Lawrence Kren
Senior Editor

The Chevy Tahoe and GM Yukon will drive a bit differently beginning with the 2008 model year. That’s when GM debuts hybrid versions of its big SUVs. A GM-BMW-DaimlerChrysler collaboration designed the hybrid system, which GM has packaged in a standard-sized automatic transmission for the SUVs. The approach simplifies integration with an existing platform and is key to the hybrid’s ability to tow loads on par with conventional SUVs.

“Mode,” in this context, describes the configuration of transmission gearing for optimal efficiency at varying speeds and power levels. Unlike conventional continuously variable transmissions, the 2-Mode full hybrid’s electrically controlled system uses no mechanical belts or bands. Shifts between modes are synchronous — no engine speed changes are necessary, so acceleration is extremely smooth.

Here’s how it works: In mode 1, at low speed and light loads, the truck operates in one of three ways: electric power only, engine power only, or a combination of the two. A pair of 60-kW ac-electric motors, residing in an automatic transmission, launch the truck from a halt to 32 mph. Acting as a full hybrid, the gasoline engine shuts off for extended periods of time when moving under electric power at low speed, say, in heavy stop-and-go traffic. The electric motors can work independently. For example, one motor may act as a generator to recharge the batteries, while the other helps launch or propel the vehicle at highway speeds. Or, the motors can assist with braking, which also helps recharge the batteries.

The electric motors couple to three planetary gear sets and clutches, forming what is called an electronic-variable transmission. The electric motors lock out the gear sets to give a fixed ratio, or they drive them in combination for other ratios. “The ability to change gear ratios in this manner is what distinguishes the 2-Mode from single-mode hybrids,” explains Mark Cieslak, GM vehicle chief engineer for full size trucks. “Single-mode hybrids use fixed gear ratios, and the electric motor always powers the vehicle. Such a single-mode hybrid capable of towing heavy loads would need extremely large motors. Packaging and cooling would be an issue.” In contrast, the planetary gear sets in the 2Mode design provide torque multiplication when needed. For example, during launch the electric motor/ gear sets are configured for maximum gear reduction and torque output.

Still in mode 1, but at speeds above 32 mph, the hybrid system transitions to V4 operation. The 6.0-liter V8 engine (337 hp @ 5,100 rpm; 367 lb-ft @ 4,100 rpm) uses Active Fuel Management that switches between four and eight cylinders, depending on load and speed. The hybrid controller calls for V4 operation as quickly and as often as possible. It does this by optimizing the planetary-gear ratio and signaling for electricmotor power based on driving conditions, actions that help the hybrid stay in V4 more of the time than its conventional counterpart. That optimal ratio may be a variable transmission stage or a fixed gear stage.

Mode 2 kicks in at highway speeds and is characterized by a fixed gear stage and either V4 or V8 engine operation. A direct mechanical- link or fixed-gear stage gives good efficiency at high loads and high engine rpm. Conversely, it’s less efficient to run electric motors at high rpm. The electric motors in mode 2 lock out the planetary gear sets and assist the gas engine, again to stay in V4 operation as much as possible. Electric assist and what is called late intake valve closing helps the AFM system run the engine in V4 at speeds to 80 mph. Full V8 engine power is available for passing, towing a trailer, or climbing a steep grade. Electric assist also permits the use of a tall 3.08 axle ratio, which lets the engine turn lower rpm at highway speeds and save fuel.

Power for the ac motors comes from a 300-V nickel-metal-hydride battery via a traction power inverter module located in the engine compartment. A 300-V airconditioning compressor and electric heater pump provide climate control when the engine automatically shuts off during stopand- go driving. The module also steps down voltage to 42 V for the electric power steering and to 12 V for lighting and service systems. Gone are the alternator, starter, power-steering pump, and torque converter. The electric motors spin up the gas engine when it is called for, then the controller adds fuel and spark to light it off. “Special software dampens out the shudder and disturbances normally associated with engine start, considered one of the biggest challenges in hybrid design,” Cieslak explains.

GM tested the 2-Mode hybrid system from –40 to 120F. The lower extreme is especially difficult because batteries don’t work well at those temperatures. The engine is able to start at –40F, though operation with electric motors is limited until the batteries recharge and come up to operating temperature. GM considered actively heating the batteries, but found it unnecessary. Transmission fluid cools the motors, helped by a large air-tooil cooler. A dedicated cooling loop moderates temperature of the power electronics.

Together, the hybrid hardware adds 400 lb to vehicle weight, 150 lb of which is the battery. Weight of the hybrid content is partially offset by elimination of the starter, alternator, powersteering pump, and torque converter. The use of lightweight seats and an aluminum, hood, lift gate, and driveshaft further trim weight.

Aerodynamics is another area that got a lot of attention. The front of the hybrid trucks sit 10 mm lower than standard models to lessen drag. The hood leading edges are streamlined, and the running boards flare to help guide airflow around the wheel openings. Other aerodynamic refinements include sharpening of the D-pillar rearward, including the taillamps, a spoiler over the rear hatch, and a closeout panel under the back of the vehicle to facilitate airflow. Lightweight, aero-efficient wheels shod with low rolling- resistance tires lessen parasitic losses. Tighter gaps between fascias, lamps and grille openings, as well as improved front-end sealing, nearly eliminate air leak paths. A wrap around fascia eliminates the conventional bumper-to-body gap, whi le a steeply raked windshield and smoother roof line help cut drag, all of which contributes to a slippery 0.34 drag coefficient and better highway fuel economy.