Two-Mode Hybrid Transmissions

Two-mode is the name for the new transmission technology that developed as a result of a joint project from DaimlerChrysler and the BMW Group. The 2008 Chevrolet Tahoe and the 2008 GMC Yukon SUVs are the first to come out with this technology. This is the first attempt at converting full-sized SUVs to hybrids.

The name two-mode means that the system operates with two modes depending on the circumstances. The first mode functions when driving at low speeds and when carrying only light loads. Under these conditions, the vehicle chooses to operate at one of three configurations:

• Only using electricity
• Only using the engine
• Or a combination of both

In the second mode, which is when the vehicle is running at highway speeds, the system then applies a second gearset to help improves the fuel economy. By using:

• 2 gearsets
• 2 60k W electric motors
• 300-volt battery pack
• V8 gasoline engine

GM claims to offer 25% improvement in the fuel efficiency over their gasoline models. Part of this improvement is considered to be a result of:

• ‘On demand’ cylinder deactivation at highway speeds
• Low rolling resistance tires(LRR) (LRR)
• Aerodynamic design
• Lighter weight hood and tailgate (now aluminum)

These improvements alone raised the EPA rating from 17 to 21. This improvement actually equates to a huge savings of two tons less CO2 being emitted for each vehicle, every year.

The purpose of this two mode system is to run the vehicle at optimum performance utilizing the least amount of fuel. To do this, the brain of the system checks how the system is operating close to 100 times every second. It makes minor adjustments depending on the results it finds. The transmission system is the key. It’s similar to a continuously variable transmission only with a fancy control system that delivers the speed at the same time it balances the source of power – either from the gasoline V8 or the electric motors.

The brain of the system checks in constantly, always making the decision as to use the electric motors or to bypass them. In city driving the electric motors operate close to 75 percent of the time. On highway driving the gasoline V8 operates close to 75 percent of the time.

This system really is the best of both worlds. During low-speed driving, the system is a regular hybrid vehicle, ignoring the V8 gasoline motor that is always available to rely solely on the electric motors. When driving on the highway, the system can utilize the electric motors to provide an extra power boost when necessary. At the same time, the V8 engine under the hood offers several fuel saving advantages such as the variable valve timing and cylinder deactivation.

It is the ability of the transmission to shut down the V8 engine by choice so it can operate on constant, fuel-saving rpm for the rest of the time. This transition from gasoline to electric and back again is not noticeable.

These automakers involved in the two-mode design are planning to incorporate the technology on several new models. This two-mode system is already on the Chevrolet Tahoe Hybrid and GMC Yukon Hybrid, both of which are available as two and four wheel drive models. For the coming year, expect to see the Dodge Durango, Chrysler Aspen, Cadillac Escalade, some of the luxury Mercedes-Benz cars and BMW models. Also look for the Chevrolet Silverado and GMC Sierra full-size pickups to receive the new two-mode hybrid technology. Expect the prices to reflect the new technology.