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GM develops prototype two-mode hybrid transmission with enhanced EV capability for plug-in hybrids

Plug-in hybrid operating modes with the 2MH4EV architecture. From Yang et al. Click to enlarge.

GM has developed a prototype of its two-mode front-wheel drive (FWD) hybrid transmission (earlier post) with enhanced electric vehicle capability (2MH4EV) for application in a plug-in hybrid; GM presented a paper detailing the work at the SAE 2011 World Congress in Detroit.

The prototype drive unit includes an additional input-brake clutch to the existing FWD 2-mode system, enabling two new electric-only modes in addition to the existing EVT (electrically-variable transmission) modes and fixed gears. The two new electric-only modes have the capability to cover the entire FTP urban cycle with a properly sized battery pack, GM says.

GM’s base FWD 2-mode system incorporates two electric motors, two planetary gear sets and a system of four hydraulically-applied wet-plate range clutches to deliver two continuously variable speed-ratio ranges (modes) and four forward fixed gear ratios. The first continuously variable mode (EVT1 or M1) covers the higher speed ratio range, while the second continuously variable mode cover the lower ratio range. The second fixed gear (FG2) is located at the ratio of the synchronous transmission point between EVT1 and EVT2. Synchronous mode transition can be smooth because the shifting clutches can be pre-synchronized in speeds by using the electric motors.

The basic 2-mode transmission has two electric drive modes (i.e., zero engine speed) EVT1 and EVT2. Due to system constraints, only the EVT1 electric mode is available at lower vehicle speeds; this low-speed electric-only mode is called EV0. EV0 has limited speed range and acceleration capability.

The 2MH4EV architecture adds an additional input-brake clutch (CB5); the damper by-pass clutch in the base transmission is removed, and its hydraulic control circuit is reused to control CB5. With this, there are two new electric-only modes: EV1 and EV2.

In EV1, the transmission is in the first continuously-variable mode EVT1 with both brake-clutch CB12R and CB5 applied. With CB5 applied and providing reaction torque at the first planetary gear set, both motors can provide positive propulsion or negative regenerative braking torque, thus enhancing electric drive capability.

In EV2, the transmission is in the second continuously-variable mode EVT2 with both rotating clutch C234 and CB5 applied, again enabling both motors to provide positive propulsion or negative regenerative braking torque.

Overall, EV1 has a higher output torque capability, with better efficiency in the lower speed, higher torque region. It will be used as a low-speed, electric-only mode. EV2 has better efficiency in the higher speed, lower torque region, and will be used as a high-speed electric-only mode.

In addition to the existing engine-on operating modes EVT1, EVT2 and fixed gears FG1, FG2, FG3 and FG4 as well as the low-speed electric-only mode EV0, the two new electric-only modes EV1 and EV2 cover the entire low-torque, low-speed region assuming a sufficient amount of battery power.

—Yang et al.

GM developed the controls for the enhanced system in a dynamic simulation model, then implemented them in the production hybrid software environment and tested them on the hardware in the loop bench, and ultimately tested and refined them in the vehicle.


  • Hong Yang, Anthony Smith, Shawn Swales, Joel Maguire (2011) Development of Two-Mode Hybrid Powertrain with Enhanced EV Capability. (SAE 2011-01-0883)


richard schumacher

Four clutches and two planetaries. How could this be better than, or even as good as, Toyota's HSD?


@richard schumacher

"Four clutches and two planetaries. How could this be better than, or even as good as, Toyota's HSD?"

Simply. It is more efficient and probably works better. Also, this is not more complicated mechanically than a 6 speed automatic transmission other than it contains the electric motors. The control algorithms are probably more complex but that is basically software.


GM is really digging into Chevy Volt gearbox development. IMHO architecture is getting more complex and expensive instead of officially stated target of cost reduction by $7 500 per unit.

Stan Peterson

I am glad to hear that GM is still developing another generation of the two-mode hybrid for FWD applications.

In the process of downsizing the hybrid transmissions system from a bus and large truck application to an automobile application, we saw the first generation as suitable only for large pickup trucks and SUVs. It was just too big and heavy for automobiles.

That first genenration was only offered in a RWD form and is still available for GM trucks. A still-borne version for the VUE# SUV was briefly introduced and withdrawn.

