Two-Mode Hybrid Explained!

[GM_Fanatic]

Wasn't sure if this had been posted, but it is a very indepth technical explanation of GM's cooperative two-mode hybrid. It actually goes into a pretty lengthy explanation of other hybrids as well.

Regardless, an interesting read if you have the time...and also if you want to brush up on your Deutsch. (The actual article is written in dual languages because of its connection to the German automakers)

http://media.gm.com/us/gm/en/technol...%20VERSION.pdf

The Two-Mode Hybrid is a full hybrid system that
enables significant improvement in composite fuel
economy while providing uncompromised
performance and towing capability. In city driving
and stop-and-go traffic, the vehicle can be powered
either by the two electric motors or by the combustion
engine, or both simultaneously. The Two-
Mode Hybrid can also drive the vehicle using an
input power-split range, a compound power-split
range or four fixed-ratio transmission gears. The
s ystem is fl exible and efficient, with smaller moto rs ,
inverter module and battery that enable numerous
cost advantages. Business cooperation promotes
an even greater potential for further cost
improvement.
The Two-Mode Hybrid system meets customer
interest and mainstream market demands by
providing the positive aspects of both hybrid and
conventional transmission operation in one
package and across a broad range of products
offered by the partners of the Global Hybrid
Cooperation. This strategic organization
recognizes the driving forces behind hybrid vehicle
development and has utilized this motivation to
collaborate the strategy, technology, and the
process by which to achieve such business and
industry advances.
Electrically Variable Transmission Architecture
Selection
Many types of hybrid systems were explored by the
partners of the Global Hybrid Cooperation in
establishing the Electrically Variable Transmission
(EVT) as the basis for the cooperation. EVTs are
currently the dominant hybrid systems in the
market place and possess a number of attractive
attributes that contribute to this leading position.
One of these attributes is smooth and continuous
regenerative braking. This maximizes the ability to
recoup vehicle kinetic energy during deceleration
meeting the high standards for refinement of luxury
brands.
Another benefit of the EVT is that it has the ability
to continuously vary the speed of the engine, like
the mechanical Continuously Variable Transmission
(CVT). The EVT can also store energy electrically in
a battery. The CVT action combined with the ability
to store energy electrically offers the unique
opportunity to maximize overall system efficiency
by enabling the operation of the engine at optimal
speed and load points. The ability to manipulate the
operation of the engine in this fashion is also
synergistic with the cylinder deactivation systems,
thus allowing the engine to increase operation with
deactivated cylinders, thus reducing fuel consumption.

[mikesright]

It truely is ingenius, isn't it! You can live a normal life, tow, haul, etc, then when you're commuting the other 90% of the time, you get much improved city AND highway mileage. Utilizing Active Fuel Management more of the time, by using the hybrid system to make more power when you need it was a great innovation, and multiplies the effects of both! Eat dust on the REAL hybrid system, Toyota!

[01cavalier]

The system really does do a lot more, making the use of both a variable transmission and a regular geared one. Am I correct in understanding that the part of the transmission with traditional gears is a four speed?

[mkaresh]

Okay, my head is now killing me, but I'm beginning to understand how this system works...

(Revised the next day, head no longer hurting so much.)

Essentially, they've taken the sort of system used in Toyota's more powerful hybrids and added an additional planetary gearset to the mechanical output path, essentially a three-speed automatic.

Info on planetary gearsets:
http://auto.howstuffworks.com/automa...nsmission3.htm

The main piece that remains unclear to me is the source of the fourth gear--I think this is provided by the third plenatary gearset which the paper doesn't directly discuss. It's also possible that all the gears are provided by the second and third gearsets working in tandem, though the paper implies otherwise. The system in the Lexus GS is most similar, but it provides two potentially fixed ratios to the GM system's four. Because these two are so widely spaced, they probably cannot be realistically be used as fixed ratios; instead the CVT probably shifts simultaneously to minimize the change in engine rpm.

