When nitrous oxide transitions to a supercritical fluid at 97.5°F, distinct liquid and gas phases no longer exist. All of the nitrous oxide in the bottle becomes a homogeneous fluid phase. Above the critical temperature, orientation of the siphon tube is irrelevant.
However, what happens as the nitrous oxide transitions to a supercritical fluid is very relevant. Click on the following link to the University of Leeds Supercritical Fluids website for pictures of carbon dioxide undergoing phase transition. Nitrous oxide will behave in a similar manner. Near the critical temperature, the liquid meniscus starts to disappear and the liquid and gas phases become less distinct.
If you are operating your nitrous system with the bottle just below the critical temperature, the siphon tube will not consistently draw liquid and you can expect surging as the nitrous oxide mass flow rate varies.
The conclusion is that the system should never be operated just below the critical temperature. Since temperature of the nitrous oxide within the bottle is difficult to measure, pressure measurements will give more accurate results. If you keep the bottle pressure at or below 1,000 PSI, you should be safe. If the bottle gets above 97.5°F in the hot summer months, you can still run your system. You will be feeding supercritical nitrous and the mass flow rate will probably drop. Unless you re-jet the system, your overall performance will decrease and the engine will tend to run rich.
NC-2™ Nitrous Controller
Our 350Z Nitrous Project Car
Nitrous Oxide Systems - Recommended Reading
Why You Need A Nitrous System Controller
Most nitrous systems are designed to operate at 900-1,000 PSI. This corresponds to 85-93°F. In the winter months (or even summer if you live in a cold climate area), you will need some means of heating the bottle. Electric bottle heaters are readily available for this purpose. Depending on the manufacturer, the bottle heater is controlled by a thermostat or pressure switch. Each approach has pros and cons, but a pressure switch gives the most accurate control. Some designs use a pressure switch located downstream of the bottle valve, as this is a convenient attachment point.
Above 97.5°F, the nitrous oxide exists as a supercritical fluid. As the bottle temperature increases further, so does the pressure. But the fluid density also decreases rapidly. There are additional effects that must be considered when the supercritical nitrous oxide flows through pressure drops in the system, expands, cools, and reverts back into a bubbly mixture of liquid and gas. From tests we have conducted, it appears that as the bottle temperature increases above 97.5°F, the nitrous oxide mass flow rate drops.
'Warm' Bottle @ 88 Degrees F = 323 HP
Nitrous Jet Calculator Requires FIVE (5) Input Values
A Progressive Controller pulse width modulates the solenoid valves and allows you to control the flow rate. The two primary applications for a progressive controller are matching nitrous system power output to vehicle requirements and independently controlling fuel and nitrous oxide flows for optimum air/fuel ratio (AFR). The Daytona Sensors™ NC-2 can be used for both applications.
If you have a high horsepower nitrous system in a vehicle with limited traction, you can use time or RPM based progressive control to reduce the power output in the mid-RPM range or off the starting line to eliminate problems with wheel spin. You can also use RPM based progressive control to reduce excessive strain on the engine in the mid-RPM range.
Nitrous Oxide Injection is an easy and highly cost effective performance modification, yet it is also one of the most misunderstood. And these misunderstandings usually result in very expensive engine damage. (Engines that go 'BOOM' are NOT a good thing)
Many manufacturers and vendors selling Nitrous Oxide systems tend to gloss over the important details and some of the systems even lack basic safety features.
Surprisingly, there is only limited information is available on Nitrous Oxide relative to automotive performance. However, here's some books that we recommend:
Learn All About Nitrous Oxide
There is at least one well-documented case of a nitrous bottle explosion that occurred due to heat. Fortunately, nitrous oxide companies are well ahead of the curve on situations like these, as any bottle produced today has a safety blow-off cap or disc installed that will safely release the pent-up pressure once it exceeds a specific psi rating before the bottle can explode. While we’d hate to speculate and falsely accuse the owner of the vehicle of any wrong-doing, tampering with or incorrectly installing the safety valves on a bottle can render these safety features useless. And in this case, the valves/discs clearly didn’t function as they were designed to.
Nitrous Bottle Install From Our Nissan 350Z Project
A further power loss occurs because the charge cooling effect diminishes.
When liquid nitrous oxide (below 97.5°F) is injected, the charge cooling effect is the result of the both the latent heat of vaporization (heat energy absorbed when a liquid is vaporized) and the expansion of the resulting gas to atmospheric pressure.
Supercritical nitrous oxide (above 97.5°F) has zero latent heat of vaporization, so the only cooling effect is from expansion of the fluid.
From Early History To Chemical Structure
& Eventual Adaption For Engine Use
What Happens Inside The Nitrous Oxide Bottle When It Reaches 'Critical Temperature?
