Nitrous oxide engine

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This article is about the use of nitrous oxide in a racing context. For other uses, see Nitrous oxide.
The white car in the near lane (closest to the camera) is purging nitrous oxide

A nitrous oxide engine is an engine in which the oxygen required for burning the fuel stems from the decomposition of nitrous oxide (N2O) rather than air. The system increases the internal combustion 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 into the fuel mixture.[1]


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, one of the pioneering companies in the development of nitrous oxide injection systems for automotive performance use.


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 33% 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 alone combines with gasoline, alcohol, or diesel fuel 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 liquid into an inlet manifold, the vaporization and expansion causes a reduction in air/fuel charge temperature with an associated increase in density, thereby increasing the cylinder's volumetric efficiency.

As the decomposition of N2O into oxygen and nitrogen gas is exothermic and thus contributes to a higher temperature in the combustion engine, the decomposition increases engine efficiency and performance, which is directly related to the difference in temperature between the unburned fuel mixture and the hot combustion gasses produced in the cylinders.

In many applications torque gains are even greater as increased fuel is burnt at a lower RPM range; this relative increase in power in the lower RPM band 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 two-, three-, or even four-stage kits). 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 are gradually introduced (as opposed to being applied to the engine and transmission immediately), reducing the risk of mechanical stress and, consequently, damage.


Cars with nitrous-equipped engines can be identified by the "purge" of the delivery system that most drivers perform 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.

Types of nitrous systems[edit]

There are two main categories of nitrous systems: dry & wet. A nitrous system is primarily concerned with introducing fuel and nitrous into the engine's cylinders, and combining them for more efficient combustion. There are four main sub types of wet system: single point, direct port, plate, and plenum bar all of which are just slightly different methods of discharging nitrous into the plenums of the intake manifold.


In a dry nitrous system, extra fuel required is introduced through the fuel injectors, keeping the manifold 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 vehicle's computer to increase the time the fuel injectors remain open during the engine cycle. This is typically done by spraying nitrous past the mass airflow sensor (MAF), 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.

Dry nitrous systems rely on a single type nozzle that only sprays nitrous through it, not nitrous and fuel. These nitrous nozzles generally spray in a 90 degree pattern.

Wet single-point[edit]

A wet single-point nitrous system introduces the fuel and nitrous together, causing the upper intake manifold 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, most makes of nitrous systems combined with unsuitable intake designs, often result in distribution problems and/or intake backfires. 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 in some cases can be more expensive and difficult to install. This is due to the process of discovering, cutting, and tapping into the fuel delivery line, many of which are at high pressure (20-40PSI.)

A wet nozzle differs in the way that it takes in both nitrous and fuel which are metered by jets to create a perfect or proper air-fuel ratio (AFR). Proper atomisation of the fuel and nitrous will ensure consistent power gains.

Newer wet nitrous kits on domestic[clarification needed] cars have become increasingly easy to install by pulling fuel via the schrader valve on the fuel rail, which is normally designated as a fuel test port. It makes plumbing and using a wet nitrous kit much simpler.

Wet direct port[edit]

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 through a direct port system. There are several different types of nozzles and placements ranging from fogger nozzles that requires one to drill and tap the manifold, to specialty direct port E.F.I. nozzles that fit into the fuel injector ports along with the fuel injectors.

A multi-point system is the most powerful 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 3,000 horsepower (2,400 kW) with only one stage, but most produce less than half that amount with two, three or even four stages.[citation needed] These systems are also the most complex and expensive systems, requiring significant modification to the engine, including adding distribution blocks and solenoid assemblies, as well as drilling, tapping and constructing plumbing for each cylinder runner. 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. A more recent improvement on the staged concept from WON is the progressive delivery system, which allows a simpler single stage system to act even better than multiple stages, delivering a smoothly progressive increase in power which is adjustable to suit the user requirements.

Plenum bar[edit]

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.

Propane or CNG[edit]

It is possible to combine the use of nitrous with a gaseous fuel such as propane or compressed natural gas. This has the advantage of being a dry system and yet still maintaining proper air/fuel mixture. Such a system requires exact choice of jet sizes and gas pressure regulation to provide a consistent pressure to the jets. Other advantages include better air/fuel mixing and distribution and less risk of knocking due to the increased octane rating of propane and CNG.