A final tweak is that it appears that the first motor might be able to provide propulsion and not simply serve as a generator.

More detail, walking through from the engine to the wheels...

If you don't know how plenetary gearsets work, you should read the article at the above link. They're quite nifty. They have three elements, which can potentially be hooked up to any mix of up to three inputs and outputs. Alternatively, one or two elements can either be permanently fixed in place or temporarily fixed in place using a clutch. In both the GM and Toyota systems the engine and one electric motor are attached to a planetary gearset, with the output shaft attached to the third element. The direction and rate with which the electric motor turns (or, in generator mode, is turned) determines the ratio between engine rpm and output shaft rpm, such that this gearset functions as a CVT. (CVTs in non-hybrids and Honda hybrids are a totally different type using a belt between two pulleys.)

(BMW's active steering system similarly uses a planterary gearset to continuously vary the steering ratio.)

The primary argument of this paper is that Toyota's system is inefficient because it generally channels a portion of the engine's output into electricity, and converting power into electricity and then back into power is only about 70% efficient, vs. the approximately 90% efficiency of a conventional mechanical transmission. Why does the Toyota system do this? Because it is relying on that electric motor to do two things: generate electricity and vary the transmission ratio. As a result, whenever the electric motor is operating to vary the transmission ratio, it is also generating electricity whether this makes sense or not.

The only situation where this is not the case is when the electric motor is not turning, and thus is essentially acting as a clutch on its element of the plantary gearset. In this situation, all of the engine's power goes through a purely mechnical path to the wheels. But if the electric motor is not turning then you essentially have a one-speed transmission, and as with any single transmission ratio it's only providing the proper balance of power and efficiency within a narrow range of speeds.

I don't know which speed this is for the Toyota system, but I'd guess it's the highest ratio, and thus when cruising along. Conversely, whenever the car is accelerating at low to moderate speeds, and thus a lower ratio is called for, a portion of the engine's output, perhaps a good portion, is channeled into electricity generation.

Hybrids don't have huge battery packs. If a lot of electricity is being generated, then if it's not to be wasted it must be immediately used. So the Toyota system requires a large, powerful electric motor to use up this electricity. As a result, at low speeds under heavy acceleration much of the power at the wheels is likely coming from the electric motor.

If the paper is correct that this path is inefficient, then whenever the car is accelerating a pretty hefty price is being paid for the CVT functionality.

The GM et al solution: add a second planetary gearset, perhaps even a compound planetary gearset (see article linked above), to the output shaft. This planetary gearset, like those used in most conventional automatic transmissions, has one input and one output. So it functions as a fixed ratio transmission much like a conventional automatic, not as a CVT. Assuming the electric motor generator is stopped, forcing the first gearset to function as a one-speed transmission, when combined with this second gearset it becomes essentially a three-speed automatic.

However, in many cases the motor/generator will be used to some extent. When the second gearset is in first gear, the system is pretty much identical to a Prius drivetrain. This is what the paper calls "input-split EVT mode." (EVT means simply a CVT based on a planetary gearset with ratios determined by an electric motor.) Shift the second gearset into second or third, and you've got the "compound-split EVT." I think the "compound" simply refers to the involvement of two planetary gearsets. The advantage here is you now have three ratios where the electric motor can stop and permit all engine output to follow a mechanical path to the wheels, and even in hybrid mode the proportion routed along the mechanical path will generally be higher, as the CVT doesn't need to operate along as wide a range. (The further you get from the fixed ratio, the more that generator is being turned.)

In both the GM and Toyota systems, elecricity is turned back into power by a second electric motor. Output from this motor joins with output from the engine. In some Toyota systems and in the GM system as well power is then routed through another planetary gearset. In both cases there's just one input shaft, and the planetary gearset essentially operates as a two-speed automatic. I think that by shifting this gearset the GM system produces its fourth fixed ratio. Because the GM system generates less electricity, this motor can be smaller.

