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Toyota details powertrain advances in Gen4 Prius; available E-Four system for all-wheel drive (not for US)

Toyota made significant in its new generation full hybrid powertrain applied in the fourth generation Prius (earlier post), reducing losses, redesigning and relocating key components and reducing weight. Extensive changes to the transaxle, engine and combined hybrid system, generate 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.

In a presentation outlining the changes to the new Prius system, Shunsuke Fushiki from Toyota’s Hybrid Engineering Management Division noted that the new engine contribues about 28% of the improvement in fuel efficiency; hybrid system management contributes about 26%; the new electric motor, 16%; new transaxle and PCU, 13% each; and new battery packs, 4%.

Engine. Although the new 2016 Prius retains the established 1.8-liter VVT-i Atkinson cycle gasoline engine (2ZR-FXE), Toyota completely re-engineered the engine, delivering results in terms of performance, fuel economy, and reductions to size and weight.

The new SULEV engine achieves a high maximum thermal efficiency of 40%. This improvement was enabled in part through the use of a large-volume exhaust gas recirculation (EGR) system and improvements in combustion efficiency and knock control.

Additionally, a redesigned air intake port improves airflow inside the combustion chamber, while the coolant passages have been redesigned to optimize internal engine temperature. The friction created by the piston skirts, rotating parts and the oil pump has been reduced, while the use of low-viscosity oil reduces friction between sliding engine components.


2016 Toyota Prius 1.9 2ZR FXE cutaway. Click to enlarge.

Fuel efficiency is also boosted by improvements to heating performance. An active grille shutter has been added to open and close the grille as necessary in response to outside temperatures. The exhaust heat recirculation system has also been improved and made more compact, lighter and efficient. This recycles exhaust heat from the engine into the coolant to be reused in the heaters and to warm the engine. Cold weather fuel efficiency has also been improved by introducing an engine coolant selector valve and adding a new system which accelerates engine heating.

Transaxle and Electric Motor. Toyota engineers also redesigned the transaxle and motor, delivering a reduction in their combined weight of 6%. The motor itself is considerably more compact (approximately 12%) and gains a better power-to-weight ratio (weight is reduced some 20%). Notably, there is an approximate 20% reduction in mechanical losses through friction compared to the previous model. The planetary gear arrangement in the reduction gear has been replaced with parallel gears, which further helps loss reduction.



2016 Toyota Prius Transaxle. Click to enlarge.

Power Control Unit. The use of a loss-reduction device in the power control unit cuts losses by about 20%. With a more compact (33% size reduction) and lighter (11% weight reduction) design, the unit can now be positioned directly above the transaxle.



2016 Toyota Prius PCU. Click to enlarge.

New Lithium-Ion or Nickel-Metal Hydride Hybrid Batteries. The new Prius will be available with either a new lithium-ion or a new nickel-metal hydride hybrid battery. While the batteries use established technology, Toyota has succeeded in increasing their energy density, which means more power can be obtained from a smaller unit.

The Li-ion pack is 31% lighter, with a 6% reduction in size; the NiMH pack is 2% lighter, with a 10% reduction in size. This in turn has allowed the battery to be relocated beneath the rear seat, increasing cargo space.

E-Four. For the first time, the Prius will feature an available E-Four (electronic four-wheel drive) system, which uses a high-output rear motor to assist the engine and front motor as needed and continuously optimizes the allocation of power between the front and rear wheels.


With a compact size and lightweight design that minimizes any negative impact on fuel efficiency, the system is positioned snugly in the rear of the vehicle to ensure plenty of legroom and luggage space. The luggage area capacity of the model featuring E-Four (with spare tire) is 457 liters, the same as the 2WD model (with spare tire).

Toyota is not offering E-Four in the US.

TNGA and Handling. The 2016 Prius is the first global vehicle to implement Toyota’s New Global Architecture (TNGA). (Earlier post.) TNGA aims to greatly improve core vehicle performance and enhance product appeal through an integrated development program for powertrain components and vehicle platforms.

The Toyota TNGA concept delivers an increase of more than 60% in body torsional rigidity in the new Prius, compared to the previous model, by using Laser Screw Welding (with a greater number of weld points), structural adhesives and uniquely shaped frame structures. In addition, the amount of high-tensile strength steel has increased from 3% to 19%. Combine these enhancements with its new double wishbone rear suspension and a lower center of gravity, and the result is a hybrid with a sporty ride.


2016 Toyota Prius package. Click to enlarge.

New updates to the hybrid system software improve the feel of acceleration, giving a smooth and direct response in a lower rpm range. Toyota has added a new feature to enhance the driving experience when Power mode is selected. An adaptive system continuously monitors the acceleration pattern and lateral G-force readings to determine if the driver is adopting a more enthusiastic driving style. This system will then adapt and implement a sportier acceleration and deceleration.

