A Nitrous Oxide Engine, or Nitrous Oxide System commonly referred to and known as NOS, is an internal combustion engine in which oxygen for burning the fuel comes from the decomposition of nitrous oxide, N2O, rather than air. The system increases the engine's power output by allowing fuel to be burned at a higher-than-normal rate, because of the higher partial pressure of oxygen injected with the fuel mixture. Nitrous oxide is not flammable at room temperature, it only becomes flammable under extensive pressure. Nitrous injection systems may be "dry", where the nitrous oxide is injected separately from fuel, or "wet" in which additional fuel is carried into the engine along with the nitrous. NOS may not be permitted for street or highway use, depending on local regulations. N2O use is permitted in certain classes of auto racing. Reliable operation of an engine with nitrous injection requires careful attention to the strength of engine components and to the accuracy of the mixing systems, otherwise destructive detonations or exceeding engineered component maximums may occur. Nitrous oxide systems were applied as early as World War II for certain aircraft engines. Terminology In the context of racing, nitrous oxide is often termed nitrous or NOS. The term NOS is derived from the initials of the company name Nitrous Oxide Systems, Inc. (now a brand of Holley Performance Products) one of the pioneering companies in the development of nitrous oxide injection systems for automotive performance use, and has become a genericized trademark. Nitro is also sometimes used, though incorrect, as it refers more to nitromethane engines. Mechanism When a mole of nitrous oxide decomposes, it releases half a mole of O2 molecules (oxygen gas), and one mole of N2 molecules (nitrogen gas). This decomposition allows an oxygen concentration of 36.36% to be reached. Nitrogen gas is non-combustible and does not support combustion. Air—which contains only 21% oxygen, the rest being nitrogen and other equally non-combustible and non-combustion-supporting gasses—permits a 12-percent-lower maximum-oxygen level than that of nitrous oxide. This oxygen supports combustion; it combines with fuels such as gasoline, alcohol, diesel fuel, propane, or compressed natural gas (CNG) to produce carbon dioxide and water vapor, along with heat, which causes the former two products of combustion to expand and exert pressure on pistons, driving the engine. Nitrous oxide is stored as a liquid in tanks, but is a gas under atmospheric conditions. When injected as a