I applaud Toyota for making the most efficient gasser on the market. With a 55 mpg EPA rating it will nearly be as efficient as Toyotas small FCV that gets 60 miles on one kg of hydrogen (costing 14 USD per kg) that is comparable to the energy content in one gallon of gasoline. Off cause, Tesla's Model S gets 100 mpge, Model X gets 92 mpge and the Leaf is 114 mpge.

However, why can't Toyota drop the outdated and polluting Nickel-Metal battery? Also the air coefficient of 0.24 is exactly the same as Tesla can do for their new SUV Model X. Toyota may not be bad but it tells me that Tesla did incredibly well with the aerodynamic design of that Model X.

1kg hydrgen = 1 gallon gasoline
http://www.hionsolar.com/n-heq1.html

Model X air drag of 0.24
http://www.teslamotors.com/modelx

This seems to be a very well engineered HEV with 55 to 60 mpg efficiency depending on drive mode used. Too bad that top of the line Lithium batteries are not used.

The current first generation (rather large and very heavy) Toyota FCEV gets 67 mpge (that's about 3X the average best ICEVs). A second smaller, lither generation, with less weight and drag, will probably get 80+ mpge by 2020 or so. Honda's new FCEVs may even do better.

VW (and many others) will most probably mass produce 500+ Km mid-size affordable BEVs before 2020 to compete against current diesel units.

Harvey:

' The new Prius will be available with either a new lithium-ion or a new nickel-metal hydride hybrid battery.'

This is all nice and well, but it won't be enough for me to buy a Prius again (even if I am very satisfied with my second gen Prius).

My next car will most likely be a full electric one.

I think Toyota doesn't understand its customer base, this is why they are going to bleed a lot of enviroment-oriented customers in the near future. Current Prius owners will simply ignore their FCV scam and buy a BEV or EREV from an other manufacturer.

If Toyota had even a half-decent EV, I would probably go with them because I like their fantastic quality. Given their lack of EVs, I will have to settle for a Nissan, a VW or maybe Tesla.

"an expected improvement in EPA estimated MPG on core models of up to 10% (i.e., up to ~55 mpg US combined), with an Eco model that will achieve an even greater improvement".
The fuel efficiency is impressive.
Also, it sounds like Toyota has improved the driving experience.
However, improving aerodynamics might adversely effect sales:
Front seat lowered 59mm - harder for elderly Prius owners to get into.
Zero improvement in rear headroom - does not appeal to families with tall teenagers.
The big disappointment is no mention of a PiP & the design changes do not provide space for a larger battery pack. The hybrid pack is small & squeezed under the rear seat. The trunk floor is lower & the under-floor storage seems to have gone.
Toyota needs a 10 kWh or larger battery to compete with the Hyundai Sonata PHEV or Chevy Volt.
It will be fascinating to see whether Prius HEV sales continue to dwarf PHEVs & EREVs or whether Hyundai & Chevy can tempt large numbers of Prius owners to switch.

This being the fourth generation of Prius and inline with those previous releases, Toyota policy once again requires a one year delay before introducing it to North American shores in order that the new manufacturing tooling can benefit from the experience as well as any possible vehicle improvements that may be forthcoming from the thousands of early-adopter Japanese motororists. An insider once told me that as a rule new model designs, as opposed to yearly refreshes, are an opportunity for Toyota to discard all their machine tools, robots etc and bring in new computers with the latest operating systems. This ensures their manufacturing environment remains both current and maintainable being that it is often difficult to obtain spare parts for legacy equipment that is more than five years old.

The computer generated model of the transaxle doesn't agree with the photograph. Seems to be upside down and back to front !

So what's new in real terms ? Well what I noticed is that the traction motor, referred to as MG2, is directly geared onto the final (differential) gear which, as we know, rotates at the same speed as the wheel axle.

The previous model (MY2010) coupled MG2 to the power-split device (PSD) speed using a planetary gear arrangement similar to the PSD itself. This particular planetary gave a 2.5 ratio step down allowing the motor to rotate as fast as its 15000 rpm limit but able to be coupled to the PSD output which maxxes out at 6000rpm. The PSD then transmits the power through a two stage reducer to give an overall 4.11 stepdown to the wheels, where the ~1500 rpm approximates to about 100mph.