To improve brake feel and control while reducing noise reduction, the all-new Prius utilizes a newly-developed active hydraulic booster for its regenerative brakes.

Body styling. The TNGA chassis allows for lower lines, while retaining the Prius’s signature triangular silhouette.

The body height has been reduced by 20mm and the high point of the roof has been moved 170mm forward, ensuring plenty of headroom for front seat passengers while also contributing to excellent aerodynamics and a 0.24 coefficient of drag (Cd). The lower center of gravity has also allowed the nose height to be brought down by 70mm and the rearmost section of the hood by 62mm, improving the driver’s forward view.

Available Advanced Technologies. The new Prius will make a significant advance in active and preventive safety measures with the available Toyota Safety Sense P package that includes a Pre-collision System with Pedestrian Detection. The system uses a millimeter-wave radar and a single-lens camera with integrated control to detect both cars and pedestrians. The package also comes with Full-speed Dynamic Radar Cruise Control.

Drivers can also receive parking assistance with the Intelligent Parking Assist, which uses ultrasonic sensors to detect surrounding objects and identify parking spaces. The driver stops the car before the open parking space and by pushing a single button, the system guides drivers to the right position for reverse parking and assists drivers in backing into the space.

Following its unveiling in Las Vegas last month, the all-new Prius will go on display from October 28 at the Tokyo Motor Show, and is slated to go on sale in Japan in December. The new Prius will arrive in dealerships in the US early next year.

Comments

mahonj

It looks like an awful lot of work for a 10% improvement in efficiency. I suppose we r
Still, at least it is better than diesel in terms of local pollution.

Account Deleted

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

Nick Lyons

Cool technology. Styling is polarizing, and keeping the gauges above the center stack instead of the driver is very disappointing. Lowered hip point is going to make this hard to climb into and out of for creaky old geezers (like me). I might be interested in the Rav 4 version.

HarveyD

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.

Rashomon2

On an energy efficiency basis, 55 mpg on a Prius from gasoline is likely a lot better than e-cars that get 100 mpg-e. The latter assumes 100 percent of the heat value of a gallon of gasoline, and there is no way the efficiency of the energy chain in charging an e-car justifies that. You have to look at energy cost of the fuel for the powerplant, powerplant efficiency (topping out at maybe 60 percent), line losses in electric power distribution, charger efficiency (maybe 85 percent), and coulomb losses in the batteries themselves. To do similar calculations for the Toyota, the energy cost of the gasoline refinement and distribution would have to be taken into account. The actual carbon cost of each, particularly assuming non-renewable or non-nuclear power generation, is likely to favor the Prius, particularly given the initial energy cost of the vehicle is certainly lower.

Davemart

Harvey:

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

soltesza

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.

soltesza

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.

sd

I was going to give Toyota credit for going with the rear electric drive giving an intermittent AWD capability but then I read that it will not be available in the US. Maybe it was not suitable (rugged enough) for use in the US. Also, Toyota, it is long past time to dump NMH batteries.

Last winter, I remember passing a Prius going up a snow covered hill in my Chevy Silverado work truck. The Prius was not going to make it and it was not even bad enough for me to switch into 4WD. I probably had better tires for the conditions and a limited slip rear differential but still I had an unloaded RWD pickup. To get around in the winter out here, you need 4 or AWD.

Chip

"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.

GasperG

The problem with plug-in version is that it doesn't offer enough EV power and range. If they make at least 8 kWh capacity and 50 kW EV power then it would be a nice evolution. And if they can keep lover price then other PHEVs then they will succeed.

But looking at pictures I have my doubts where they will install big battery, rear double wishbone doesn't help here.

T2

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 sur 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 Pham

@T2,
I have a little problem comprehending your posting.
IMHO, the principle of Hybrid Synergy Drive in the power split planetary gear set remains unchanged from previous generations of Priuses.

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.

The MG2 always acts like a motor when powered by the engine or the battery. The MG1 always acts like a generator to convert engine power to torque in the MG2.

1) When torque requirement in the transaxle is higher than the engine can put out, then the engine will turn faster than the PSD, and this will spin the MG1 FORWARD faster and generates electrical power to feed to the MG2 to augment the torque of the PSD.

2) When the torque requirement of the transaxle is less than that produced by the engine at peak thermal efficiency, then the engine will turn slower than the PSD unit, and the MG1 will turn BACKWARD to oppose the engine's rotation and forcing the engine to develop high torque, while still generate electrical power to feed to the MG2 so that it continues to turn faster than the engine.

T2

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


T2

So I examined my charts and notes from nearly ten years ago and particularly the equation MG1 = 3.6 X ICE - 2.6 X MG2

and realised that under certain forward driving conditions MG1 does reverse its direction. So the finishing statement from my last post is incorrect and I need to do a reset on that post.

What I should have said was: We have a small disagreement over the operating modes of MG1 and MG2. I think MG1 and MG2 under certain forward driving conditions swap roles and here's why.

