We drive Mazda’s Norwegian hydrogen-powered RX-8

Yes. You read that correctly. Mazda has been experimenting with hydrogen as an energy carrier for a long time. In fact, I wrote about it way back in Tech Tidbits, July 1992. Not long afterward, Mazda offered me a drive of a special hydrogen-fueled rotary-powered Miata MX-5. More than a decade later, March 2004, Sam Mitani drove and was able to report on an RX-8; this one, dual-fueled, either gasoline or H2.
And, indeed, the latest version of this RX-8 HRE (as in “Hydrogen Rotary Engine”) is what I drove recently. The HRE has had lease-only availability in Mazda’s home market since early 2006. The car is also taking part in Norway’s Hynor Hydrogen Highway Project, in which seven stations support a 360-mile route from Stavanger, in the southwest of the country, across to Oslo.
Why a hydrogen rotary?
It turns out there are two excellent technical reasons for fueling a rotary with hydrogen, both based on combustion dynamics. "With a rotary's hmmmm, as opposed to a conventional reciprocating engine's boing boing, note that a rotary's engine's intake port is nowhere near its combustion chamber." Thus, unlike in a recip engine, the intake charge isn’t subjected to hellacious heat of the previous combustion.
This is of particular importance with hydrogen, because the stuff is 10 times more flammable than gasoline. A major challenge in hydrogen recip engines is “flashback,” essentially preignition of the H2 encountering hot surfaces. A rotary engine sidesteps this completely.
Another beneficial aspect of rotary combustion is its inherently greater swept volume. Intake and compression of a recip engine involve 360 degrees of crankshaft rotation. By contrast, the rotary’s eccentric shaft travels through 540 degrees to accomplish these same two cycles, thus giving a more consistent mixture of fuel and air.
In summary, rotary combustion and hydrogen are made for each other—more or less.
The “less” part has to do hydrogen’s volumetric shortfall as an energy carrier, combined with a rotary’s inherent paucity of low-end torque. As a general rule, if an engine has dual-fuel capability, it gives up almost half its gasoline output when running on hydrogen. In the RX-8’s case, a gasoline-fueled 206 bhp drops to 108 on H2; torque diminishes from 164 lb.-ft. to 103. What’s worse, the RHE is tuned to deliver this reduced output in either of its dual-fuel modes.
Alas, the RX-8 HRE’s performance is decidedly sluglike. This feels all the more disconcerting because the car’s driveability on either fuel is exemplary. What’s more, the rotary’s phenomenal smoothness in revving is retained—only the kick is halved.
What’s Mazda to do?
If it were me, I’d dedicate the engine utterly to hydrogen. Its high octane—effectively 130+—would permit really abundant forced induction, gaining back a lot of the lost output. Also, in forgoing the dual-fuel jazz, I’d replace the RX-8 HRE’s currently minuscule 6 lb. of H2 at 5000 psi with perhaps double the tank volume and also take advantage of state-of-the-art 10,000-psi storage.
The car would still be an excellent “bridge,” a much-needed transitional vehicle in helping solve the chicken/egg challenge of hydrogen mobility. With proper rotary smoothness and power, it would be all the more entertaining as well.
(Purely as an aside—and because people have asked—the rotary concept isn’t amenable to diesel. This has nothing to do with anything we’ve discussed here. Rather, the problem is the infeasibility of arranging a rotary’s compression ratio high enough for diesel compression ignition.)