It’s getting harder to believe automakers when they say they can’t meet tougher fuel-economy standards. And it’s their own fault. Not only are they rolling out hybrid vehicles and developing advanced hybrids that can be plugged in: they’re also demonstrating, at least in concept cars, advanced combustion technology that could cut fuel consumption.

This week General Motors announced two drivable concept cars that use homogeneous-charge-compression ignition (HCCI), which provides fuel-economy advantages similar to those provided by diesel engines, but without the extra pollution. HCCI could reduce fuel consumption by 15 percent. The vehicles go up to 55 miles per hour on HCCI, shifting to conventional spark ignition for higher speeds.

Recently, MIT researchers demonstrated that such vehicles could run on ordinary gasoline, which could help the technology be quickly adopted.