Nitrous is a slang term for nitrous oxide (N2O), commonly used by drag racing classes like Pro Street, Top Sportsman, and Pro Mod. Nitrous oxide is an oxidizing agent used to increase an engine’s power output by allowing for faster burning of a fuel (usually gasoline). Nitrous can be used with alcohol in the mud racing categories.
Nitrous oxide boosts engine performance in a number of ways, some of which are not fully understood. Its primary effect seems to be exerted through its ability to release oxygen at high temperatures. When nitrous oxide decomposes, a single mole will release 1/2 mole of oxygen, 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. The greater the oxygen saturation, the higher the pressure and the greater the power released. However, peak cylinder pressure alone does not determine engine performance.
Nitrous oxide is stored as a liquid in tanks, but because of its low boiling point it will vaporize when it enters a cylinder during the intake stroke. As it boils, the cylinder temperature will drop, reducing the pressure during the compression stroke and thus reducing power loss. This drop in intake manifold temperature also increases the density of the air/fuel charge, thereby increasing the cylinders volumetric efficiency.
There is a third way in which nitrous improves engine performance. When N2O breaks down to release oxygen, nitrogen (N2) is also formed. Nitrogen gas contains molecules with extremely stable triple bonds, and so the formation of nitrogen is very exothermic. Because N2 is generated during the engine’s power stroke, nitrous boosts power by increasing the temperature inside the cylinder by the formation of diatomic nitrogen.
The original company in nitrous oxide injection was NOS. Today, there are several competing companies in the field, including BOSS NOSS, NOS, ZEX, Compucar, Top Gun, Nitrous Pro Flow, Nitrous Express, Nitrous Works, Cold Fusion, and Edelbrock.
Nitrous systems can increase power by 45% or more, depending on configuration, and are usually built in one or two stages. All Pro Mod cars and some Pro Steet cars use three stages, for additional power.
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 - air or gaseous nitrous oxide in the line will cause the car to "bog" for an instant until liquid nitrous oxide reaches the intake.
There are three types of nitrous systems: "dry", "wet single-point", and "wet multi-point". A nitrous system is primarily concerned with introducing fuel and nitrous into the engine’s cylinders, and combining them for most efficient combustion. A fourth type called a plenum bar system sprays the nitrous into the plenums of the intake manifold.
In a "dry" nitrous system, extra fuel required is introduced through the fuel injectors, keeping the upper intake dry of fuel. This property is what gives the "dry" system its name. Fuel flow can be increased either by increasing the pressure in the fuel injection system, or by modifying the vehicles’ computer to increase the time the fuel injectors remain open during the engine cycle. This is typically done by spraying nitrous past the MAF sensor (Mass Air Flow), which then sends a signal to the vehicle’s computer telling it that it sees colder denser air, and that more fuel is needed. This is typically not an exact method of adding fuel. Once additional fuel has been introduced, it can burn with the extra oxygen provided by the nitrous, providing additional power.
A "wet single-point" nitrous system introduces the fuel and nitrous together, causing the upper intake to become wet with fuel. In carbureted applications, this is typically accomplished with a spraybar plate mounted between the carburetor base and the intake manifold, while cars fitted with electronic fuel injection often use a plate mounted between the manifold and the base of the throttle body, or a single nozzle mounted in the intake tract. However, the intake must be designed for wet flow (for example, carburetors also require a wet flow intake), as distribution problems or intake backfires may result. Dry-flow intakes are designed to contain only air, which will travel through smaller pipes and tighter turns with less pressure, whereas wet-flow intakes are designed to contain a mixture of fuel and air. "Wet" nitrous systems tend to produce more power than "dry" systems, but are correspondingly more expensive and difficult to install.
A "wet direct port" nitrous system introduces nitrous and fuel directly into each intake port on the engine. These systems are also known as direct port nitrous systems. Normally, these systems combine nitrous and fuel through several nozzles similar in design to a "wet single-point" nozzle, which mixes and meters the nitrous and fuel delivered to each cylinder individually, allowing each cylinder’s nitrous/fuel ratio to be adjusted without affecting the other cylinders. Note that there are still several ways to introduce nitrous via a direct port system. There are several different types of nozzles and placements ranging from fogger nozzles that require you to drill and tap your manifold, to specialty direct port efi nozzles that fit into your fuel injector ports along with your fuel injectors.
A multi-point system is the most powerful and efficient type of nitrous system, due to the placement of the nozzle in each runner, as well as the ability to use more and higher capacity solenoid valves. Wet multi-point kits can go as high as 1,100 horsepower (820 kW) with only one stage, but most produce that much power with two or three systems. These systems are also the most complex and expensive systems, requiring significant modification to the engine, including adding a distribution block and solenoid assembly, as well as drilling, tapping, and building plumbing for each cylinder intake. These systems are most often used on racing vehicles specially built to take the strain of such high power levels. Many high-horsepower race applications will use more than one nozzle per cylinder, plumbed in "stages" to allow greater control of how much power is delivered with each stage. A two-stage system will actually allow three different levels of additional horsepower; for example, a small first stage can be used in first gear to prevent excessive wheelspin, then turned off in favor of a larger second stage once the car is moving. In top gear, both stages can be activated at the same time for maximum horsepower.
Another type of system is called a plenum bar system. These are spraybars that are installed inside of the plenums of the intake manifold. Plenum bar systems are usually used in conjunction with direct port systems in multi-stage nitrous systems.
The same technique was used during World War II by Luftwaffe aircraft with the GM 1 system to boost the power output of aircraft engines. Originally meant to provide the Luftwaffe standard aircraft with superior high-altitude performance, technological considerations limited its use to extremely high altitudes. Accordingly, it was only used by specialized planes like high-altitude reconnaissance aircraft, high-speed bombers and high-altitude interceptors.
British World War II usage of nitrous oxide injector systems were modifications of Merlin engines carried out by the Heston Aircraft Company for use in certain night fighter variants of the de Havilland Mosquito and PR versions of the Supermarine Spitfire.