Does the Prius have a transmission?

Sorry, but this is an incoherent post. If only 1/3 of the power from the ICE can make it to the wheels in the Volt's parallel mode and the Volt is unable to power split the ICE into a series path then where exactly do you propose that the remaining 2/3's of the ICE power goes?

Both cars require the motor attached to the Sun gear to provide that eCVT controlling function. In the Prius this is the smaller motor that is primarily a generator. In the Volt this is the larger motor that is primarily the drive motor. Both cars would go nowhere on ICE power if this Sun gear motor were allowed to spin freely.

The obvious answer is that the Volt is not at all like the Honda IMA system because it does, in fact, power split in parallel mode and it can and does vary the power flow through the series path (somewhat like the Prius) to vary the effective ratios (eCVT). Although the Prius and Volt both use a planetary gear power splitting arrangement the components are hooked up to the gears in a different arrangement which leads to different limitations and optimal paths.

 

And you think this somehow prevents MGa from generating electricity and placing load on the ICE?

When C2 is disengaged, the Volt is in series-only operation. This does not mean that when C2 and C3 are engaged that there cannot also be a series path in addition to the parallel path.

You're hung up on the idea that only the Sun gear can generate electricity but the series path flows in a different direction on the Volt because of how things are hooked up the planetary gears.

 

Best hybrid mpg is probably around 45 mph.

Best EV efficiency is around 20 mph.

 

MGa will never spin too fast since it spins at the same rpm as the ICE which is limited to 4800 rpm.

 

Those maps and graphs show when the Volt's transmission chooses to switch between single motor and combined motor operation etc. That's different from the question of what is the most efficient speed to drive at.

From a US national labs study of the Chevrolet Volt... I question some of the other numbers in it, but these numbers below (from page 2 of the document) look about right and are at least good for making comparisons between speeds. The most efficient speed for EV mode driving is probably around 15-20 mph which would be roughly consistent with results published by Tesla for the Roadster and Model S.

Energy Consumption at Steady-State Speed, 0% Grade

10 mph
219.5 Wh/mi

20 mph
160.8 Wh/mi

30 mph
180.0 Wh/mi

40 mph
207.8 Wh/mi

50 mph
253.1 Wh/mi

60 mph
294.2 Wh/mi

70 mph
361.6 Wh/mi

80 mph
411.4 Wh/mi


http://avt.inel.gov/pdf/EREV/fact2013chevroletvolt.pdf

I haven't seen carefully gathered steady state data for hybrid mode efficiency with the gas engine at different speeds. My guess of 45 mpg is based purely on personal experience as an observant Volt driver. I haven't attempted to measure this carefully.

It might be the case that there is a slower "most efficient speed" as well around the 20 mph point when the EV efficiency aspect of series mode is best. I should try measuring the efficiency of hybrid mode at 24 mph (which is always series mode and is the lowest settable cruise control speed) and 40 mph which is fast enough to switch to power-split.

The key factors are probably aerodynamic resistance (slower is better) and the efficiency of power splitting with near optimal engine rpm and load vs. the conversion losses of series mode operation.

 

If you are redlining the ICE at 4800 rpm at 60 mph you are almost certainly in series mode so MGb is spinning at whatever speed is implied by the planetary gear ratios to the wheels. I haven't personally worked through those calculations and don't have time to do them now.

I'm not sure what you mean about motor vs generator being dependent upon the direction of spin. If that were true then single motor BEVs like the Model S and the LEAF wouldn't be able to do regenerative braking.

My understanding is that the real constraint on power splitting mode is due to the planetary gear ratios and the torque output of MGb vs. MGa and the ICE on the ring gear. In order for MGb to apply all of its output to the wheels, the ring gear has to be locked and the MGa and the ICE have be unclutched from the ring gear. I haven't worked the numbers myself but I've seen sensible sounding people say that.

There is a high level description of this stuff at:

Solving the Volt’s Extended Range Combined Mode Power Split

One if these days, when I have more time, I plan to work my way through the various equations that describe the various aspects of the Prius and Volt transaxle designs.

 

I agree that a couple of those graphs are confusing in isolation. I'll go back over the weekend and look at the full paper in order to place them in their full perspective.