The existing mild hybrid system that GM offers on the Saturn Vue and Aura uses a combined starter and heavy-duty alternator that are belt-driven off the engine. In the Two-Mode system a pair of 60kW electric motors are integrated into a new electronic variable transmission. A 1.8 kWh, 300V nickel metal hydride battery pack supplied by Cobasys sits under the second row seat. The transmission is mated up to a 6.0L V-8 that has variable valve timing and displacement on demand that can shut-off four cylinders under light loads.

The Two-Mode name refers to the two distinct operating modes that are available. The first mode is used under lower-speed light load conditions. Based on conditions and battery charge level, the system can operate in either battery only mode, engine only or combined mode. This first mode is pretty much the same as the other existing full hybrid systems on the market from Toyota and Ford. In this lower speed mode the transmission functions with continuously variable ratios as the power flow is blended between the engine and electric motors. If the battery has power, the truck will pull away from a stop in almost complete silence propelled only by electricity and keep going for about half a mile.



As the charge level of the battery drops, the engine almost imperceptibly fires up and keeps you moving. Unlike the Altima Hybrid that had a distinct shudder when the engine kicked in under light throttle conditions, the only indication of the Tahoe V-8's activity came from the exhaust note and gauge cluster. The exhaust was by no means loud but was mostly noticeable due to the effort GM expended to quiet down the rest of the vehicle. With the capability of running in electric mode, NVH was clearly an issue that needed to be addressed.

One of the biggest sources of noise in traditional setups is the vibration of all the accessories that are mechanically driven off the engines. Two of the biggest loads are the air conditioning compressor and power steering pump and GM has largely silenced these by electrifying them. The air is now chilled by a 300V compressor while the rack mounted electric power steering assist runs off of 42V. The power control module that manages the flow of electricity between the battery and motors has DC power outputs at 300V, 42V and 12V for the various systems on the vehicle.

From the driver seat the primary visual indicators that you're driving a hybrid are in the gauge cluster and center console. The central display features a power-flow indicator like the one in the Altima that lets you and your passengers know whether the vehicle is sending power to the wheels from the battery, engine or both and also when the power is flowing in the other direction. The tachometer now also has an auto stop position just below zero when the engine has shut down. An economy gauge sits to the left of the tach and adjusting your driving to keep the needle pointed straight up will yield the best results.



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, pressing the brake pedal of GM truck resulted in using up half of the travel before any noticeable deceleration occurred. Now it feels like applying the brakes of a race car with decel starting almost as soon as you touch the pedal. It's not grabby, just very responsive.

After a brief drive at the GM proving ground in a pre-production prototype, it was clear that if nothing else the changes done to create a hybrid full-size SUV have resulted in the most refined large truck GM has ever built. The mantra repeated during the presentation was seamless integration. It's incredibly quiet even with the engine running. There are no noticeable transitions between driving modes and it all just seems to work. GM is still doing some tweaking to calibrations, but they are very close.



According to GM, the 6.0L hybrid Tahoe will get about twenty-five percent better fuel economy on the combined EPA city/highway cycles compared to the 5.3L standard vesion and do forty percent better in urban driving. The Two-Mode Hybrid Tahoe/Yukon will be the first hybrid full sized SUV when it goes on sale this fall and the only hybrid with a 6,000 lb towing capacity and eight passenger seating. When asked why not mate the hybrid system with the smaller engine or even the Duramax diesel engine, they gave the usual answer about not discussing future product plans. They did however indicate they are working on a full range of options from the mild hybrid system in the Aura and Vue through the Two-Mode system and on to the plug-in hybrid Vue and E-Flex. This is just the first step and for a vehicle that represents a large segment of the American market it is a very important one. Once production versions are ready later this year, we'll be sure to do a more thorough evaluation and find out how well this all works in the real world.