Q: What is the energy content of ethanol versus gasoline?
A: Ethanol has an energy density of 30 MJ/kg or 24 MJ/L, whereas gasoline (or petrol) has an energy density of 46 MJ/kg or 34 MJ/L. When comparing the fuels for use in reciprocating engines, it is important to consider the higher octane rating of ethanol, which permits operation at higher compression.
Q: How do liquid fuels differ for jet engines, diesel engines, racing engines, and passenger vehicles?
A: Quite a few factors go into consideration when optimizing a fuel for a particular application. These might include cost, energy density, cold-weather vaporization characteristics, water miscibility, octane rating, oxygen content, hot-weather vaporization characteristics, and pollution (all along the production and use chain). Gasoline, or petrol, is typically composed of molecules with 5–12 carbons, but is also often supplemented by ethanol. Kerosene, jet fuel, and diesel fuel are similar, with carbon counts of 8–15 or so. Drag-racing engines often burn nitromethane; it carries along its own oxygen supply, much like an explosive rather than a fuel.
If you are interested in learning a bit more about gasoline, you may find this link from Chemical and Engineering News interesting.
Q: What does nitrous oxide do in racing-car engines and/or in welding processes?
A: Nitrous oxide is N2O; it is a gas at standard temperature and pressure (STP). Since it carries one oxygen atom per molecule, it is a more concentrated source of oxygen for combustion than air. One can achieve more power per unit volume by burning excess fuel in the presence of nitrous oxide than in air.
Hence, some automobile enthusiasts use nitrous-oxide injection in the combustion chambers (engine cylinders) for faster acceleration, and welders and analytical chemists use nitrous oxide / acetylene flames to achieve higher temperatures than would be possible with air.