General Motors (GM) has taken the lead in producing affordable and practical hybrid light-duty vehicles. Two innovations coming from GM are the “dual mode” hybrid transmission and the “BAS” or micro-hybrid drive system. The two-mode transmission is a self-contained hybrid electric drive unit which can be easily fitted to exiting rear wheel drive vehicles.

The dual mode unit will be offered in the 2007 model Chevy Tahoe and GMC Yukon. The dual mode transmission is the result of a joint development effort between GM and DaimlerChrysler. The hybrid transmission requires a NiMH battery pack rated at 300VDC and features two 60,000-watt electric motors. In mode one, the unit utilizes its electric drive capability extensively to maximize fuel economy while operating in an urban/suburban drive cycle. This is when idle stop, electric launch, and regenerative braking would be used. Mode two is used for driving at higher speeds and interstate cruising. GM claims this system will provide a 25 percent improvement in fuel economy over a conventional Tahoe.

Detail of the GM/DCX two mode hybrid drive unit coupled to a GM Vortec V-8. Note connections for the two three-phase 60kw electric motors. Control electronics are mounted in a separate module. Photo courtesy of Rich Cregar

The other exciting development from GM is the introduction of the first American-made micro-hybrid, the Saturn VUE. GM calls its micro-hybrid system “BAS” for belt-alternator-starter. As previously discussed in the NAFTC eNews, a micro-hybrid abandons the 12V starter and replaces the conventional alternator with a specialized alternator-starter, which is mounted where an alternator is normally found and utilizes the engine serpentine belt drive. This type of hybrid drive has been available in Europe for several years.

This unit provides many of the same benefits of a mild hybrid at a greatly reduced cost. The Saturn VUE hybrid, called the “Green Line,” is being offered as a 2007 vehicle for around $23,000. This is far cheaper than other hybrid models on the market today. GM claims the VUE Green Line will provide a 20 percent improvement in fuel economy over a conventional VUE. The BAS micro-hybrid system will also be offered in 2008 in the Chevy Malibu.

GM’s BAS micro-hybrid system will provide the vehicle with the advantages of idle stop, which will shut the engine off when not needed such as while waiting for a stoplight. The BAS unit will become a three-phase electric motor and quickly restart the engine when the brake pedal is released. This provides a major gain in fuel efficiency. The BAS system utilizes a 36V NiMH battery pack to provide the three-phase A/C BAS unit with the energy needed for engine start.

While driving, the BAS system will also provide regenerative braking during which the unit will become an alternator tasked to recharge both the 36V and 12V batteries. The 12V voltage supply for the auxiliary battery is produced by a DC/DC converter. The energy stored during regenerative braking can then be used by the BAS unit in motor mode to provide up to 115 ft.lbs of additional drive torque to the 2.4-litre four-cylinder engine. This is done to improve fuel economy during acceleration and cruising.

Other advantages of the BAS system include an intelligent charging strategy with which the control electronics will call on the alternator to produce electricity primarily during coasting and deceleration and a “torque smoothing” system which calls upon the BAS motor to provide drive torque during light load periods. During this mode, the engine fuel injectors will be shut off to provide additional fuel savings.

These new models from General Motors will provide consumers with two additional, affordable choices when considering the purchase of an efficient advanced technology vehicle. They also will provide technicians with two additional types of hybrid drive systems that will need to be learned and understood in order to service and maintain these vehicles. You can read more about this in the NAFTC eNews article "General Motors Debuts Two New Hybrids" in this month's NAFTC eNews.