which substane is useful as fuel for cars hydrgen or octane?

Answer 1

Both are useful. Octane has the advantage that it is a liquid, and doesn't need compressing, but has the disadvantage that it gives off a great deal of CO2. Hydrogen needs a heavy cylinder to store it in, but the big advantage would be that the only product is water.

Answer 2

Hydrogen is nice in that, if used in a fuel cell, it generates useful power with high efficiency and the only waste product is water. But hydrogen will never be a suitable fuel for vehicles, for the following reasons:
- It is expensive to make. The cheapest source of industrial hydrogen is natural gas; the hydrogen has only a fraction of the energy of the gas.
- It is difficult to transport. Hydrogen seeps into cracks in steel, embrittling and weakening it, so it cannot be transported in standard pipelines.
- It is difficult to store. As a gas, its energy density is very low; a standard welding cylinder full of hydrogen at 2200 psi contains about as much energy as a half gallon of gasoline. Tanks proposed for vehicular use operate at as much as 5000 psi, which makes them large and heavy. In liquid form, hydrogen is the second coldest substance there is; it has to be stored and transported using special materials and techniques which can withstand the extreme temperature without cracking.
- It is difficult to deliver. One cannot reasonably imagine one's elderly aunt filling up an automobile with a substance which has to be delivered at 5000 psi.
Bottom line: if we had an unlimited source of cheap hydrogen, the best thing to do with it would be to combine it with coal and make octane, for which we already have the infrastructure in place.

Answer 3

hydrogen