Reliability concerns[edit]

A piston which was cracked due to nitrous use.

As with all modifications to increase power, the use of nitrous oxide carries with it concerns about the reliability and longevity of an engine. Due to the greatly increased cylinder pressures, the engine as a whole is placed under greater stress, especially the parts involved with the combustion chamber. An engine with components not able to cope with the increased stress imposed by the use of nitrous systems can experience major engine damage, such as cracked or destroyed pistons, connecting rods, or crankshafts.

Even if the engine is up to the task, severe damage can occur if a problem occurs in the fuel system; an engine running with nitrous oxide depends heavily on the proper air to fuel ratio to prevent detonation from occurring. For example, if the engine's fuel supply were to be reduced, this would cause the engine to run lean by whatever degree the fuel delivery was reduced, which can lead to engine knock or detonation. Depending on the engine, this may only need to occur for a matter of seconds before major damage occurs.

It is essential not to reach a fuel cut rev limit as this will also momentarily restrict the fuel flow to the engine and as nitrous is still being injected into the engine without the additional fuel the engine will again run lean and cause detonation.

Some mechanism to disable the nitrous system when knock is detected by a knock sensor would be beneficial. Ignition timing must also be watched closely when using nitrous oxide. It is said[by whom?] that, on a large car engine, for every 50 horsepower of nitrous used, the ignition timing must be retarded by two degrees.[citation needed] This is recommended for any stock type application. It is also recommended[by whom?] that high octane fuel (92 octane minimum) be used to avoid detonation.[citation needed]

Good optimisation of enrichment fuel is essential otherwise the fuel can 'drop out' and puddle in the intake tract, potentially causing a backfire. With a properly designed nitrous injector and correct placement of the nozzle (not too far from the intake entry point and away from any abrupt bends and restrictions in the intake tract) backfires can be avoided.

Street legality[edit]

Nitrous oxide injection systems for automobiles are illegal for road use in some countries. For example, in New South Wales, Australia, the Roads and Traffic Authority Code of Practice for Light Vehicle Modifications (in use since 1994) states in clause that The use or fitment of nitrous oxide injection systems is not permitted.[2] Nitrous is legal in street driven automobiles, only if the feed line from the bottle is disconnected or if the bottle is dry (empty) or simply closed.[citation needed]. In Great Britain, there are no restrictions on use of N2O, but the modification does have to be declared to the insurance company, which will undoubtedly require a higher premium for Motor Vehicle insurance or could refuse to insure. Regulations in Australia vary by State, but is banned in New South Wales.

In Germany, despite its strict TÜV rules, a nitrous system could be installed and used legally in a street driven car. The requirements for the technical standard of the system are similar to those of aftermarket natural gas conversions, especially for the gas bottles. Since the car still has to meet its emission standards, which depend on the car's year of construction, it is easier for older cars.

Racing rules[edit]

Sanctioning bodies in motor sports allow nitrous oxide use in some classes. It is legal in IHRA competition in Pro Modified, Top Sportsman, and Top Dragster. It is legal in the NHRA only for Pro Modified and in certain classes of bracket racing cars.

In 1976, NASCAR fined many drivers for doing so; in June 1998, the NHRA suspended Pro Stock driver Jerry Eckman and car owner Bill Orndorff for a year, stripped the team of all points, and imposed a fine for violations. The team closed down shortly after the suspension. Eckman later served as a crew chief on Pro Stock teams, but waited until 2012 to regain his competition license in order to help tune for the team he was working.


A similar basic technique was used during World War II by Luftwaffe aircraft with the GM-1 system to maintain the power output of aircraft engines when at high altitude where the air density is lower. 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.

See also[edit]


  1. ^ "Nitrous: Everything You Need to Know". 2011-09-27. Retrieved 2013-07-11. 
  2. ^ Code of Practice for Light Vehicle Modifications. Roads and Traffic Authority of New South Wales. 1994. ISBN 0-7310-2923-2. 

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