David Patience
1 HN recommender · recommended in 4 months, 2023–2024
4:37 · 286K views · Apr 29, 2020
A short animated explanation of the power split device in Toyota-style hybrids, a planetary gearset that works as an eCVT. It shows how engine and motor power are combined.
HN people link it in hybrid versus EV debates as a clear view of how simple the hybrid drivetrain really is.
5 comments from Hacker News
More YouTube videos about the eCVT... 1) Better -- much simpler, and above all shorter -- video explanation of the Toyota system: this video . 2) Another, a little longer, but very pedagogical and beautifully animated explanation: [LINK] . Eight years old, but apparently the "power split device" worked the same even then. Bonus -- short conversation in the comments, arriving at the same conclusion I did above: @TimpBizkit 3 weeks ago I thought the prius had a cvt 1 reply @aspecreviews 3 weeks ago It doesn't. The "CVT" is electrically simulated. Heh, yeah. 3) Titled something about "Launch Gear", but actually on the whole CVT thing (from a channel I semi-regularly watch): [LINK] . 4) A good basic look-through of the various types of CVT and their history: [LINK] . 5) Long and thorough explanation of the Prius powertrain: [LINK] . Uses a cutaway of the actual transmission, so good for getting a more concrete picture than e.g. #2 above. But... Long. 6) Another one from WeberAuto like in my parent comment, this time on the (apparently very similar) Honda system: [LINK] . So likely to be about as good as the previous one, but... 25 minutes more of this? Naah, I've had enough. There were also some from Scotty Kilmer et al, but... The less said, the better. The ones I'd most recommend are #4 for general CVT stuff and #2 for the Prius e-CVT's operating principle specifically. Well well, all in all that was quite the rabbit hole.
> but an appropriate gear ratio should handle that. See Prius's "Powersplit Device". I'd describe the Powersplit device to be a combination of generator/alternator, starter/electric motor, reversed-differential (2x power inputs -> 1x driveshaft), and effective gear-ratio. All in one planetary gearset. EV motor2 determines the speed of the car. The ICE motor can spin at any speed that the computer determines to be useful. If EV Motor2 is 0-rpm, then the ICE is 100% in generator mode (2000rpm but the car isn't moving: all the energy goes to charging the battery). If the EV motor is at 10mph but ICE is off (0-rpm), then its 100% electric drive mode. And any combination in-between is possible. EV Motor1 (a smaller, weaker motor) controls the 3rd set of gears (I think the planet gears?? I forget), which determines how ICE relates to EV (changes the effective gear-ratio) [LINK] --------- So yeah, the PSD allows the ICE to always function at the appropriate speed (which is either 0rpm or ~2500rpm for efficiency). While the combination of EV-motor1 (changes effective gear ratio of ICE) and EV-motor2 (hard-wired to the final speed) handle the different speeds the user wants in practice. All in like, 15 gears or so. this video --------- I do think that the Prius (and Prius Prime) have surpassed the Volt's design, and the proof is in the pudding. Prius Prime has 52mpg, a figure far more efficient than the Volt ever had. Prius Prime also has 220 horses today for a 0-to-60 time of 6.6 second
All the "complexity" of a hybrid (or at least... a Prius) is approximately 15 gears that replace the alternator/starter/transmission. The Toyota Power-split device is a planetary-gearset connecting 2x electric motors/generators with 1x ICE engine. But a regular ICE car needs 2x electric motor/generators *anyway*. #1 is called the starter (turns Lead-Acid battery pack into cranking force to start the engine), and #2 is called the alternator (turns engine RPMs into electricity to charge the battery pack). Then you need a gear-shifter to have the ICE car handle a variety of different speeds. Change #1 and #2 to be bigger, stronger. Then make the effective-gear shifting a function of the relationship of #1 vs #2 RPMs, and you got a Toyota Power Split Device. -------- Its so efficient and usable, that Ford has copied the design, as well as Stellanis's Chrysler Pacifica. As it turns out, a Hybrid barely has any more complexity over a regular ICE in practice. Its just a *different* set of gears, but really not a big deal. this video Over the last 25 years of Prius, the Hybrid / Power-split Device has rarely failed. Prius transmission / generator / starter is incredibly reliable. Its just gears. All in all, the "complexity" of a Hybrid is roughly the same level of complexity as an automatic-transmission wet-clutch device.
> You've got everything that can fail in a fairly high tech ICE drivetrain, plus everything that can fail in an EV drivetrain You should seriously look up the drivetrain of hybrids. The planetary gearset is a single set of gears that functions as the alternator/generator, the starter/EV motor, the transmission, and the engine driveshaft. Its not "different" or "new" components. Its a singular design that accomplishes what 3x different parts of the car used to do seperately. There's a reason why Toyota Hybrids are $23k, its a lot cheaper and more efficient to do it this way. this video You needed the starter anyway. When its beefed up to a full-size EV motor that can drive the car, its more powerful and more durable. You needed the alternator anyway. When its beefed up to a full-sized generator, its more powerful and more durable. When the ICE engine operates at its ideal RPM (rather than directly connected to the road), it can be made cheaper, more reliable, smaller, more efficiently than when its the only thing moving the car. Hybrids are superior to ICE, and Prius shows how to do it with more reliability. Now yes, Chrysler is terrible with reliability, but that's nothing to do with Hybrids and everything to do with Chrysler.
Toyota's eCVT? Not at all. The gas engine can drive the front wheels directly. this video Conceptual explanation of the system. [LINK] Weber Auto giving an extensive hands on explanation.