its is fuel efficient and pollution free
But it's inflammable too ...
Because oxygen isn't fuel. A fuel (generally) is something that burns with oxygen.
For the same reason you can't have powdered water.
Bad example. We get a lot of powdered water where I live in the winter...
Yeah, but you don't have to add water to reconstitute it from the powder.
If we carried only O2, what would we react it with to extract energy?
We have been burning hydrocarbons, (such as wood, peat, coal, fat or oil), in air for thousands of years. The air contains oxygen which is consumed by the reaction to produce H2O and CO2. So over time we have come to expect chemical energy to be released by oxidation of a fuel. The oxygen is not the fuel, it is the oxidiser of the fuel.
Hydrolysis of water generates a fuel, H2, and an oxidiser, O2. We can then recover some of the energy invested by reacting them together. But we do not need to carry the oxygen on our vehicle as there is more than sufficient in the atmosphere. Because O2 weighs many times that of H2, our total vehicle weight is reduced by carrying the fuel only, without the oxidiser.
As said by others, to create combustion, you need a fuel and an oxidizer. Oxygen is the most common oxidizer and plenty of it is found in air. Fuel is usually something made with carbon and/or hydrogen.
But adding oxygen is already used to enhance performance in race cars, although not in its pure form.
The first method is with the addition of nitrous oxyde (N2O). The concentration of oxygen is more dense with nitrous than with air alone, so more fuel can be added within the same engine displacement, leading to more force acting on the piston.
The second method is to used nitromethane (CH3NO2) as a fuel. Nitromethane is a fuel (because it has C & H) that actually carries oxygen so less air is needed and - just like for nitrous - more fuel can be burned within the same combustion chamber volume.
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