What is energy density?
Why a litre of diesel and a litre of ethanol are not the same amount of energy.
Energy density is how much usable energy a fuel carries per unit of volume or weight. It explains most of the practical differences between fuels.
Diesel is dense, which is why heavy equipment uses it. Ethanol carries roughly two-thirds as much per litre. Hydrogen is superb by weight and poor by volume, which is why it must be compressed so aggressively.
When comparing fuels, compare cost per unit of energy delivered — not cost per litre. A cheaper fuel that carries less energy is not necessarily cheaper to drive on.
Educational overview only. Before acting on any of this, review the safety fundamentals and local regulations.