instead of a liquid. Some vehicles have been converted to natural gas, this started long ago, and there are some stations already selling natural gas at the pump. Natural gas is much preferred to propane because of its superior octane rating. Hydrogen has an impressive octane rating, I believe more impressive than natural gas. IIRC, it's about 140.

Small economy vehicles with small engines are already in production, it seems it would be less expensive for a manufacturer to modify the fuel delivery system than to design a completely new technology.

Ford had a Focus converted to hydrogen.

As I understand the fuel, because two hydrogen atoms meet one oxygen atom, the largest problem is the amount of hydrogen that must be carried on board. By way of comparison, gasoline mixes with oxygen in a ratio of about 12 (oxygen) to 1 (gasoline). Hydrogen is 1 (oxygen) to 2 (hydrogen): H2O. To traverse any distance, a lot of hydrogen has to be carried in tanks when compared to gasoline, so the efficiency of the engine is critical.

The only reason I've been able to concieve that we don't already have hydrogen powered cars is lack of hydrogen at stations and generalized foot dragging by industry to use what they already sell us. It's probably cheapest to drill for crude oil.

My main point is that it should be cheaper for industry to convert existing engines to hydrogen, than to devise an entirely new technology. Once filling infrastructure is in place, there's nothing to stop a more 'efficient' automotive technology from evolving from it, such as fuel cells, hybrids, or something else.