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GM Previews First Two-Mode, Front-Wheel-Drive Hybrid Transaxle

by Jack Rosebro

2modefwd2
General Motors 2MT70 FWD two-mode hybrid transaxle, as seen from engine side. Click to enlarge.

At the recent SAE 2009 World Congress in Detroit, General Motors engineers presented the first front-wheel-drive (FWD) iteration of their two-mode hybrid transaxle, termed the 2MT70 (earlier post), scheduled for its first application in the Saturn 2009 VUE 2-Mode Hybrid light sport-utility vehicle later this year (earlier post).

The two-mode design, itself originally adapted from a hybrid transmission designed by Allison for transit buses, has up to now been exclusively incorporated into rear-wheel-drive (RWD) transmissions with the intent of reducing the fuel consumption of heavier and larger passenger trucks and SUVs while preserving towing capacities.

Expanding The Two-Mode Portfolio. The 2MT70 transaxle, which is extremely compact at just 381mm (15 inches) in length, has the potential to bring two-mode architecture to a much wider range of vehicles. The unit is designed to be scalable across a range of FWD vehicles with engines ranging from a 2.0L four-cylinder turbo-diesel to a 3.6L direct injection, spark ignition V6 gasoline engine, while requiring no more space than a conventional six-speed automatic transaxle.

GM Developing Additional Full Hybrid System for Smaller Vehicles
With the FWD two-mode targeted at larger sedans on up into crossovers and SUVs, and the RWD two-mode applied in full-size SUVs and light trucks, GM has a product gap for an advanced full hybrid offering for smaller, lower-torque engines in more compact vehicles.
To address that, GM is developing a third full hybrid system targeted at those applications, said Larry Nitz, GM Executive Director of Hybrid Powertrains, during an interview at SAE 2009 World Congress.
Nitz would not disclose if the third full hybrid system is a direct extension of the two-mode transmission, only saying “Stay tuned.”
Nitz also noted that the coming next-generation of the mild GM Hybrid System (earlier post), which in its current form is applied in the Malibu, VUE and Aura, will offer equivalent power to Honda’s IMA system in the new Insight.
Nitz also said that the next-generation GM Hybrid System was “aligned with Honda’s perspective” on cost-effectiveness and performance.
—Mike Millikin

As the acceleration of a heavier vehicle requires more torque, and as the two-mode design uses drive motor B exclusively to produce all reverse torque, scaling the transaxle for a larger or faster vehicle would typically involve extending the length of drive motor B and redesigning the end casting of the transaxle to accommodate the required increase in motor length.

Design parameters of the current design include a 50% improvement in EPA composite fuel economy as compared to a conventional Saturn VUE, as well as a 7.5 second 0-60 mph (0-97 km/h) acceleration performance, as compared to a 6.9 second acceleration for the baseline vehicle.

Transaxle Operation. The transaxle incorporates two 55 kW active-cooled AC motor-generators, each connected to a planetary gearset and capable of providing either (1) a continuously variable speed and gearing range, or “mode”, or (2) motive power. The first mode blends power from the IC engine and the electric motors according to demand at up to 40 mph (64 km/h), while the second mode controls engine gearing above 40 mph.

Conventional series-parallel transaxles and transmissions employ one planetary gearset to provide variable speed and gearing outputs. While a second planetary gearset is sometimes used, it is typically fixed or two-speed. GM refers to such designs as “single mode”.

By comparison, GM’s two-mode system uses a combination of engine speeds, motor speeds, and a system of clutches to manipulate the planetary gearsets and provide two variable ranges, as well as a compound split in which the two ranges influence one another.

A compound split powerflow allows either motor-generator to control gearing by varying its speed; the other motor can then act as the vehicle’s traction motor. The clutches can also be used to create four fixed forward gear ratios. Two of the clutches are capable of braking, while the remaining two are conventional on-off units.

Motor-Generators. To maintain the same form factor as that of a conventional FWD transaxle, yet preserve the co-axial arrangement of the 2MT70’s two motor-generators, project engineers had to design electric machines that are as thin as possible, yet provide adequate torque.

While practical rotor and winding diameter is limited by available tunnel space in RWD series-parallel hybrid transmissions, restricting motor-generator performance and requiring elevated rotor speeds and compound gearing, the physical constraint for motor-generators in narrow FWD transaxles is the length of the rotors as well as the stator windings.

To make the transaxle as longitudinally compact as possible, GM engineers specified concentrated windings for both electric motors.

Hybrid and electric vehicle motor-generators normally use stators that are wound as either distributed or concentrated windings. In distributed windings, the coils of an AC motor’s stator winding span more than one tooth of the stator core. Distributed windings have often been preferred in the past for traction motors due to their low noise and smooth torque.