Let's say we accelerate the Prius to its electronic speed limit of 100mph. Just before it hits 100mph the ICE will be doing 5000rpm, MG2 will be doing 6000rpm and MG1 will be doing 2400rpm via the above equation. Two other facts the engine will always be made to run at max torque (80lbs-ft) where possible and it takes 49Hp to remain at 100mph (scan guage). Max output of the 1NZ-FXE engine is 76Hp @5000rpm so 49 Hp will require a new engine rpm of 49/76 X 5000 = 3223 rpm. Plugging this into the equation along with MG2 at 6000rpm reveals that :

MG1 will need to rotate at (11603 - 15600 = - 3997) Yes that's minus 3997 rpm so it will need to go backwards as you suggested earlier Roger.

MG1 @ its rated 22lbs-ft will drive as a motor ~17Hp into the PSD. Meanwhile the mechanical output of the PSD which remains steady at ~57lbs feet @ 6000 rpm representing ~ 66Hp. However Mg2 will no longer be augmenting this with 38HP but will now be swapping roles with MG1, MG2 now acts as a generator and will strip 17HP away from the PSD power just before it enters the 4.113 reducer to drive the wheels (23" dia). This will leave the requisite 49Hp going to the wheels needed to maintain the cruise speed at 100mph.

The 17Hp from MG2 now converted to electricity is then supplied to MG1 that is needing it to drive into the PSD as I just mentioned. I think I can now come from out under table !

I apologise for not using metric but a US audience is more favorable towards imperial measurements. You might be wondering where that 38Hp came from. Well the full power of the MY2004 Prius is 104HP ( 76Hp from the engine and 28 Hp (21Kw) from the NiMH battery). 66Hp comes directly from the PSD ([email protected]) and the balance of 38Hp is supplied by MG2 which draws electrical power from the battery (28Hp) and MG1 (10 Hp). Bear in mind that MG1 is only able to rotate at 2400 rpm at this time so generates but a fraction of its 42.2Hp (@ 10,000rpm) in this case just 10.2Hp.

The important thing is that I was assisted in being able to prove the power balance electrically in terms of the volts and amps of the thing thanks to the Oak Ridge National Laboratory posting the PDF of their tear down in the spring of 2004.

It is useful to know that with the pedal to the metal from rest, MG1 eclipses 10,000 rpm as soon as the ICE has zipped up to 2777 rpm. MG1 stays at that rpm until 51 mph. At 51mph the ICE - until then having been mechanically constrained by road wheel rpms through the PSD gearing equation - is able to finally attain 5000rpm and its max power. The ICE remains at constant max rpm and full rated torque until it reaches 100mph the electronic limit. This gives the Prius a performance more akin to a 2.4L engine than its actual 1.5L would suggest.

From 51mph MG1 begins ramping down to 2400 rpm @ 100mph. It should be noted that the fact that traction motor MG2 is only assisting with 38HP as the 100mph is approached is solely a limitation of the HV battery supply. However because the generated power from MG1 0-51mph is 42Hp and merged with the 28Hp from the battery means that the 67Hp motor is well provided for during that initial half of the acceleration ramp.

When you understand the constraints of the Prius Synergy Drive you realise how much better a pure series hybrid could be without those constraints.

One last thing the PSD is really a TSD (Torque Split Device)

The 57 lbs ft is the ICE torque times 2.6/3.6
where 80 X 2.6/3.6 =57.63 lbs-ft

The Sun gear torque of MG1 = ICE Torque X 1/3.6 =22.37
lbs-ft

The 2.6 is the tooth ratio of the PSD = Ring gear / Sun gear = 78/30

The 3.6 is from the effective planetary ratio between the ring and the carrier if the ring is stationary.

Planetary gear ratio = tooth ratio + 1


T2


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.

Sasparilla

Henrik - "However, why can't Toyota drop the outdated and polluting Nickel-Metal battery? "

Probably because there are advantages to the NiMH for a Hybrid vehicle - the primary one being that it doesn't loose capacity over time like a Li one does. If I was buying a hybrid to keep for 10 years and had a choice on the Prius between a NiMH and a Li, I'd take the NiMH as I wouldn't have to worry about its capacity later on and its (and the associated car's) value.

My guess would be the normal hybrid has the NiMH and the plug-in has the Li like before (course if Toyota wants to go anywhere saleswise with the plug-in they need to drastically increase the range offered).

T2

@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.

Roger Pham

@T2,
I'm glad that we're on agreement on the functioning details of the new Prius. Thanks for the wealth of information and your careful analysis on the HSD.

The beauty behind the elegantly simple Prius' new HSD transaxle design is in its reliability, with very few part to fail and to replace. I've heard that the mechanical CVT in many Nissan models aren't so reliable. Toyota is able to achieve both very high efficiency and very high reliability and durability at the same time. The final-drive pinion gear is big and robust, so should be quite durable.

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