German Supertuner RUF Building an Electric Porsche - Page 3

Joseph · 10-23-2008, 03:33 PM

Something to add.

I know it sounds odd that electric motors are more efficient at high rpms. My understanding is that at lower rpms, the current (amps) vs. voltage proportion is higher than it is at higher rpms. So basically, low rpms use more current but less voltage and higher rpms use less current but more voltage.

High current and low voltage is less efficient. I don't know why, but I know for certain that is true. A wire containing 100 amps at 50 volts will create much more heat that 100 volts at 50 amps.

Also, there's nothing wrong with transmissions. They improve performance a whole lot, but the problem is reliability. Super high rpm and super high torque puts too much stress on the transmission. Plus, it's a disaster on the clutch because the "engine" can't wind down quick enough. A gas car will lower rpms in an instant, allowing for a smoother shift; but an EV spinning at 14k rpm will still be spinning very quickly when you shift it.

TEG · 10-23-2008, 03:51 PM

Quote:

Originally Posted by Joseph

...Also, there's nothing wrong with transmissions...

Turning more gears hurts overall efficiency. Tesla realized a range improvement when they went from 1.0 to 1.5...

stopcrazypp · 10-23-2008, 04:16 PM

I think there's a balance in terms of efficiency. Is the efficiency that you gain from having the option of the lower lower gear greater than what you have lost from having a transmission?

The biggest positive I see from the transmission is just the option to have a higher top speed. But using the existing transmission on the car is something I expect more from homemade conversions than a professional tuner. If it's like other conversions I have seen, you can't actually switch gears easily while driving. So we still don't have a true manual multi-speed transmission for EVs yet.

TEG · 10-23-2008, 07:15 PM

Quote:

Originally Posted by stopcrazypp

...The biggest positive I see from the transmission is just the option to have a higher top speed...

I have said a bunch of times that I think EVs shouldn't be geared for high top speed. It just shows off their shortcomings. Limited energy storage results in very short ranges at high speed. EVs are a lot about efficiency, and wind resistance starts to cut into that big time at speeds > 90 MPH.
Besides, look how the Roadster didn't fair so well when the German auto journalists took them out on the Autobahn. The eMotor started to get too hot and reduced top speed back down to something closer to 100MPH. Tesla needs to gear the Roadster for brief excursions to 125MPH top speed because they sell a product that has to have a decent 1/4 mile time for image reasons. For something not aspiring to be a "supercar" (of sorts) an 85MPH top speed could be much more reasonable. Why complicate things when a high rev AC motor can get you up to US Freeway speeds with only 1 gear. Direct drive all the way, thank you very much.

Joseph · 10-23-2008, 09:40 PM

Transmissions are not necessary by any means, and are completely impractical for efficiency and reliability reasons for a normal production car. However, if you're building some unique, oddball EV (sorta like Ruf's EV Porsche) keeping the transmission has its merits. You sacrafice reliability, (clutch, gears wear out) usability (EVs don't shift gears well) and efficiency (all those extra gear will nock off about 5-10% of your range) BUT you get a higher top speed.

For the automotive enthusiast who has enough time and cash to waste going after this EV Porsche, that's all that matters.

doug · 10-24-2008, 01:53 AM

Quote:

Originally Posted by Joseph

However, if you're building some unique, oddball EV (sorta like Ruf's EV Porsche) keeping the transmission has its merits. You sacrafice reliability, (clutch, gears wear out) usability (EVs don't shift gears well) and efficiency (all those extra gear will nock off about 5-10% of your range) BUT you get a higher top speed.

For the automotive enthusiast who has enough time and cash to waste going after this EV Porsche, that's all that matters.

This was pretty much my point in mentioning the transmission above. Having 6 gears is overkill, seems you could skip half of them, and I really doubt top gear is all that useful. But my main point is that this is a fine shortcut for an enthusiast conversion project, but seems like a poor effort from "professionals."

TEG · 10-24-2008, 08:37 AM

Yes, well, as Tesla can attest it takes real money to make your own gearbox!

William3 · 10-24-2008, 10:43 AM

Quote:

Originally Posted by TEG

Tesla needs to gear the Roadster for brief excursions to 125MPH top speed because they sell a product that has to have a decent 1/4 mile time for image reasons.

With a claimed qtr mile time of 12.9, I'd guess that the Roadster's trap speed at the end of the qtr mile is around 105mph.

bill · 10-24-2008, 12:25 PM

Quote:

Originally Posted by TEG

I have said a bunch of times that I think EVs shouldn't be geared for high top speed. It just shows off their shortcomings. Limited energy storage results in very short ranges at high speed. EVs are a lot about efficiency, and wind resistance starts to cut into that big time at speeds > 90 MPH.

Good Point..!!
Your last mail about "It's the battery, stupid" really drives home the message. Before we invest in gearboxes, we need the battery performance. Top speed can wait. From what I read here, it seems most of us who are buyers, or future buyers won't be on the track looking to drive 200MPH. The single speed box is a Super Asset for us.

DaveD · 10-24-2008, 03:45 PM

When Tesla announced that they were dropping the two-speed transmission, and going with a single-speed gearbox, I was personally very happy with that decision. Simple to operate and more reliable. Mind you, I don't like automatic transmissions; I've never owned a car with one, and I never want to. But I'll happily take a single-speed gearbox any day.