GM's Displacement on Demand - "Impossible" without throttle-by-wire.
Warren, MI—General Motors took a big step toward fulfilling the promise of by-wire technology recently, as it announced plans for two million of its engines to incorporate so-called "Displacement On Demand technology" by 2008.
Displacement On Demand, which allows eight-cylinder engines to transform themselves to four-cylinder motors and six-cylinder models to effectively convert to "threes," is important for GM because it enables the company's vehicles to boost their fuel economy between 8% and 20%.
GM engineers note, however, that the technology would have failed if not for throttle-by-wire technology. Throttle-by-wire, they say, enables the engines to switch back and forth smoothly from six cylinders to three, or eight cylinders to four, without any input from the driver. "It makes the change transparent, so the drivers have no idea whether they're in an eight-cylinder or a four," says Jeff Allen, engineering group manager in advanced controls at GM Powertrain.
Using Displacement on Demand, the engine can ratchet down from say, six cylinders to three, whenever the vehicle no longer needs brisk acceleration. To make the changeover seamless, the electronic throttle needs to "understand" the vehicle speed and engine load, so it can make its moves at precisely the right moment. When solenoids in the engine move the valve lifters, shutting down the prescribed exhaust and intake valves (see figure), the electronic throttle repositions the throttle blade, enabling the remaining cylinders to maintain the engine's torque and speed without any noticeable changes.
Engineers say that consumers won't accept the technology if they feel the car lurch or, worse, if they have to pick up the slack for the lost cylinders by stepping harder into the accelerator. "You want the driver to leave his foot in the same position and never know the engine has switched," Allen says. "So you open the throttle blade slightly to let more air in and reduce the pumping losses."
Engineers say that most drivers will never know that the trusty link between their foot and the throttle has been severed, and that's the way they want it.
That role of "quiet enabler" is one that by-wire technologies are likely to repeat many times over the next several years. Automotive engineers say they need steer-by-wire and brake-by-wire technologies as enablers for such features as lane keeping, collision avoidance, and ultimately, autonomous driving. Throttle-by-wire is expected to quietly play a key role in those forward-looking technologies, too, just as it has in Displacement On Demand. "It's definitely an enabling technology," Allen said. "It's going to let us do a lot of things that we couldn't do otherwise."
General Motors' Displacement On Demand program is believed to be one of the broadest implementations to date of so-called "by-wire" technology, in which computer-controlled electric motors replace traditional mechanical linkages.