Finally, since engine weight and volume affect vehicle design, and consequently in-use vehicle power requirements, those factors, as well as engine efficiency, must be taken into account in evaluating overall hybrid vehicle efficiency. John McCarthy has an informative page on many factors involved in using hydrogen for automobiles. The Cappella Cargo van model vehicle will burn a hydrogen fuel that when burned will not emit carbon dioxide. Mazda lent the vehicle to the Hirohata Steel Mill for use as their company van and drove it for at least 20,000 kilometers on public roads . The fuel was stored as metal hydride .
Some metal hydrides can store even more volume than liquid hydrogen.
MX-5 metal hydride, Miata body, front twin rotor, rear drive. Hydrogen has a low ignition temperature so the standard reciprocating engine is prone to pre-detonation and backfiring through the intake ports. The rotary engine separates the intake ports from the combustion area so there are no hot spots in the intake phase. Furthermore, the HR-X2 uses hydrogen injection into the engine after air intake. Thus it is encouraging that it performs as well as it does.