For more than a century, air cars have remained a quixotic quest of engineers—an idealistic exercise with little long-term likelihood of entering mass production. As fuels go, air has obvious upsides: It’s ubiquitous, clean, and, best of all, free. But air requires energy to store energy because it must be compressed, limiting the utility of an all-air car. Two engineers from French automaker PSA Peugeot Citroën thought they could overcome that problem by pairing two tried-and-true technologies: a gasoline engine and hydraulics. To test the concept, they formed the Hybrid Air Program in 2010 and connected the engine of a subcompact car to a commercial airplane’s hydraulic system. “We were trying to push the project against a lot of people who didn’t trust the fact that we would succeed,” says engineer Karim Mokaddem.