Left: a BMW's hydrogen filler cap. Right: fuel filler cap, Autovision Museum, Altlußheim, Baden-Württemberg, Germany.
A bivalent or bi-fuel engine is a combustion engine that can use two types of fuel, and a multivalent or multi-fuel engine is a motor that can use more than two.
Diesel engines are, by nature, bivalent, because they can use petrodiésel and biodiesel, although many engines can have reliability problems with high proportions of biodiesel.
Explosion engines, in addition to gasoline, can use fuels such as compressed natural gas, liquefied petroleum gas, ethanol or hydrogen.
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