GM Powertrain displaces on demand

Eaton Corp. developed the Switching Roller Follower and the Solenoid Control Valve Assembly for GM's displacement-on-demand (DOD) technology. DOD engines will debut in 2004 as part of GM's Vortec V8 engine family.

In choosing a vehicle, most customers select a powertrain that will meet their needs in maximum operating conditions. Many truck buyers, for example, demand powerful V8 engines to handle the heavy load without sacrificing performance. But when a vehicle equipped with such an engine is driven for routine transportation purposes—such as typical city driving—the engine operates at a small fraction of its capability at almost closed throttle conditions. This impaired volumetric efficiency causes poor fuel economy.

General Motors plans to equip its large trucks and SUVs with "displacement-on-demand" (DOD) engines that, according to the company, boost fuel economy by about 8%, based on an EPA testing procedure, and up to 25% in certain real-world driving conditions. The engines will debut in 2004 as part of GM's Vortec V8 engine family. GM initially plans to produce more than 150,000 of them in the first year. Production will then be increased over the next several years to nearly 1.5 million units in 2007.

DOD saves fuel by using only half of the engine's cylinders during most normal driving conditions. The system automatically and seamlessly reactivates the other cylinders when the driver needs the engine's full capabilities for brisk acceleration or load carrying. "We have figured out a way to make our V8 engines work smarter," said Sam Winegarden, GM Powertrain Chief Engineer of Vortec V8 engines.

The engine is always started on eight cylinders. But once the vehicle is moving, the powertrain control module (PCM) activates DOD. While actual fuel economy gains depend on driving style, vehicle options, and load-carrying conditions, the potential customer benefits are significant. For example, a full-size SUV with a current cruising range of more than 645 km (400 mi) per tank of gasoline could gain as much as 160 km (100 mi) per tank with DOD.

DOD marks a dramatic evolution in engine technology pioneered by GM 20 years ago. In 1981, GM implemented cylinder deactivation technology that was limited by a cable throttle, mechanically controlled transmission, and a fraction of the computer power currently available. Today's engine computer is a 32-bit machine (vs. 8-bit) with an internal clock that has roughly 25 times the speed, 50 times the computing power, and 100 times the memory of the 1981 controller. Electronic throttle control and electronically controlled transmissions are now available—proven technologies that allow seamless DOD operation. Together, these devices permit precise engine torque control at all times and more accurate information used by the transmission for shift-point selection and actuator control settings.

DOD enables the working cylinders to achieve higher fuel efficiency through better thermal, pumping, and mechanical efficiency. To make the technology seamless to customers, GM is developing improvements in powertrain electronic control systems, intake and exhaust noise silencing, and powertrain mounts. Modifying existing electronic and mechanical controls minimizes cost and complexity.

The PCM determines load conditions based on inputs from vehicle sensors. When loads are light, the control module sends a signal to close both intake and exhaust valves for half of the cylinders, thus cutting off their air and fuel supplies. The valves are reopened to provide all-cylinder operation for brisk acceleration or for hauling heavy loads.

Sensors also tell the PCM what the engine is doing at any point in its cycle—for example, which cylinder has fired and where the valvetrain is located at that particular point. The module interprets the information to control complex functions such as fuel injection, electronic spark control, and electronic throttle control. The PCM also monitors driver commands, several engine sensors, and mathematical models to determine the right time in the engine cycle to initiate the process. Aided by DOD algorithms, it then controls spark advance, fuel injectors, electronic throttle, and four valvetrain control solenoids in precise coordination to achieve seamless transition between V4 and V8 operation. The sequential reactivation of cylinders to increase engine output happens so quickly that there is an immediate increase in engine output.

In addition, DOD leverages an existing oil pump system to provide hydraulic pressure to control the system. The mechanical actuators are special hydraulic lifters, each with a spring-loaded locking pin. In normal operation, when the camshaft rotates against the hydraulic lifter, it opens and closes either an intake or exhaust valve.

The special lifter, developed by Eaton Corp., is designed so that one section can collapse, or telescope, into the other section. The two sections can be either coupled or uncoupled by means of the locking pin. When cylinder deactivation is initiated, hydraulic pressure is used to dislodge the locking pin and collapse the lifter, thus closing the valve. In reactivation mode, the removal of hydraulic pressure causes the locking pin to return to its latched position to restore the lifter's normal function.

The activated state of the special lifters is the default mode. In V4 mode, every other cylinder, in the firing order, is deactivated. Thus, in a V8 engine, the process would affect the outer two cylinders on one bank and the inner two on the opposite bank. The cylinder deactivation-reactivation operation is accomplished in a fraction of a second, making the transition seamless and transparent to the driver.

According to Winegarden, GM's overhead valve engines are particularly well suited for DOD. The engines have a flexible architecture that has been adapted for racing, trucks, and passenger cars in markets worldwide. "With two valves per cylinder, only two actuators per cylinder are needed," Winegarden said. "The system is intelligently simple and compact. By contrast, while DOD certainly has merit for overhead-cam, multi-valve engines, four valves per cylinder require four actuators per cylinder. Additionally, the electrohydraulic (or other) control network must be packaged within the tight confines of the cylinder head assembly, adding both complexity and cost." Other applications for DOD are being considered, including six- and four-cylinder engines as well as overhead-cam engines.

DOD is just one of several advanced powertrain technologies GM is developing to address fuel economy. "Other promising powertrain technologies include continuously variable transmissions, direct-injection gasoline, port deactivation, manual transmission auto shift, and variable valvetrain," Winegarden said. "GM also is continuing work on many alternative propulsion systems."

- Jean L. Broge