Engines Exposed: Nitrous Oxide Is Performance in a Bottle


Source: Thinkstock

While I’ve always been interested in machines, I didn’t really get excited about cars until I saw The Fast and the Furious. During that first race scene, Brian O’Conner (Paul Walker) presses two red buttons on his steering wheel labeled NOS 1 and NOS 2. When he checks with the laptop on the passenger seat to determine if the engine can handle NOS 2, it sounds an alarm and states “Danger to Manifold.” He ignores this and hits the NOS 2 button, which brings him within a car’s length of victory, but causes significant damage to his engine (and causing the passenger floorplate to fall out for some unknown reason). Having minimal knowledge of cars at the time, I assumed that manifold was a verb, i.e. the engine was manifolding under the stress of the race. Learning what a manifold was started my love affair with cars.

All nostalgia aside, this scene leaves a few questions unanswered for people who aren’t in the know. What is NOS? Why did it make his car accelerate? If it makes his car accelerate, why is too much of it a bad thing? And, perhaps most importantly, why is the driver of the white car so bad at Gran Turismo?

Source: iStock
Source: iStock

In the movie, “NOS” is frequently used synonymously with “nitrous oxide.” “NOS” unquestionably sounds a lot more appropriate for folks who say things like, “You can have any brew you want… as long as it’s a Corona,” but it’s actually a brand name for Nitrous Oxide Systems. Nitrous oxide is a compound with two Nitrogen atoms bonded with one Oxygen atom in a linear molecular shape. Contrary to what the film might lead us to believe, nitrous oxide is not flammable, it’s an oxidizing agent. Rather than get into the chemistry, just know that it provides more oxygen to the engine.

Purge Nitrous Express
Source: Purge Nitrous Express

As I’ve discussed before, air is the bottleneck to power. But it’s important to realize that “air” is a mixture of a lot of different compounds. The two that are most abundant are oxygen (O2) and nitrogen (N2). By mass, they make up 23% and 75% of the air around us, respectively. Nitrous oxide, on the other hand, is 36% oxygen by mass, but isn’t flammable until it decomposes into nitrogen (N2) and oxygen (O2). The nitrogen remains inert and the oxygen is combusted with fuel. Given that there’s 57% more oxygen available by mass, it’s possible for more fuel to be combusted. Really, this is the same concept as supercharging and turbocharging, but it’s accomplished chemically rather than mechanically.

It’s important that the nitrous oxide is in a liquid state because its increased density makes it possible to store more of it in the tank. But nitrous oxide doesn’t exist as a liquid at atmospheric conditions, so to force it in a liquid state, it must be cooled to -128 degrees Farenheit, or the pressure in the storage tank must be increased significantly. At 70 degrees Farenheit, that means the pressure must be increased to 760 PSI before it will condense.

And due to its extremely low boiling point, the liquid will boil almost instantly when it experiences a pressure drop. This process of boiling absorbs heat from its surroundings. Given that nitrous is injected into the intake manifold, its environment is the air that is going into the engine. By absorbing heat from the air, it reduces the temperature, which increases the density of the air entering the cylinder and lowers the temperature, helping to prevent detonation. But nitrous oxide has a tendency to boil in the lines between the storage tank and the engine. This is problematic because it delays the liquid’s injection into the engine after the system has been activated. To prevent this, drivers will purge the lines before a race begins.

Direct Port Manifold Nitrous Express
Source: Nitrous Express

There are several different kinds of nitrous delivery systems. In a “dry” system, the nitrous oxide is injected into the intake manifold, usually upstream of the mass airflow (MAF) sensor, independently of the fuel system. This is the simplest kind of nitrous arrangement, and it relies on a single injector. A “wet” nitrous system combines delivery of fuel and nitrous into one system. This allows the nitrous oxide system to control the fuel delivery while it’s being injected. There are two different kinds of wet systems: single-point and direct port (seen above). Single point refers to a single injection point in the intake manifold, while direct port means that there is an injector in each runner of the manifold. Direct port systems can be tuned to a much higher level of performance compared to the simpler single-point style.

Nitrous oxide is one of the simplest ways to add power to your car, but it’s not without risk. Increasing the amount of oxygen in the combustion chamber can cause serious engine damage if the air-fuel ratio becomes too lean. Detonation can occur, which leads to hot-spots and high temperatures, which leads to faster detonation and other big problems. It’s also important to safeguard agains this – while nitrous is being injected, significant engine damage can occur in under three seconds during uncontrolled detonation of a lean fuel mixture.

Ja Rule was wrong when he said that Brian has “…enough NOS in there to blow himself up. Period,” given that it’s a non-flammable compound. But he was right that it could wreck his engine, as it did by frying his piston rings, or junk his entire car if the tanks were to rupture. Nitrous may sound like a pretty volatile substance, but treat it with care and respect, and it’s one hell of a harmless performance booster. After all, remember its two most common uses: as a propellant for whipped cream dispensers, and as the anesthetic known as laughing gas, are designed to make life better. And if the nitrous doesn’t make you laugh, Ja Rule’s facial expression as Brian passes him definitely will.

Like classics? It’s always Throwback Thursday somewhere.