But now, that is all changed. The topology of the new generation gear path is to route MG2's power directly onto the final gear thus avoiding the more tortuous path through the motor planetary and the two stage reducer. Obviously this allows elimination of the motor planetary altogether as well as eliminating the 6% torque loss from that component and the 7% loss through at least one of those stages of the existing 4.11 reducer. From the photograph the relative sizes of the final gear and motor pinion suggest a 10:1 stepdown ratio may be in effect so expect MG2's rpms to continue to be as high as before.

MG2 acts as a motor while accelerating in the forward direction, it also acts as a motor when vehicle operation is required in reverse. And of course MG2 acts as a generator when acting as a brake in regen mode. At all other times it is acting as a generator which includes travelling at a constant forward speed on a level road. It is required to behave as an electronic brake to strip off surplus power coming from the PSD and feed it to MG1 (the generator) which acts as a motor and assists the engine to provide maximum torque through the PSD. This is the true Prius action, its purpose is to make the engine think it is always climbing a steep hill at 18mph or whatever when reality is that it is travelling at 60mph,say, on a level road. The engine is made to run at full torque,wideopen throttle, but at the lowest rpm that will provide the power to move the vehicle at whatever roadspeed is desired. These extra low rpms are commensurate with obtaining exceptionally low frictional losses in the engine which equals great mileage.

There is going to be some debate about this latest design move which will include lots of talk about virtual power. Virtual power that can always be made to exist when two electrical machines are connected back to back with their shafts coupled together. The problem here is that virtual power does not accrue virtual losses. That would be nice. On the contrary it generates real losses. In this case even more losses than the earlier designs since the virtual power loop extends from the PSD right down through the two stage reducer to the final gear iself and then comes back to MG2 through MG2's pinion.

To recap. In the first two North American generations of Prius (MY1999 and MY2004) MG2 was coupled directly to PSD planetary ring gear.
Zero mechanical loss in this path obviously. But MG2 limited to 6000rpm meant a high torque motor with large frame size required.

In MY 2010, MG2 was coupled in via its own planetary. (the planetary carrier being locked stationary with gearbox frame) The ring gears of the two planetaries were joined together. A real loss attributed to virtual power would now occur when the virtual power passed through the motor planetary when MG2 was in its generating mode.

In MY2017 as stated above .....virtual power loop extends from the PSD right down through the two stage reducer to the final gear iself and then comes back to MG2 through MG2's pinion.

Roger I am sure that you don't want to spend much time on this either.

I'll agree that Toyota has not made any changes to its original concept regarding the PSD operation, so explanatory arguments should remain unchanged.

Incidentally I found a small error in your second paragraph. The chain drive was in fact removed going to the third generation while your statements make it appear to be still present on the third generation.

Anyway I think we both agree that MG2's planetary first appeared on gen three and has now been removed going to gen four.

In my recap I apologise for omitting to mention anything at all regarding the sprockets and gear chain because their removal doesn't change the geartrain direction in the same way that knocking out a shaft with two gears on it would have done and besides the ratio was close to 1:1. ( a 35 tooth to a 36 tooth I believe). Tyring to keep things simple that's all.

We have a small disagreement in the direction change of MG1. I think it doesn't and here's why ... to be continued

Correction:

The 3.6 is the effective planetary ratio between the sun gear and the planet carrier if the ring is stationary.

I forgot to point out that the 17Hp (in this particular example) that is sent to MG2 mechanically from the PSD and then returned to MG1 electrically is the virtual power I was mentioning from an earlier post.

There may be some who may be interested in the software that controls this system. I cannot comment on the coding of the ECU but I can say that the accelerator pedal controls the engine speed which represents power unlike other cars on the road which are motivated by variable torque demands.

