As the name suggests, it involves supplying an engine with nitrous oxide. This contains more oxygen than the atmospheric air which the engine usually ingests; when injected into a cylinder, the additional oxygen is released and more fuel can be burnt - boosting the engine's output.
Wait, don't you mean NOS?
Nitrous is often referred to as 'NOS' - but that stands for 'Nitrous Oxide Systems', a company that produces power-adding set-ups for street and race cars. The company was founded in 1978 and bought out by Holley Performance Products in 1999, just before the brand and system rose to worldwide fame in 2001's 'The Fast and Furious'.
NOS is not the only company that makes such set-ups, however; other nitrous system companies include Wizards of NOS, Nitrous Express, Nitrous Outlet, ZEX and Edelbrock.
How does nitrous oxide injection work?
The chemical compound nitrous oxide consists of two atoms of nitrogen and one of oxygen. In automotive applications it is stored in liquid form in a tank, under high pressure, and - in the simplest of set-ups - its flow into the engine is regulated by a valve.
This valve, which is typically electronically controlled, releases the nitrous through a nozzle into the engine's intake manifold, from where it flows into the cylinders. When it is heated during the combustion process, it releases oxygen and nitrogen gas.
With more oxygen in the cylinders, more fuel can be burnt and more power produced. The amount of approximate power added by the nitrous, provided the fuel supply is regulated to suit, is controlled by metering jets that fit into the nitrous nozzle.
Activation of the system is usually controlled by an arming switch, which prevents accidental operation, and a triggering set-up. This is usually a throttle-activated switch, tripped when full throttle is applied, or a simple push button.
This can result in dramatic increases in power, however. Consequently, many more advanced set-ups use progressive controllers, which gently ramp up the flow of nitrous by pulsing the electronic solenoids that control the valves. Myriad configurations, control systems and triggering options are available.
The nitrous also boils to a gas upon injection into the intake manifold. This dramatically cools the intake charge and increases its density, granting further potential power increases.
What are the downsides?
Besides the fact that nitrous oxide usage is limited by the size of the bottle, incorrect tuning can cause significant engine damage - incorrect application of nitrous can result in catastrophic detonation, for example, or cause extremely lean mixture if the fuel system cannot keep up. This causes excessive temperatures, damaging the engine's components.
A properly tested and calibrated system can be a reliable method for achieving higher engine outputs for short periods of time, though - and is often far less expensive than other methods of increasing power to similar extents.
Can I use nitrous in a diesel engine?
Diesel engines can benefit significantly, and in a safe fashion, from nitrous oxide injection. The set-ups do differ, however, due to the difference in fuel supplies. A nitrous oxide installation on a diesel engine will only inject nitrous, as additional fuel cannot be supplied to the intake manifold.
Consequently, electronic or mechanical modifications have to be carried out to get the extra diesel required injected straight into the cylinders via the existing injectors. In some cases, an additional supply of alternative fuel - methanol, for example - may be added to deliver the extra fuel required for increased power production.
Different types of nitrous oxide injection
Dry: A 'dry' nitrous oxide system only injects nitrous into the engine's intake system, with the additional fuel required typically being supplied by altering the fuel delivery of the engine's original fuel system.
Wet: These systems inject both petrol and nitrous into the intake system, allowing for substantial increases in power without having to alter or rely on the original fuel system. Both single- and multi-point wet systems are common, the latter usually being referred to as 'direct port' nitrous injection. These are more complicated but, when it comes to metering fuel and nitrous, more accurate.
A brief history of nitrous oxide injection
Nitrous oxide itself was discovered in the late 1700s and, thanks to its anaesthetic properties, has been used in medical applications since the mid-1800s.
It was later used as an oxidiser in early rocket engines - a role it still fulfils today - and made the leap to piston engine applications, in earnest, during World War II; during the war, the Germans used it to boost the output of aircraft engines to deliver improved high-speed, high-altitude performance.
These developments led to its eventual application in automotive applications, with American race teams applying nitrous in the late 1950s to achieve significant power gains.