Post #2133052 · Elektrine

2026-05-07 02:18 UTC

@TechConnectify oh man, I have some thoughts on this. prepare for a long-ass subthread. I adore the brutal simplicity of power split devices, told myself that as soon as Toyota made a hatchback that was interesting to drive and had one, I’d buy one… and then like ten years later they did (they benchmarked the Mk7 Golf’s handling for the Gen 4 Prius), so I did. and mine ended up being the first month of production, at that. so, the huge Achilles heel of an input-split power split device is that MG1’s rev limit affects the transmission’s operating range, both at low to moderate vehicle speeds with the engine running, and at high vehicle speeds with the engine off. in the former case, MG1 has to spin forward for the engine RPM to increase. MG1 will reach its rev limit before the engine reaches its horsepower peak RPM, unless the vehicle is going at quite high speeds - in my Gen 4 Prius, I calculated this as being anything under 88 MPH. this leaves a lot of performance on the table, especially at low speeds - at a stop, the engine’s well under 3000 RPM maximum due to this. in the latter case, MG1 has to spin backwards for the engine to remain at 0 RPM when the vehicle is moving. this is honestly a lot less of an issue, especially in a non-PHEV, for a few reasons - the situations where zero engine power is called for while going at high speeds are a lot rarer, higher MG1 RPM in newer models (especially with the 2016+ systems) shifts it up to a higher vehicle speed (purely based on MG1 rev limit, my Prius could hypothetically go 95 MPH with the engine off), and there’s other reasons why the engine may need to be spinning anyway (non-PHEV versions of Hybrid Synergy Drive drive the oil pump off the planet carrier, and therefore the engine must be spinning for the transmission to receive proper lubrication and cooling). as I understand, the former case is a significant reason why Nissan e-Power and Honda i-MMD are serial (or serial+parallel) systems - with the ICE disconnected from the wheels, the MG1 power rating is the only limiting factor on ICE power output at low speeds, and much better low-speed acceleration is possible. (Toyota’s solution for this in some premium models was initially the Multi-Stage Hybrid system, that just put a 4-speed planetary transmission behind the power split device, and in newer models where they’re concerned about this, has been to ditch the power split device entirely for a purely parallel hybrid configuration using a 6, 8, or 10-speed planetary automatic with a MGU in place of the torque converter.)

Replies (1)

@bhtooefr@snack.social 2026-05-07 03:00

@TechConnectify so, on the serial implementations. purely serial hybrids have another Achilles heel than just conversion losses: the size (and weight, and cost) of the MGUs. note that the Toyota MG1 and MG2 are pretty small - the fact that a significant percentage of ICE torque goes through a mechanical path, bypassing the serial path, means that MG2 needs to only be sized to take battery+MG1 power, not battery+full ICE power as a serial hybrid would need. Nissan’s e-Power system, which is purely a serial hybrid, isn’t terribly inefficient at low speeds as I understand - a bit worse than the Toyota system in comparable models, but not much - but falls completely on its face at non-Japanese (that is to say, faster than 100 km/h) highway speeds. the real reason why Nissan made it was they had Gen 1 LEAF motors lying around, and they could reuse them in a serial hybrid system. your Gen 1 Volt wasn’t purely a serial hybrid, and it had a few different modes it could operate in, one of which was a single-speed parallel hybrid mode, specifically to avoid the conversion losses of serial hybrid operation when on the highway. (the other notable improvement over a pure serial hybrid was that it could disconnect MGA from the ICE and connect it to the wheels, when high electric power was requested, so that MGB could be somewhat smaller/lighter/cheaper.) as I understand, the big efficiency problem the Gen 1 Volt had wasn’t really the hybrid system, it was that the engine tuning was not complete and they didn’t get a chance to optimize efficiency, only to get it to comply with emissions (and that they run super high EGTs to keep the catalytic converter hot, and then just keep running at those super high EGTs all the time). that said, a better comparison may be to Honda i-MMD - similar engine efficiency to Toyota’s best, and a similar serial-hybrid with a single-speed parallel hybrid mode. this does… pretty well, despite not having the Gen 1 Volt optimization of an undersized traction motor and repurposing the generator as a second traction motor in EV mode. …and then there’s the Chinese automakers. as far as I can tell, most of the Chinese systems are essentially developments of the Honda i-MMD or Mitsubishi Outlander PHEV (same idea as i-MMD) concept, with the addition of a 2-4-speed gearbox (possibly a DCT) to allow the ICE to connect to the axle at multiple gear ratios. BYD’s system is, I believe, “what if we just did the Gen 1 Volt system but better?” these seem to be doing shockingly well, but I’m sure some of this is BYD and Geely breaking 46% peak thermal efficiency, when the best Toyota can do is 41%.

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