I want cars to continue having internal combustion engines, but become zero-emission vehicles by ceasing to offload combustion products, except at refuelling time.
This favours a low per-unit-energy sum of oxygen mass and fuel mass, or otherwise said, a low ash mass. On this scale water and water-plus-CO2, the ashes respectively of hydrogen and gasoline, both lose to B2O3. This is among the reasons I believe motorists will eagerly upgrade to boron when they get the chance.
G. R. L. Cowan, homepage.
G. R. L. Cowan, How fire can be tamed, Int. J. Nuclear Hydrogen Production and Applications 3 (2008), 235–248.