The flammability limits of hydrogen are extremely
wide, spanning from 4 to 75 Vol-% (of hydrogen in air), or
in equivalence ratio terms: 0:1 G G 5, thus making
hydrogen ideally suited for lean-burn operation ð G 1Þ.
Also, the laminar flame speed, a physicochemical mixture
property affecting the fuel burn rate, of a hydrogen–air
mixture at ambient and stoichiometric conditions is by a
factor of 4 to 6 higher than that of gasoline. At lean
conditions (around 0:5), the laminar flame speed of