My first reading of the GM paper guessed that the final planetary gearset was upstream of where the electric motor's output joins the engine's output, but after reading Lexus' even less detailed description of its system in the GS I don't think this is the case. So the Lexus system isn't as different from the GM system as this paper implies. However, it only offers two fixed ratios to the GM system's four, and doesn't explicitly operate in conventional automatic mode, probably because it has fewer ratios to play with and they're very far apart, 3.9:1 vs. 1.9:1. The entire Lexus system--CVT, two motors, two rather than three gearsets--is housed within a single transmission-shaped casing just like the GM system; so the GM system is not doing anything new in this regard.

In both systems the second electric motor handles regenerative braking. Only in the GM system can the first electric motor operate in propulsion as well as generator mode.

Cost and weight tradeoffs are hard to assess. There's an additional planetary gearset (two additional compared to the Prius), but the second electric motor is smaller. My first thought was that three sets of planetary gears involves a lot of cost and complexity, but the new six-speed automatic developed with Ford uses three planetary gears.

I'm sure I'm misunderstanding or missing some essential pieces here. Anyone else care to take a whack at it?

[hammerhead]

You hit the nail on the head.It's all due to physics,the planetary gears diameter is equal to the circumfrance of the electric motors main gear ratio-thus increasing performance without boosting cost!!!!!

[WishIhadatruck]

That was very interesting. Obviously it has been too long since I've studied gears since I kept forgetting what each kind is. Oh well, it was fun to read anyway and can't wait to own one someday. When I wear my wagon out (several years from now) I hope I can get a good deal on a used 4wd Suburban with this system. It better be E85 at the same time. Can't wait to see what real world mpg turns out to be.

[jlt0x]

OK, great. My wife will say, "So what! Put it in a 300C or STS with AWD and lets see how well it works."

We know it's an ingenious invention, but the pubilc wants more applications and "real-world" tests and figures to back up this monumental collaboration. We'll see the 2-mode hybrid trans in a Durango & Tahoe. We want a scaled-down version in an Impala SS, 5-series, and an E-class. Bring-it-on.

[johnny smallblock]

So GM designed a revolutionary hybrid system that sits inside the transmission, but they made the tranny a 4-speed?

Please tell me I'm misunderstanding something!!!

[mkaresh]

It's far from clear under which circumstances and how often this transmission will operate like a conventional automatic. As pointed out in my revised lengthy explanation, the Lexus GS has a similar hybrid system within a case similar to that of an automatic transmission. The difference is that the Lexus has only two very widely spaced ratios to combine with the CVT, while the GM transmission has four. Four combined with a CVT should be plenty. By combining four fixed ratios with the hybrid system's CVT, there should be a very broad spread of ratios with a large continuous range for each fixed ratio.

Theoretically this system is quite interesting, and should have significant benefits. But the proof is in the execution.

The Honda system isn't discussed. It uses a single electric motor and purely mechanical path (its CVT is the usual pulley type, not a planetary), so it avoids the inefficiencies attributed to the Toyota system. The problem with it is that the vehicle cannot practically be driven based on the electric motor alone, and this motor tends to be much smaller than that in the Toyota system. Much cheaper and simpler that the Toyota or Dual Mode system, but not as good for driving in bumper-to-bumper traffic. Probably about as good in typical suburban conditions, though.

[kenman923]

I hope something like that makes it to market.

[GM_Fanatic]

Originally Posted by kenman923

I hope something like that makes it to market.

No sense in hopeing, it WILL be in the 08 Tahoe.

[crooner214]

Thanks for the explanation, Mkaresh, but now MY head hurts!

[pinyon]

What make my head hurt is thinking of all the technology that our beloved General has. Just think of a class leading Malibu with...

2.8L HF V6 VVT AFM DI E85 Dual Mode Hybrid that got 40+ mpg with great performance

Even if they used the 3.5 HV will they please just make it and bury the competition.