Now a second generation version in FWD format for automobiles is apparently going forward. Furthermore it appears that it is being modified to incorporate and utilize what a larger PHEV-sized battery allows.

The Toyota HSD is relatively primitive, and constrained for which vehicle sizes it can be applied. If implemented finely, this GM hybridization offers significant improvements for a much larger range and size of vehicles to which it may be applied.


After more than 10 years of Prius sales, it is good to see something that might be used in the Malibu. I would imaging patents played a big part, Ford Escape and Nissan Altima both licensed from Toyota.


SP...did you know the Toyota's Prius won first place for the most efficient drive train?


This is a variation of the transmission used in the Volt..

Toyota also has that type of transmission, its used in their higher power Lexus hybrids, called Lexus Hybrid Drive.. it basically blends two mechanical gear sets with a Prius type transmission.


KISS = keep it simple and stupid.

This overly complex GM system might look good in the sales brochure, but the Prius HSD is a wonder of elegant simplicity that vehicle owners will thoroughly enjoy when their vehicle ages. GM owners otoh will see skyhigh repair bills.

richard schumacher

Anne has nailed it. Long-term reliability of this thing does not look good.

Stan Peterson

Harvey D,

The Prius drivetrain is NOT the engineering end-all and be-all. It is merely a good first attempt. So it won an award. BFD.

By Whom, and For What? If awarded by the Toyota Prius Dealers it isn't worth the proverbial "...bucket of warm Spit..." On the other hand an award by the SAE, a peer group of Engineers, means a great something.

The Prius fanboys always seem to crawl out of the woodwork when any advance seems to threaten their "baby". Still the limitation of HSD, seem to be threatening and delaying the PHEV version of the Prius; and the fuel economy numbers don't ever seem to get to triple digits for what is still only an aerodynamically optimized, C-segment, HEV-oriented hatchback.

The PHEV-oriented Volt, despite EPA denigration including changing their own measurement standards im mid-stream, seems routinely capable of producing gasoline fuel economy in the triple digits, without hyper-mileing or other grotesque and unusable schemes.

Isn't that the real issue? Isn't the issue Hydrocarbon consumption, availability, economic blackmail, and general sustainability now that toxic pollution is essentially solved, and no longer a genuine concern?


Just because the verbal description sounds complex, I can not conclude that this device will be unreliable. The previous story showed a picture and I would say that it is probably well thought out. The fact that they have to go around a patent mine field does not help.


4 clutches can't be good. The current two mode hybrid has lag in power delivery while shifting between the two modes (around 40mph). Search "escalade hybrid 0-60" on youtube and see the video uploaded by mpgomatic.


@ANNE +1

Not to take away from her comments but Anne has disclosed elswhere that she happens to drive a Prius !!

The two mode has been discontinued by GM and is not currently used by Chrysler or BMW either. I believe these were the three companies that shared in the initial development. Hardly encouraging...

OTOH it bears mentioning that even the more complicated Lexus hybrids ship in quite low numbers. Lexus buyers are loathe to risk reliability issues more so since they would be exclusively locked in to Lexus repair centers and Lexus prices.

Increasing the amount of mechanical engineering as this new GM system employs is a move in the wrong direction. In the automobile industry it is well known that the addition of sophisticated electronic interfaces into modern automatics have made them increasingly harder to diagnose. Dealer garages are reluctant to expose themselves to warranty repair and are turning towards service policies which recommend complete replacement with factory refurbished transmissions. At this point the owner of a vehicle more than 8 years old may choose to avoid a significant expense and abandon the vehicle.

So regarding the Prius, though it may still be one of the most complicated vehicles on the road, it does avoid use of hydraulic shifing of a multi-ratio gearbox and the associated clutch mechanisms both of which have the usual wear issues.

In short, no matter what you would think, the complexity of the Prius actually makes it more reliable. And I and a few others didn't see that coming either.


The auto industry is a tough one, it is capital intensive and competitive. They have to produce devices that last a long time under a severe service life.

It is much easier for investors to just flip houses or bid up the price of commodities in hedge funds. Manufacturing is seen as too risky and if the car industry did not exist I am not sure it would be started in the U.S. 100 years later.

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