Friday, May 21, 2010

ABG First Impressions: Two-Mode hybrid Chevy Tahoe and GMC Yukon — Autoblog Green

ABG First Impressions: Two-Mode hybrid Chevy Tahoe and GMC Yukon — Autoblog Green

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Where this new system differs from others on the market is the second mode. As speed and load increase, the transmission which features four clutches transitions from variable ratio mode toward one of four fixed gear ratios. When this happens, power flow is diverted completely away from the electric motors and directly through the gears for even greater efficiency. While the motors are about eighty-five percent efficient, power flowing through direct mechanical gearing is more than ninety-five percent efficient. If even more power is needed such as climbing a hill while towing a trailer or boat, the motors can also kick in at higher speeds.

In order to improve the long term durability of the motors, transmission fluid flows through cooling jackets around and through the electric motors to keep the temperature down. In addition the copper windings of the motors are made with extra thick wire to minimize resistance and heat build up.

All this extra hardware adds mass to a vehicle as evidenced by the extra 300 lbs that the Altima Hybrid carries compared to a standard model. GM made a major effort into mass reduction on the hybrid, switching to aluminum for the hood and tailgate as well as other parts. They also added lighter weight front seats that are also thinner than the regular models. That had an added benefit of increasing rear leg-room. New lighter weight wheels also shaved about 5lbs per corner. All together the diet was enough to neutralize the 400 lbs added by the hybrid drive-train and keep the overall weight essentially unchanged.

The most noticeable external difference is in the front clip which has a significantly deeper air dam and is more sealed off. Additional aerodynamic tweaks around the wheel openings, running boards and the rear fascia and spoiler all contribute to a drag coefficient reduction from .37 to .34. Although all of this helps, when you are dealing with a vehicle that has the frontal area of an average garage, it only goes so far.

The calibrations of the 6.0L V-8 have also been optimized to make the most of the hybrid system. The GM V-8s have had a displacement on demand system for several years, but the presence of the hybrid system means that the motors can provide a performance boost when needed. That allows the engine to run in four cylinder mode much more often in medium load conditions than it does currently. The driver information display in the main cluster lets you know whether the engine is running of four or eight cylinders and it's definitely the only way you'll ever know, because the change-over is imperceptible.

The final major mechanical change to the hybrid trucks is a new electro-hydraulic brake system supplied by TRW. The vacuum brake booster normally found on the Tahoe is now supplanted by the new system which uses a high pressure hydraulic pressure accumulator to provide boosted braking. Extra solenoids in the system also control the blending of friction braking and regenerative braking. The regenerative braking can provide up to 0.3-0.4g of deceleration which will cover about 90-95 percent of brake applies in the real world. In the not so distant past