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The higher heating value assumes the water produced condenses and gives back its latent heat; the lower heating value assumes it leaves as vapour. Each kilogram of hydrogen makes nine kilograms of water, and vaporising that costs 2.442 MJ/kg at 25 C. Petrol at 46 MJ/kg with 14% hydrogen therefore delivers about 42.9 MJ/kg in an engine that exhausts steam.
Water formed
water = 9 x hydrogen mass fraction
Lower heating value
LHV = HHV - 2.442 x water formed
Almost always the lower value, because exhaust leaves hot and the water never condenses. Condensing boilers are the exception and are rated on the higher value.
Water is H2O: 2 g of hydrogen makes 18 g of water, so mass increases ninefold.