Nitrous Oxide Injection in Diesel Engines

How Nitrous Works to Boost Performance in Diesel Engines

Nitrous oxide can be used as a performance enhancer in internal combustion engines, including diesels. The principle of operation is the introduction of additional oxygen into the combustion chamber. Nitrous oxide is a compound containing two nitrogen molecules and one oxygen molecule bound together (formula N2O). When subjected to high temperatures (~1100° F), the compound breaks down into elemental nitrogen and oxygen gas. While nitrogen gas is inert, oxygen is not and its presence increases the performance potential of the combustion process by permitting additional fuel to be injected and combusted. Nitrous oxide injection is not legal for street use - this modification is intended for off-road use only and is therefore only suitable for competition use.

What is Nitrous Oxide Injection?

Nitrous oxide is bottled and stored in its liquid form. When injected into an engine, the pressure differential between the manifold absolute pressure and bottle pressure causes nitrous to transform into its gaseous state. As a result of this state change, the temperature of the nitrous is reduced considerably. Therefore, in addition to increasing the concentration of oxygen in the combustion chamber, nitrous oxide injection also provides significantly cooler intake air temperatures. The cooler incoming air charge is therefore denser, further increasing the oxygen available during the combustion process. In diesels, this is particularly beneficial as it can significantly reduce exhaust gas temperatures in high performance applications. Nitrous oxide injection can also be used as an aid in reducing turbocharger spool times by increasing the mass of exhaust gases flowing through the turbocharger at low engine speeds.

Nitrous oxide injection, used as intended, is relatively safe for use in diesel engines. Diesel engines tend to be significantly more robust than a standard gasoline engine and therefore can withstand higher cylinder pressures without the tendency to experience a catastrophic failure. However, every engine has its breaking point and a system should be carefully sized per the limitations of its intended application. Additional concerns with nitrous oxide injection systems in diesel engines include:

Turbocharger over-speeding - nitrous oxide injection works almost instantaneously, and a sudden increase in exhaust flow may cause the turbocharger to reach speeds significantly higher than intended, thus increasing the propensity for failure. The use of a wastegate will considerably reduce this risk.

Engine running lean condition - nitrous oxide injected in large quantities may lean the engine out and cause it to stutter and/or misfire. If this condition is experienced, the fuel injection rate needs to be increased or the nitrous injection rate needs to be decreased. On related terms, an engine may experience a condition in which the compressed mixture is too cool for complete combustion to occur.

Nitrous backfire - in the advent of a severe lean condition a cylinder may misfire, pushing fuel and air out the exhaust system. As a result of heat produced by a sequential combustion event, this mixture can be ignited before or within the turbocharger. Nitrous backfires are inherently dangerous to turbochargers are may cause catastrophic damage. This reiterates the fact that nitrous kits must be properly tuned to match the characteristics of their intended application.

Types of Nitrous Oxide Systems

Wet Nitrous Oxide System
A "wet" nitrous system is one that injects both fuel and nitrous oxide into the intake manifold. This is typically found in carbureted gasoline applications and the addition of fuel is necessary to maintain a stoichiometric air-to-fuel ratio. Wet nitrous systems are not found on diesel engines, as fuel cannot be drawn into the cylinder during the intake stroke; only air can be present in the combustion chamber during the compression stroke, and fuel is injected directly into the cylinder at or near the top of the compression stroke after the air has been compressed. Due to the fundamental operation of a diesel, a wet nitrous injection system is simply and inherently incompatible.

Dry Nitrous Oxide System
A "dry" nitrous system is one that injects nitrous oxide into the intake manifold. Dry nitrous oxide systems are compatible with diesel engines. A number of kits are available for Cummins, Duramax, and Power Stroke diesel engines. The quantity of nitrous injected for a given set of conditions is dictated by the size of the nozzle orifice and the number of nozzles installed for a particular setup. Nozzles are typically installed through the intake elbow or air horn of the engine.

Direct Port Nitrous System
Direct port nitrous injection is a wet system that injects nitrous oxide and a metered amount of fuel directly into the combustion chamber. Direct port systems are not compatible with diesel engines for the same reasons a standard wet system is incompatible. In gasoline engines, direct port systems are favorable in fuel injected applications; this is the most accurate and powerful delivery method.

Nitrous oxide systems can be designed as either a single nozzle system or a staged system. A staged system relies on a series of nitrous nozzles and solenoids. A multiple nozzle system typically allows greater control of power in race applications. In drag racing, for example, it is often beneficial to reduce power at takeoff and gradually increase power output once the risk of a loss of traction has been reduced. Example - a single "shot" of nitrous may be used during staging in order to help spool the turbocharger. After the 60 foot mark, a second stage of nitrous is triggered, increasing performance to its maximum potential.

The injection of nitrous oxide is typically triggered by a controller, which allows a driver to modify the parameters at which the nitrous system is activated. These parameters may include engine speed, accelerator pedal position, vehicle speed, turbo boost pressure, or time since takeoff.

Nitrous Oxide Pros and Cons

Pros Cons
Extremely high performance potential Not legal for street use
"On demand" power increase Risk of nitrous backfire
Reduced turbo spool times Relatively expensive, refill stations may not be readily available in all areas
Lower exhaust gas temperatures (EGT) Higher cylinder pressures increases wear