Direct injection is not a "bolt-on" technology that can be added to any powertrain, said Kevin Quinlan, Delphi Automotive LLC's general manager for gasoline engine management.
A direct injection engine requires a new cylinder head. Then engineers also must consider a smaller engine, different gear ratios and turbocharging.
"If you really want the maximum fuel economy benefit, you have to rethink your overall powertrain strategy," Quinlan said. "You could bolt it on, but then your fuel economy improves only 1 or 2 percent."
Once an automaker adopts direct injection, further improvements are possible. Direct injection works well with start-stop systems, which turn off the motor whenever the vehicle stops.
Direct injection lets engines restart more quickly, so motorists are hardly aware of the transition. "It sets you up for start-stop," said Barb Samardzich, Ford's vice president of powertrain engineering.
On Dec. 27, Ford announced plans to eventually equip its North American cars and SUVs with start-stop systems that will improve fuel economy by 4 to 10 percent.
What's next? Suppliers are tinkering with stratified charge injection systems, which would further improve fuel efficiency.
The injectors spray the fuel into one portion of the ignition chamber, which creates a leaner burn. However, that requires the injector to spray the fuel more precisely. It also generates more nitrogen oxide, which in turn requires an upgraded catalytic converter.
Given the added costs of stratified charge injectors, "people are looking for a more cost-effective solution," Quinlan said. But Quinlan and others agree that conventional direct injection is a technology whose time has come.
"Most automakers are looking at direct injection," said Kregg Wiggins, Continental's senior vice president of North American powertrain operations. "And they are adopting it at a faster rate."