If cars rely on pressure caused by heated atoms then why do the fuel mixture need to be high energy? If I heated any gas up wouldn't they no also cause extreme pressure inside the combustion chamber? Would there need to be more gas in order to pull this off? What's the reason?
It's as simple as economy of space. The higher the density of fuel, the farther you can go on a tank of gas. Sure you could use lower energy fuel, but then you'd be filling up more often, or your vehicle would require a larger fuel tank to go the same distance.
For instance, ethanol (E100) fuel only has 73-85% of the energy density of gasoline. As a liquid, it still takes up the same amount of space as gasoline (petrol), but won't get you as far.
A second thing why other fuels aren't used, like hydrogen, are four fold:
- The infrastructure isn't setup to use it
- The auto industry isn't equipped to deal with it (large cost to change)
- Harder to contain hydrogen (need special tanks to hold it which are bulky)
- Hydrogen is more expensive to produce in quantity than is gasoline (you have to put more energy into producing hydrogen than you get out of it when used)
I use hydrogen as a good example, because 1kg (or 2.2lbs) of hydrogen has the same energy density as a gallon of gasoline. The thing is, though, a gallon of gasoline weighs about 6.9lbs per gallon. If the industry were able to make the switch and do it economically, it would be a great alternative to gasoline.