Some (deep) notes on the Gen 5 transaxles

I used to be deep into this stuff when the Prius was new, and I had months to kill while on a Gen 2 waiting list in 2004. It's coming back to me now I've got a Gen 5 on order. (I knew Toyota would crack and produce a Prius I'd be happy to swap a G2 for one day...)

As Professor Kelly at Weber Auto hasn't done a video for the G5 yet, I've been deep digging into the New Car Features, to see what's going on in the new transaxles.

Not a lot, is my preliminary answer.

Rather boringly, the PA10 (1.8 HEV), PB10 (2.0 HEV) and PB12 (2.0 PHEV) transaxles used in the Gen 5 are very similar to the P610 in the Gen 4.

The transitions from G2 to G3 and from G3 to G4 had some significant transaxle design changes. This seems more like the G1 to G2 transition - a refinement that simply improved the individual components. I can't actually see any structural changes since G4 - the layout seems the same as the P610. But all the bits are bigger and stronger.

The only fundamental change I see is that the G5 PHEV has dropped the one-way clutch at the engine flywheel. Both HEV and PHEV G4 had the same 53kW MG2, and the engine clutch allowed the PHEV to add extra power from MG1 without spinning the engine backwards, giving a total of 68kW (92PS) in EV mode.

G5 actually has different specced MG2s - 83kW for the 2.0 HEV and 120kW for the PHEV. That 120kW (163PS) means MG1 doesn't have to help in EV mode any more, so the clutch is gone, and we're back to traditional MG2-only EV mode.

What's not clear in the docs is what makes the PHEV's MG2 120kW rather than 83kW. The Japanese brochure says it's the same motor (1VM), and it appears to be mechanically the same size in the PB10/PB12 diagrams. Two possible factors are that they've raised the specced voltage from 600V in the HEV to 650V in the PHEV, and that it can end up running faster, due to the PHEV's increased final drive ratio, but those don't seem to be enough on their own.

Other changes are just to gearing ratios, as far as I can see. The motor rpm limit seems to be staying at 17000rpm same as the G4, and the drive ratios shift a bit - increasing for the HEV, apparently mainly just to accommodate the larger wheels, and decreasing for the PHEV, possibly because there's now much more torque in the PHEV MG2, so it can afford to get closer to the HEV ratio, and raise the max speed closer to 180km/h.

The detailed numbers for those who care:

Core planetary gear remains 78:23:30, as in all Prius generations.
Final drive ratio = 74:20 (3.700) on G5 1.8 HEV, 77:21 (3.667) on G5 2.0 HEV, 78:20 (3.900) on G5 PHEV, compared to 73:21 (3.476) on G4 HEV, 71:19 (3.947) on G4 PHEV.
Counter gear ratio = 49:53 (0.925) on PB10 and PB12 (2.0 engines), compared to 53:65 (0.815) on P610 and PA10 (1.8 engines).
MG2 reduction ratio = 53:16 (3.313) on PB10 and PB12 (2.0 engines), compared to 65:17 (3.824) on P610 and PA10 (1.8 engines).
Resulting MG2:wheel ratio = 10.838 for G4 HEV, 11.535 for G5 1.8 HEV, 11.229 for G5 2.0 HEV; 12.307 for G4 PHEV, 11.944 for G5 PHEV.
Resulting speed for 17000rpm MG2 = 187km/h for G4 HEV, 185km/h for G5 1.8 HEV, 193km/h for G5 2.0 HEV, 165km/h for G4 PHEV, 182km/h for G5 PHEV. (Limiter is clamped to 180km/h for all G4/G5 HEVs, and set to 162km/h for G4 PHEV and 177km/h for G5 PHEV, which seems consistent).
Inverter/motor voltage = 650V in G5 PHEV, 600V in the rest of G4/G5.
MG2 power = 53kW for Gen 4, 70kW for G5 1.8 HEV, 83kW for G5 2.0 HEV, 120kW for G5 PHEV.
MG2 torque = 163Nm for Gen 4, 185Nm for G5 1.8 HEV, 206Nm for G5 2.0 HEV, 208Nm for G5 PHEV.
Theoretical MG2 effective torque = 1.8kNm for G4 HEV, 2.0kNm for G4 PHEV, 2.1kNm for G5 1.8 HEV, 2.3kNm for G5 2.0 HEV, 2.5kNm for G5 PHEV.

 

Okay, one more minor difference. Not mega-exciting, but looking for anything here. There's a change to the oil pump mechanisms in the transaxle, improving its EV capability.

P610 uses a combination of a sling type lubrication mechanism (from the output) to literally throw oil up to catch tanks, and there is also an engine-driven oil pump. The engine-driven oil pump sends more oil to the motors through a heat exchanger to cool the oil.