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Nitrous Oxide Injection
Here's Some Solid Information & Tips We Have Assembled To Help You During Your Build
NC-1™ and NC-2™ Nitrous Controllers
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Nitrous Bottle Heaters And The Effect of Bottle Temperature On Performance
Our Nitrous Oxide System Jet Calculator
by Dave Vizard
'Supercharging, Turbocharging and Nitrous Oxide Performance'
by Earl Davis (2002)
It is fairly obvious that a low bottle temperature will result in reduced nitrous oxide flow and a drop in performance. But what happens in the hot summer months? As the bottle temperature increases, the pressure and resulting nitrous oxide mass flow rate will also increase until the temperature reaches the critical point at 97.5°F. Liquid flow through a metering jet is proportional to the square root of pressure. At the critical point, the pressure will be exactly 1,051 PSI. For a system that was designed to operate at a nominal 900 PSI, this corresponds to an increase in nitrous flow of about 8%. If the system was jetted rich, with enough extra fuel to provide some safety margin, this should not pose a problem.
The conclusion is that bottle temperature must be maintained in an optimum range as recommended by the system manufacturer.
In areas with hot summers, you might benefit from a combination bottle heater/cooler. Contact the kit manufacturer for options.
Our calculators above are based on data published in the National Institute of Standards and Technology® Database 69 - June 2005 release. Most vendors list a recommended pressure range for their systems.
Applications For A Progressive Nitrous Controller
Nitrous system performance is dependent on BOTTLE PRESSURE. Below 97.5°F, pressurized nitrous oxide exists as both liquid and gas phases in equilibrium. As the bottle is discharged, a small amount of the liquid phase will vaporize and the resulting gas phase will occupy the free volume. The pressure will be entirely dependent on TEMPERATURE. If the bottle is maintained at a constant temperature, the pressure will also remain constant as long as there is some liquid nitrous oxide. If the bottle is rapidly discharged, such as may occur in a high horsepower application, vaporization of liquid nitrous oxide to fill the free volume will cause the temperature and pressure to drop.
THERE IS A CRITICAL TEMPERATURE & PRESSURE in chemistry, referred to as the CRITICAL POINT, above which a single substance cannot exist as a liquid or gas. Above this point, the substance becomes what is referred to as a supercritical fluid, with properties in between those of a liquid and gas. The critical point for nitrous oxide is 97.5°F and 1,051 PSI. Supercritical fluids are compressible similar to gases. Above 97.5°F, nitrous bottle pressure will depend both on temperature and density. At any given temperature above 97.5°F, the pressure will drop as nitrous oxide is discharged and the density decreases. Operating your system with the bottle above the critical point temperature can lead to erratic and unexpected results. You can use our Nitrous Bottle Pressure and Temperature Calculators to calculate the pressure and temperature relationships for any values below the critical point.
Do You Really Need A Nitrous Controller? - Take A Look At The Quote Below!
'Hot' Bottle @ 106 Degrees F = 302 HP
The graphs on the left show the results from two of our dyno tests on our 2006 Nissan 350Z Project Car with a 100 HP nitrous system are shown below.
The first test was conducted with the bottle at 88°F.
The second test was conducted with the bottle at 106°F and resulted in a drop of 20 HP.
A normally aspirated baseline run for this vehicle is about 240 HP.
by Joe Pettitt
'Nitrous Oxide High Performance Manual'
by Trevor Langfield (2006)
By independently modulating the fuel and nitrous solenoids, you can control the AFR. This is especially useful for late model fuel injected vehicles.
The NITROUS TO FUEL RATIO for most commercial kits is set in the 5.5-6.0 range (very rich) to provide a generous safety factor. A higher nitrous to fuel ratio in the range of 7.0-8.0 will generate more power. Before you recalculate the fuel jet size using a higher ratio, you should check your air/fuel ratio (AFR) using a wide-band sensor system. If your measured AFR values are below 11.5-12.0 when the system is activated, you can try using a smaller fuel jet. Go down one jet size at a time until your AFR values reach 11.5-12.0.
Nitrous Bottle Temperature Makes A Big Difference
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The following is taken from a past posting on the My350Z.com online forum. We couldn't have said it better ourselves:
"Well last night I was getting on my car, I’m only spraying a 50 shot because I haven’t had time to get the 100 shot pills....... but anyways I hit the rev limiter because I don’t have a window switch and wasn’t really paying attention. Now the car sounds like it’s misfiring and when it decelerates it sounds real bubbly. I’m thinking I might have bent a valve or something."
Basics Of A Nitrous Bottle Warmer & Words Of Caution
Read About Our Nissan 350Z Project Vehicle Nitrous Installation - CLICK HERE
The NITROUS HORSEPOWER TARGET is the increase in engine horsepower that can be expected when the nitrous system is activated. Note that some vendors and manufacturers sell kits with ratings based on rear wheel horsepower. To calculate jet sizes based on rear wheel horsepower, enter an engine horsepower value that is 15-20% higher.
Daytona Sensors™ Interactive Online Nitrous Oxide Calculation Tools
A Nitrous System Jet Calculator can help you estimate jet sizes for your system. You can use the calculated values as a starting point for tuning your system. Most Nitrous Jet Calculators we found on the Internet appear to be based on old formulas.
We based our calculator on actual nitrous and fuel flow tables published in the 'Supercharging, Turbocharging and Nitrous Oxide Performance' book by Earl Davis (see link above).
NOTE: The early book by Dave Vizard, (well-known & knowledgeable automotive engineer), includes extensive background information on Nitrous Oxide chemistry. However, it is somewhat out-of-date as far as applications.
Nitrous Bottle Pressure & Temperature Values