication, and for what benefit? I suppose it would allow Tesla to keep pace with the high powered ICE engines at higher speeds, but I don't see that providing enough benefit to justify the added cost & complication. The torque may be very high in automotive terms, but certainly not in terms of general geared machinery.

As an aside, the AWD Teslas effectively have a dual speed gearbox. They just run the motor that's in the best location on the efficiency/power curve.

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There's a lot of potential benefit, mostly in either low end torque or ultimate high speeds. It's true that an electric motor has a much broader range and can start from 0 rpm, but there are definitely areas where it is more efficient than others. If I remember right, the efficiency of the Volt's drive motor drops almost 10% from the mid rpm middle power peak to the limits of the normal operating range - and by doing the 2 motor EV mode and dropping the main motor rpm by half, they pick up 4% overall system efficiency on the highway.

Have a look at this chart WarpedOne published in a prior discussion on the subject:
The real top speed of the Model S - Page 6

Speeds over ~70 mph start cutting greatly into the efficiency and power output of the Tesla drive motor (this was before the D announcement - I'd be interested to see a P85D and an S85D plotted on the same chart) - limiting the car's top speed to something well below it would otherwise have. Having a taller gear could change all of that (though those speeds are still very inefficient due to simple aerodynamics, and dangerous on most roads.)

The question is always about the tradeoffs - weight, cost, space. Potentially some of the mass of the transimssion can come out of making a smaller, higher RPM motor, but the total package still has more weight and cost, and more chances to fail. Most OEMs believe that on the balance the tradeoff isn't worth it. They certainly have more data to decide with than I do.
Walter

Formula E uses a 5 speed sequential Hewland gearbox, but they also max out at 200kw.

Saghost said:

Have a look at this chart WarpedOne published in a prior discussion on the subject:
The real top speed of the Model S - Page 6

Speeds over ~70 mph start cutting greatly into the efficiency and power output of the Tesla drive motor

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That chart shows that a multi-speed transmission would allow the Tesla to stay with-in its peak powerband, however it says nothing about efficiency, as it's not a plot of efficiency.

And the article in this thread claims the main improvement will be on the NEDC cycle which is low speed (not the speeds mentioned in WarpOne's charts). I imagine also the type of motor (induction, PM, or switched reluctance) will play a big role also.

stopcrazypp said:

That chart shows that a multi-speed transmission would allow the Tesla to stay with-in its peak powerband, however it says nothing about efficiency, as it's not a plot of efficiency.

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Agreed. From what I've read Back-EMF should be eating a lot of efficiency in those rpm ranges, but the chart doesn't cover or discuss that.

The BMW i8 uses a two-speed gearbox on the front axle in order to provide both low speed torque and high speed ability.