The G4 PHEV might not run the engine for a prolonged period, so it adds an electric oil pump that's presumably only turned on when necessary. That pump increases flow in the transaxle, but as far as I can tell it does not send the oil through the heat exchanger.

The G5 transaxles all drop the sling lubrication for dry sump lubrication. They have an output-driven oil pump and an engine-driven oil pump, again with the engine pump sending oil through the heat exchanger in the HEV.

Then instead of an extra electric pump, the G5 PHEV adds 2 control valves allowing the output-driven oil pump instead of the engine-driven pump to send oil through the heat-exchanger-to-motor path. This means we can achieve full cooling and lubrication capability in EV mode, and it can actually choose whether to use the engine-driven pump or output-driven pump or neither for heat exchange at will, giving it a choice of cooling+lubrication levels at any time.

 

Interesting question. From the NCF specs:

P410: 92kg
P610: 81.3kg (both HEV and PHEV)
PA10: 85.6kg
PB10: 93.5kg
PB12: 93.5kg

Comparison with other models:

P710: 95.9kg (Camry/ES300h/RAV4 2.5 HEVs)
P810: 111.8kg (RAV4/NX450h+ 2.5 PHEVs)

 

Best I can do is this pair of hard-to-compare numbers:

2ZR-FXE: 90kg (with water and oil fully filled), from Prius G3 specs - not shown in G4 or G5 specs
M20A-FXS: 113kg (without coolant and oil)

 

No, it wouldn't. If you're accelerating in HEV mode, MG1 is doing its traditional job of balancing engine force - it captures power from the engine by resisting the portion of the engine's force that goes through the transaxle to MG1 and sends the power to MG2 for it to push with. No difference between generations.

The clutch in G4 is there to permit MG1 to contribute battery power while the engine isn't running without turning the engine backwards. (MG1 needs to push backwards, and it needs the engine to lock in place to act as a pivot in the power split device to send force forwards to the wheels). It boosts EV power only. But the G5 has far more EV power from MG2 than G4 had from MG1+MG2 combined.

Animated thingy here that might help you visualise what MG1 is doing:

Toyota Prius - Power Split Device

I do find it hard to believe the G5 could have less acceleration at any speed than the G4 - its power numbers are so much higher on every part of the system.

 

Yes, that's an old page with limits based on the G2 motor-generators. In the PHEVs the reverse rev limits on MG1 are considerably increased, together with other gearing changes, raising the 42mph engine-off limit up to 84mph. The diagram still shows why there is an engine-off limit, even if the value has shifted.

But I was referring you to it for an illustration of how the power split device works, and what the dual motor clutch thing in the G4 is about.

There are mechanical assembly differences, but the one thing that's remained utterly constant from G1 to G5 including PHEVs is the power split device with its 78/23/30 gear ratio, and the resulting power transmission.

The functional differences are outside the power split device - differently powered engines and motor-generators, different rev limits on the motor-generators, a reduction gear on MG2 to use a higher-rpm lower-torque motor since G3, different final gear ratios, and the G4 PHEV's "don't let the engine go backwards" clutch - which lets the unchanged PSD be used in a different way.

 

@Max_B - glad you found it helpful. I've had a look at your PTC thread.

I made my own non-live nomograph by adjusting the ratios in Toyota Prius - Power Split Device locally - also not published.

One thing I'm curious about that you might be able to find out is what are the rev limits on MG1 and the ICE now? Toyota aren't publishing those, and I've not attempted any live OBD monitoring myself.

Looking at my own nomograph, it seems to me that a traditional 10000rpm limit on MG1 could be quite constraining on engine power, not letting the engine rev up fully until high speed.

But then as observed on this YouTube review - "it's funny that the full power comes in after like 70 mph".

A couple of other threads discussing details of the G5 PSD that might be interesting to you:

Best OBD? | PriusChat (edit: oops, actually mainly G4, but still might be interesting)
42 mph? | PriusChat

 

I see some discussion about ratio of MG2 rpm to km/h over there.

My calculations - based on the numbers already listed above:

G1 = 36 (rpm / km/h)
G2 = 35
G3 = 63
G4 HEV = 91
G4 PHEV = 103
G5 HEV (19") = 88
G5 PHEV (19") = 94

Main potential area of squiffiness there is that I'm using a "theoretical" wheel circumference based on tyre geometry numbers [pi * (wheel diameter + 2 * tyre width * aspect ratio)]. Don't know how that compares to the actual rolling circumference you'd see under load.

 

Agreed on those calculations, but where are you getting 6000rpm for ICE engine from?

Previous generations have been limited lower - 4500rpm in G1, 5000rpm in G2, and I don't know about later generations.

5000rpm from ICE at 70mph would be 10000 rpm on MG1 - maybe 5000rpm is the limit still.