For release: Sept. 27, 2007

CHEVROLET TAHOE AND GMC YUKON HYBRID: SOPHISTICATED HYBRID POWERTRAIN
 

Until now, consumers looking for an efficient utility vehicle had to sacrifice a great deal of cargo-hauling and trailer-towing capacity in return for improved fuel economy — they could never have both. This is because of the limited load capabilities of less-advanced transmission systems found on most hybrid passenger vehicles today.

All that has changed with the introduction of GM’s new electrically variable transmission (EVT), which offers the best of both worlds — a hybrid system that blends continuously variable operation (for low-load driving situations) with fixed-gear operation (for high-load conditions such as towing or highway driving).

Drawing on its vast experience in developing hybrid bus propulsion systems, GM designed the EVT to provide the best combination of city and highway fuel economy. Used in concert with the Vortec 6.0L Gen IV V-8 engine with Active Fuel Management, this all-purpose system is well-suited to provide excellent economy and full-size SUV passenger-carrying, cargo-hauling and trailer-towing capabilities.

Furthermore, the EVT is designed to bolt directly to the standard four-wheel-drive transfer case found on the gasoline-only models for true four-wheel-drive capability.

The Tahoe and Yukon Hybrid drivetrain is made up of the following major components, each of which works together to provide seamless, economical and comfortable operation that goes virtually unnoticed by the driver and passengers.

Electrically Variable Transmission (EVT)Electrically Variable Transmission (EVT)

The key to GM’s all-new EVT is its unique assemblage of two 60 kW electric motors, three planetary gearsets and four traditional hydraulic wet clutches. This arrangement allows continuously variable operation, as well as providing four fixed gear ratios (with operation comparable to that of a standard electronically controlled automatic transmission).

The reason GM engineers chose this design is because of the operational characteristics of electric motors, which are very efficient when turning at low speeds, but much less efficient as motor rpm increases. Current hybrid passenger vehicles run their electric motors almost continuously throughout the entire drive cycle, which can be very inefficient under high loads and at highway speeds.

The opposite is true with GM’s EVT, which can activate any of its four hydraulic clutches to allow power to be transferred via the fixed-gear ratios whenever high load conditions are experienced.

A sophisticated Hybrid Optimizing System (HOS) constantly receives torque-based data from the powertrain and other vehicle systems, and then determines the most efficient means of propelling the vehicle — either via electric power, gasoline engine power or a combination of the two. The EVT is like having two transmissions in one — continuously variable drive for light-load conditions and fixed-ratio drive for high-load situations.

Relationship between the system's two electric motors, three planetary gearsets and four hydraulic clutchesThe EVT schematic shown illustrates the relationship between the system’s two electric motors, three planetary gearsets and four hydraulic clutches (C1-C4). Also shown is the transmission hydraulic pump and input shaft damper, which takes the place of a traditional torque converter.

By locking the first motor, the HOS keeps the output shaft operating at the same speed as the input shaft — this is called “input-split” mode and is basically the same as that used by most current hybrid vehicles. This approach provides either high fuel efficiency or high power, but not both. It is for this reason that GM added a second, or “compound-split” mode, which adds another electric motor as well as other hardware.

It is important to note that hybrid systems commonly in use today are much less sophisticated than GM’s system because their operation is based solely on the rate of airflow into the gasoline engine. On the other hand, GM’s all-new hybrid is more comprehensive, using torque-based inputs to continuously determine the optimal propulsion configuration of the EVT throughout the entire driving cycle.

All functions of the EVT are controlled by the HOS, which constantly searches for the optimal transmission operation (using either variable or fixed ratios) to meet current operating conditions. The HOS also bases its decisions on allowing the Vortec 6.0L V-8 to take full advantage of its Active Fuel Management system and remain in V-4 mode as long as possible for maximum fuel economy.

While the gasoline-only Tahoe and Yukon models also offer Active Fuel Management, this all-new hybrid system allows the vehicle to run in V-4 mode more frequently and for longer periods — because of the equivalent 30 horsepower of available electric boost. This is the key to the Tahoe and Yukon Hybrids’ 25- to 30-percent improvement in combined city/highway fuel economy.

Operation of the EVT is highly adaptable to driving conditionsOperation of the EVT is highly adaptable to driving conditions. As such, there is no set order to the application of modes or fixed ratios for any given scenario. The chart above attempts to show the general interrelation between these functions.

In Mode One (M1), the EVT provides infinitely variable drive up to 1.70:1, while Mode Two (M2) provides ratios from 1.70:1 to 0.50:1. These two modes include all instances of electric-only (Auto Stop) and electric-gasoline hybrid operation. Depending on driving conditions and vehicle load, a fixed first-gear (G1) ratio of 3.69:1 is available (such as for pulling a large trailer) instead of the variable M1 ratios.

Similarly, a second-gear (G2) fixed ratio of 1.70:1 may be selected by the HOS, if needed, before transitioning to the variable M2 ratios. As road speed increases, and depending on driving and vehicle load conditions, variable-ratio M2 operation can be switched instantly to a third-gear (G3) fixed ratio of 1.00:1, and finally to a fourth-gear (G4) fixed ratio of 0.73:1, which is used mainly for steady-state highway cruising.

During any facet of electric-gasoline hybrid operation, the HOS will continually search for the best drive ratio — variable (M1, M2) or fixed (G1-G4) — which will allow the gasoline engine to operate in its most efficient, V-4 mode. (Driving the vehicle in reverse is always done in electrical-only, though the gasoline engine may continue to run depending on conditions.)