The Prius engine's torque is controlled by MG1. The system knows that MG1's line current should be 100 amps when MG1 is exerting 22 lbs-ft at the sun gear. And 3.6 times 22 = 80lbs-ft, the rated torque of the 1NZ-FXE engine. To keep the system in torque balance the ring gear takes remainder, in fact the lion's share of the torque which would be 2.6/3.6 of the engine's 80lbs-ft = 57.63lbs-ft to be exact.

As I said ealier the power split device is more accurately described as a torque split device but outside the engineering power split is close enough.

The fuel injection system delivers the amount of fuel to the engine that will cause MG1 to hold its speed when delivering or receiving 100 amps. A bit of a controversial statement but it is in one of the Prius' patents that engine torque is governed by monitoring MG1's line current.

There is a rule pertaining to MG1. There must be a guard band around MG1's operating speeds when close to zero rpm requiring MG1 to rotate at a minimum of 100rpm in whichever direction it happens to be turning. May be something to do with low speed cogging. Naturally the engine speed will be adjusted away from optimum to accommodate this.

There is a minimum engine speed also. My notes mention that the engine is parked at 1211 rpm for power delivery of up to 8.5kw @ 67n-m beyond which the engine speed must be raised to take advantage of 80lbs-ft. This is probably due to the effect of the Atkinson camming on this engine that limits low speed torque.

The Prius is undoubtably the most complex car on the road. It is also the most reliable since it doesn't rely on an electro-hydraulically operated multi-ratio gearbox. Even those in the transmission repair industry have come out with statements that around fifty per cent of modern automatic gearboxes have faults which are uneconomic to repair. On the other hand the Prius system is alleged to lend itself to modular repair. Anyway that's enough with the commercials.

@Roger Regarding the gear train I agree with your description after having expanded the computer rendition which was a bit fuzzy on my monitor.
You wrote :

The changes are that the planetary gear reduction of the MG2 in the Gen 3 Prius is removed, as well as the drive chain from the power split device to the differential unit, replaced by a pair of simple helical spur gear sets to reduce drag, weight, and cost. Both the MG2 and the PSD are meshed to a larger spur gear of an intermediary gear set, the latter having a smaller spur gear meshed with the final gear of the differential unit.

I can now see that the (yellow) intermediate gear rotates the (orange) pinion, keyed on its shaft obviously. This pinion then drives the (blue) final differential gear and (not shown) half shafts assembly. Your description is spot on. My understanding wasn't helped by the way Toyota described it on their website :

"The transaxle also boasts a more compact design, and achieves approximately 20 percent loss reduction compared to the current Prius. This was made possible due to multi-shaft positioning of the motors, along with the relocation of the final drive reduction gear onto the same axis as the drive motor."

It looks like the pinion drive permits the ~ 4:1 ratio to be accomplished in one step from the pinion to the final gear whereas it used to be accomplished in two steps consisting of two gearsets of 30/44 times 26/75. At least they haven't lengthened the critical path between MG2 and the PSD as I had originally thought. Incidentally, mesh efficiences for gear ratios of ~1:1 are known to be 95% and for ~1:3 they hover around 93%. The original transaxle had been known to have an efficiency of 22% so this is welcome news. The chain drive used to absorb 6% and now there is this 7% saving from eliminating yet another gear interchange.

Back in 2005/6 some hypermiling enthusiasts were fitting electric pre-heaters to their transaxles to raise the transmission oil temperature to help reduce gear friction during the first two miles. In case any one is interested the PRIUS_TECHNICAL_STUFF thread documented this in Yahoo Groups.

The question now concerns the longevity of the pinion that is fitted to MG2. In the previous design MG2 was sped up ~2.5 times in order to downsize it. At that time its motor pinion became the sun gear for the planetary reducer that is now removed. One advantage of a sun gear is that as it sits balanced between the gears of the planetary gearset it is protected from lateral bearing forces, as the forces from all those gear meshings are neutralised from top, below and the sides. When there is just a single point take-off as is now then this puts lateral pressure both on the motor bearing and on the bearing at the other side of the pinion. That there is some overhang on the pinion shaft because of its length can be observed.

Not much more to be writ, there has been no rebuttal over the Virtual Power operation of the Prius and I take it that no-one has been offended by my description thereof.