Hacker News Reader

Comment by mywittyname | Hacker News Reader

I'm surprised ZF/Porsche didn't turn this into an eCVT, as an eCVT (a la Prius) has probably the same complexity, but with the added benefit of having more ratios. I suspect it would also reduce shock loads on the transmission as a result of shifting.

I wish the article would have gone into this, because I suspect Porsche said no more for marketing reasons rather than technical ones, as CVTs are universally hated in car performance circles.

View on HN

The main advantage of the eCVT is the ability to seamlessly combine the power from two motor-generator units and an ICE (as in the Prius) while holding the ICE in the RPM range you want it. Here, there is only one electric motor and keeping it at a certain RPM range (other than below it's max) isn't really important. While a CVT will get you smooth ratio changes, you are still constrained by a minimum and maximum gear ratio, a range which is relatively small compared to this kind of two speed (in this case you can have two gear ratios that are super far apart, as an electric motor makes the same torque regardless of the RPM, unlike an ICE, where you want your gear ratios close to help stay in the powerband). As for CVT's being universally hated in performance circles, they're only universally hated by the same kind of people that hate flappy paddle dual clutch transmissions, i.e people who don't care or know what is actually faster, more reliable, and more efficient in different use cases, but only care about a clutch and an H-pattern shifter.

> and keeping it at a certain RPM range (other than below it's max) isn't really important.

Yeah it is, because electric motors have RPM bands where they are most efficient. The whole point of a adding a two-speed transmission in the first place was to increase efficiency, range, and top speed. If they managed a 5% improvement with a two-speed, I'm betting they could at least double that with an eCVT.

CVT transmissions can’t handle the power. They’re also universally hated by everyone except people who merely use a car to get from A to B. A Porsche owner is the polar opposite of someone who would tolerate a CVT.

someone this close minded would likely not be considering electric in the first place (and would probably whine about porches being mostly v6 and not v8/10/12)

Close minded? Huh. You won’t find a CVT in anything but a budget economy vehicle. If you want performance, you don’t want a CVT. If you want to prioritize efficiency over everything else (including reliability), a CVT is fine.

https://en.wikipedia.org/wiki/Williams_FW15C

CVTs grant you the ability to maintain peak power at all speeds for acceleration and you don't have any losses to shifting. If you look at recent Civics for example Sport trims with more horsepower and a manual trans will lose out or tie CVT models in 0-60 and 1/4 mile times despite the CVTs having bad launch characteristics. And this doesn't even touch on how torquey they can feel at higher speed relative to a standard trans at the same HP.

I won't question your claims about reliability but CVTs are actually pretty performant within their segment, they just dont make the Vroom Vroom shifty noises that some car enthusiasts equate to performance.

The primary issue with CVTs (as others have mentioned) is that they can't be strapped to engines that make more than ~200 HP.

yes - close minded.

if you want performance - you want performance however you get it.

if someone makes a performant cvt, you want that. if they don't, you dont.

someone bound up in performance==x will not buy 'y', period.

It seems like the advantage of a CVT only really apply to internal combustion engines since they can only really go from about 500 to 8,000 rpm, and in that range they only have peak power or peak efficiency in small ranges. EV motors can go from 0 to 18,000 rpm.

A CVT can't handle as much torque as a gear-based transmission.

The e-CVT has nothing to do with actual CVTs, it's a planetary "torque split" gearbox that's very robust.

That could simply be due to the fact that CVTs have never been really used and developed for high performance applications, since car enthusiasts hate them. But there was an F1 car with a CVT before they were banned.

CVT != eCVT

An eCVT is a planetary gearbox which uses a pair of electric motors to control the rotational speed of the ring and planetary gears to achieve an overall gear ratio anywhere between a minimum and maximum ratio.

It's largely the same thing as what this two-speed gearbox is, but it leverages electric motors to provide a smooth transition between two ratios, instead of using (presumably) a set of clutches and auxiliary gears to select one of two fixed ratios.

This is why this question jumped out at me immediately. On the surface, they are going with a really, really archaic, complex design over a more simple, all-electric design which is ideally suited for their application.

I can't help but think ZF started with an eCVT, then retrofited it back to being a two-speed. They must have had a good reason for this. And I wonder if it's a patent/license issue, or if there's some technical reason this wouldn't work. And I'm disappointed the article doesn't go into details on this.

Wouldn't an eCVT require an additional motor?

Yes, but only two small ones to control the rotational speed of the ring / planet gears.

I'm not convinced it's in the same ballpark of complexity as a 2-speed gearbox, which would need just a simple two-way friction clutch.

Reply on news.ycombinator.com