The have-it-all hybrid

Automotive Design & Production, Sept, 2005

The mile-long straight is bordered by a turn-around at either end. We are at the larger of the two where the asphalt is painted with concentric circles, ready to set off--at the blazing top speed of 35 mph--in GM's Graphyte concept SUV. Dana Kaplinski, assistant chief engineer for the rear-drive two-mode hybrids, GM Powertrain, is sitting in the front passenger's seat describing what the two-mode hybrid would be doing under these circumstances, how it would respond under varying real-world conditions, and the intricacies of software algorithms designed to find the most efficient use of available torque. Though the vehicle functions mainly as a teaser of where GMC truck design may be headed and is devoid of an actual hybrid drivetrain, it is indicative of how GM will launch its most capable hybrid technology.

"The two-mode system will launch in 2007 on the full-size Chevy Tahoe/GMC Yukon," says Kaplinski, "where we expect it to show a 25% fuel economy improvement over the non-hybrid model." That figure is based on the Federal Test Protocol (FTP) every vehicle must complete to get its EPA mileage rating. It's also a percentage increase Kaplinski believes most buyers should see in everyday use. "Our control strategy allows us to shift torque to get the best result with the lowest fuel economy penalty, and the hybrid system works at higher speeds to extend the operation of Displacement on Demand--a.k.a., "cylinder deactivation"--beyond its normal capabilities, and provide assist when there is a demand for full power."

The two-mode system adds a second set of gears and a pair of compact electric motors to an automatic transmission. There is no need for a CVT or Miller-cycle engine to get the best from the system as the engine controller determines the torque level necessary for the driving conditions, and chooses the best combination from the engine, transmission, and motors. "Displacement on Demand gives us the fuel economy benefits you find on single-mode hybrids, without having to create a unique engine specification that alters our logistics and build sequence," says Kaplinski. By concentrating on a "full speed range" system, the drain on the batteries and electric drive motors promises to be lower, while adding fuel economy benefits that extend beyond stop-and-go city driving. "Our system won't ask the driver to adapt to its needs, but will adapt itself to the driver's demands while giving the best available power and fuel economy," says Kaplinski.--CAS