However, distributed winding coils extend considerably beyond the stator core on which they are wound. Ultra-thin motor-generators, such as those mounted between the engines and transmissions of Honda hybrids, utilize concentrated windings which have one coil wound around each stator tooth in a solenoidal configuration. Concentrated windings are becoming more common in electric drives: for example, Toyota departed from their usual practice of using distributed windings to provide the generator (MG1) in the 2010 Toyota Prius with concentrated windings.

Damper-Bypass Clutch. In addition to the two clutch-brake assemblies and two rotating clutch assemblies, the 2MT70 uses a damper-bypass clutch to reduce vibration during engine spin-up and shut-down. As with most series-parallel transaxles, the 2MT70 employs the rotational inertia and spring action of a damper disc, mounted between engine and transaxle, to filter out high-frequency engine torque oscillations while transferring mean engine torque.

To minimize vibrations caused by the damper itself, many hybrid powertrains spin the engine up aggressively during engine start to minimize damper oscillations. Some hybrid powertrains also employ pressure pulse cancellation, energizing a motor-generator to oppose excessive driveline vibrations caused by engine cylinder firing events. However, such solutions can restrict engine start/stop control strategies as well as motor-generator control strategies.

2MT70 project engineers elected to develop a third solution: a damper bypass clutch that directly couples the engine and transmission during stop/start transitions. This, combined with a pressure pulse cancellation strategy, was found to provide smooth operation. The bypass clutch is released during normal operation, restoring the damper function.

Saturn’s Fate
In the revised Viability Plan accompanying the bond exchange offer (earlier post), GM accelerated the timing for the resolution of its strategic review of Saturn to 2009 from the earlier 2010 to 2011 timeframe.
GM is currently reviewing purchase proposals for Saturn, but noted that if a sale does not occur, it intends to phase out the Saturn brand by the end of 2009. The current viability plan does not include production and sales to dealers of Saturn products beyond 2009.

GM representatives indicated that the two-mode technology will stay with the company regardless of the fate of brands such as Saturn, and that they are already working on the second generation of two-mode drives in both RWD and FWD variants, with a particular emphasis on cost-cutting.

Although company representatives declined to disclose the production cost of the 2MT70—one engineer joked that the unit is “just south of ‘prohibitive’”—the projected price for the Saturn VUE two-mode hybrid is expected to retail for less than US $30,000 when it goes on sale this summer.

Resources

  • Hendrickson et al.: General Motors Front Wheel Drive Two-Mode Hybrid Transmission (SAE 2009-01-0508)

Comments

Ron F

Sounds quite a bit more complex than the THS III system from Toyota. The clutches might give the 2MT70 some advantage in steady-state highway driving and towing. I suppose that is how GM differentiates their hybrid offerings, and might be more in tune with the US market and GM's perceived focus on larger vehicles in any weight class.

SJC

Toyota has the Highlander hybrid SUV with a V6 engine. I would say the VUE is competing with that. 2009 looks like the end of Saturn if no one buys them.

ExDemo

Great progress.

ejj

It was easy enough for GM to kill off Pontiac, I can't imagine why they would keep Saturn (Opel under the skin).

Will S

The two-mode design has up to now been exclusively incorporated into rear-wheel-drive (RWD) transmissions with the intent of reducing the fuel consumption of heavier and larger passenger trucks and SUVs while preserving towing capacities.

While there is a genuine need for some vehicles like this, it's obvious this is an attempt to keep alive the heavy truck/SUV recreational market, where people "need" to tow boats, jet-skis, motorcycle, etc that themselves consume copious amounts of gasoline.

Reducing our petroleum independence requires more of an introspective look at our lifestyles, not just at tweaking SUVs. That's difficult for some people to do...

Jesse 67

I like the engineer's comment on cost, "Just south of prohibitive". Well no wonder, considering how complicated it is. Sounds like the engineers think this as well. How much extra efficiency does this actually gain over say a series hybrid (without extended range capabilities) which would need an engine, a generator, a fairly simple battery a motor and a single or maybe two speed gearbox?

If it works well then you can have your cake and eat it too but people are getting the wrong idea here, efficiency does not have to be complicated and is usually simpler than the status quo. Efficiency means less, not more. The shining light at the end of the tunnel is the battery electric with a battery, controller, motor and single speed gearbox, simplicity.

Also, just read the side note about the third hybrid system GM is developing for small cars, that sounds a little more promising.

Patrick

Jesse,

I think the best layout for a serial hybrid would be one in which you intend to use electric motors on separate axles to provide all wheel drive. Eliminates the mass, friction and complexities of a central drive shaft and center diff and replaces it with the complexities of software (which while tougher to get operating correctly can always be incrementally improved in the field). I see less gain in having a single axle, two-wheel drive serial hybrid.

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