When towing at low speeds and from a stop, the HOS can apply full engine torque as well as electric motor-assist for additional pulling power. This gives the Tahoe and Yukon Hybrid models the greatest towing capability of any hybrid passenger vehicle on the market.

The chart to the left illustrates the various events that can be expected during vehicle operation in typical highway-driving and low-speed scenarios.

Because real-world driving conditions vary widely, the chart above illustrates two hypothetical driving scenarios (highway and low-speed) and the possible order of EVT modes and gear ratio selections that could be occur as the HOS searches for maximum operating efficiency based on vehicle load and driving conditions.

Unlike less advanced hybrid systems, this new GM system occasionally shuts down the electric motors, allowing the EVT to function as a conventional automatic transmission. Typically, this fixed-ratio operation occurs at highway speeds or when hauling heavy loads, and can actually be more efficient than operating in electric-gasoline hybrid mode under the same conditions. This is because under high-load situations, when the fixed gears are in use, the electric motors can be used to generate electricity — or, if needed, they can be called on to supply additional torque for improved performance.

Energy storage system (ESS)

Providing power to the EVT’s two electric motors is a 300-volt nickel-metal hydride Energy Storage System (ESS). This battery pack is located under the second-row seat, where it takes up virtually no additional space and does not interfere with second- or third-row ingress/egress — the fold-and-tumble capability of the second-row seat is maintained, and when the seat is flipped forward, a flat load floor is provided from the liftgate forward to the front seats.

The primary function of the ESS is to provide power (300 volts) to the EVT via the Traction Power Inverter Module (TPIM) and to store captured energy produced during regenerative braking. The ESS can also be charged, when necessary, by the gasoline engine via one of the two electric motors when operated in generator mode.

In addition to supplying power to the EVT, the ESS also provides power to the air conditioning compressor and the Accessory Power Module (APM), which converts the high-voltage supply to 42 volts for the electric power steering system, and 12 volts for the vehicle battery and other 12-volt electrical accessories.

Battery pack durability and reliability is maintained via optimized charge and discharge cycles, as well as a dedicated cooling system that draws air from the passenger compartment. As part of the vehicle’s emission control system, the ESS is warranted for eight years/100,000 miles.

The ESS also has numerous safety features that prevent over-charging, over-heating, unintended access to high-voltage components and infiltration from liquid spills.

Regenerative braking

As full hybrids, the Tahoe and Yukon use fully blended regenerative braking to capture energy that would otherwise be lost during vehicle deceleration. By using one or both of the EVT’s traction motors as a generator, braking energy is converted to electrical energy and stored in the ESS for future use to propel the vehicle.

The regenerative brakes are used along with the standard hydraulic brakes to slow the vehicle and/or bring it to a stop. Depending on the amount of braking force required, the hydraulic brakes may not even be used, such as during mild deceleration when slowing to allow space for merging traffic near a highway on-ramp.

When additional braking is called for, based on a change in the position of and/or the force applied to the brake pedal emulator, the hydraulic braking system will be called on to assist the generator(s) in slowing or stopping the vehicle. The Anti-lock Braking System (ABS)/Electronic Stability Control (ESC) modulator used on the Tahoe and Yukon Hybrids has been adapted to allow this interaction between the hydraulic brakes and the regenerative braking system.

Initial DecelerationThe graph to the right shows how initial deceleration may not even require application of the vehicle’s hydraulic brakes, which are added only when regenerative braking is not sufficient to provide the necessary amount of stopping power.

The use of cooperative control between the regenerative braking system and the hydraulic brakes results in excellent braking control and maximum energy recovery. The system also provides feedback in the form of brake pedal resistance, which gives the driver the same feel as would be experienced with a normal hydraulic braking system.

Regenerative braking has the additional benefit of extending the life of the friction materials used in the hydraulic braking system, as well as improving braking performance in the form of shorter stopping distances.

Vortec 6.0L Gen IV V-8 engine

Vortec 6.0L Gen IV V-8 engineChosen over the existing Tahoe and Yukon 5.3L Gen IV V-8 engine, the Vortec 6.0L V-8 gasoline engine has more favorable torque characteristics than its smaller-displacement cousin. This was especially important because GM engineers adapted the 6.0L V-8 to operate with late intake valve closing (Atkinson-cycle combustion process) for reduced pumping losses and better overall fuel economy.

Using flat-top pistons, cylinder heads borrowed from GM’s 5.3L high-output V-8 and a 10.8:1 compression ratio, the Vortec 6.0L V-8 produces 332 horsepower (248 kW) at 5100 rpm and 367 lb.-ft. of torque (497 Nm) at 4100 rpm, yet requires only regular unleaded fuel.

In addition, the Vortec 6.0L engine features variable valve timing to control late intake closing, and Active Fuel Management, which allows four of the eight cylinders to be shut off during periods of light load. Gasoline-only V-8 Tahoe and Yukon models with Active Fuel Management benefit from the economy of V-4 operation, but because of the lack of electric boost, they cannot remain in V-4 mode as long as the Vortec 6.0L in these new hybrids.

A key contributor to fuel economy is the gasoline engine’s Auto Stop mode. Once the vehicle reaches 0 mph, the gasoline engine is automatically shut down. By leaving the engine off and allowing the vehicle to move only under electric power, such as during heavy stop-and-go traffic, fuel consumption is greatly reduced.

However, when extra power is required, such as for wide-open-throttle acceleration from a standing stop, the Vortec 6.0L V-8 is seamlessly restarted so it can deliver the necessary power and torque. In this case, the engine is restarted effortlessly from the Auto Stop mode using the EVT’s powerful internal electric motors; there is no traditional starter motor.

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