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.

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.

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)