The old joke "Hydrogen is the fuel of tomorrow -- and always will be" isn't keeping Mazda from jumping on the H2 bandwagon and stuffing a dual-fuel rotary under the hood of the next RX-8. They might even dub it the RX-9.

Wankels are sweet engines that really scream at full throttle, but they get lousy fuel economy and aren't terribly green. In an effort to clean things up a bit, the next-gen production rotary reportedly will be based on the hydrogen/gasoline engine in the RX-8 Hydrogen RE (pictured).

Just make sure you aren't trying to chase down that ZR-1 under hydrogen power.

According to Auto Express, running on the most common element in the universe robs the car -- which gets 228 hp out of a 1.3-liter engine -- of 20 percent of its power, so Mazda's engineers envision owners opting for hydrogen (those who can find it, anyway) only during city driving.

Mazda's been playing with hydrogen since 1991, when it unveiled the HR-X concept at the Tokyo Motor Show. It developed the RX-8 Hydrogen RE five years ago and started road-testing it in 2004. A trunk-mounted tank holds 74 liters of gaseous hydrogen at 5,000 PSI; a direct-injection system feeds it directly into the rotor housing. "Because existing parts and production facilities are used," Mazda says, "the innovative engine can be built at relatively little cost."

But can it be sold at relatively little cost? We'll see in 2012.