Nitrous oxide (N2O) is a chemical compound used as an oxidizing agent to increase an internal combustion engine's power output by allowing more fuel to be burned than would normally be the case.
Nitrous and NOS
Nitrous is often used as an abbreviation for nitrous oxide, also referred to as NOS. The term NOS is derived from the initials of the company name Nitrous Oxide Systems, one of the pioneering companies in the development of nitrous oxide injection systems for automotive performance use.
Overview
When nitrous oxide decomposes, a single mole will release 1/2 mole of oxygen gas, allowing an oxygen saturation of 33% to be reached. Air, which contains only 21% oxygen, permits a maximum saturation of only 21%. This oxygen combines with hydrocarbons such as gasoline, alcohol, and diesel fuel to produce carbon dioxide and water vapor, which expand and exert pressure on pistons.
Nitrous oxide is stored as a liquid in tanks, but because of its low boiling point it vaporizes easily when released to atmosphere. When injected into an inlet manifold, this characteristic causes a reduction in air/fuel charge temperature with an associated increase in density, thereby increasing the cylinder's volumetric efficiency.
When N2O breaks down in the engine's combustion phase, the oxygen atoms are freed from their bond to the nitrogen atoms in an exothermic reaction, contributing to the overall power increase.
Nitrous systems can increase power by as little as 0.5 hp (0.37 kW) or as much as 3,000 hp (2,200 kW), depending on the engine type and nitrous system type. In many applications torque gains are even greater as increased fuel is burnt at a lower rpm range and is what causes the significant improvement in acceleration. All systems are based on a single stage kit, but these kits can be used in multiples (called 2, 3 or even 4 stage). The most advanced systems are controlled by an electronic progressive delivery unit that allows a single kit to perform better than multiple kits can. Most Pro Mod and some Pro Street drag race cars use three stages for additional power, but more and more are switching to pulsed progressive technology. Progressive systems have the advantage of utilizing a larger amount of nitrous (and fuel) to produce even greater power increases as the additional power and torque is gradually introduced as opposed to being applied to the engine and transmission immediately, reducing the risk of mechanical stress and consequently damage.
Fans can easily identify nitrous-equipped cars at the track by the fact that most will "purge" the delivery system prior to reaching the starting line. A separate electrically operated valve is used to release air and gaseous nitrous oxide trapped in the delivery system. This brings liquid nitrous oxide all the way up through the plumbing from the storage tank to the solenoid valve or valves that will release it into the engine's intake tract. When the purge system is activated, one or more plumes of nitrous oxide will be visible for a moment as the liquid flashes to vapor as it is released. The purpose of a nitrous purge is to ensure that the correct amount of nitrous oxide is delivered the moment the system is activated as nitrous and fuel jets are sized to produce correct air / fuel ratios, and as liquid nitrous is denser than gaseous nitrous, any nitrous vapor in the lines will cause the car to "bog" for an instant (as the ratio of nitrous / fuel will be too rich) until liquid nitrous oxide reaches the intake.
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