Degrees
Fahrenheit Bottle Pressure (PSI)
-30 167
-20 203
-10 240
0 283
10 335
20 387
32 460
40 520
50 590
60 675
70 760
80 865
97 1070
109 1150
Perhaps a cold bottle was to blame for Brit getting spanked?
Effects of temperature on Flow by Holztman Engineering
http://www.holtzmaneng.com/
As mentioned above, nitrous oxide at atmospheric conditions is a gas, having a boiling point of -127F. If you tried to carry it around in its gaseous state, the storage bottle would be much bigger than the snowmobile. That is why it is pressurized, the increased pressure raising its boiling point, causing it to liquefy. One important point to remember; the pressure existing in a bottle of nitrous oxide has essentially nothing to do with how much nitrous oxide is inside. It does not make any difference whether the bottle is full or there is only a tablespoon-full inside, the pressure is the same at any given bottle temperature. Well, this sounds like it would be the ideal situation. If the pressure does not change with the amount of nitrous in the bottle, then for any fixed size nitrous jet, the nitrous flow will be constant. Constant nitrous flow is good. So what is the problem?
Nitrous oxide vapor pressure changes drastically with temperature. Therefore, even though the bottle pressure does not change with bottle fill, the bottle pressure does change with changes in bottle temperature. Some specific examples will help. Nitrous oxide stored in a bottle at 0F has a pressure of 283 PSI. Another way of saying this is that nitrous oxide at 0F has a vapor pressure of 283 PSI, or conversely, the boiling point of nitrous oxide at 283 PSI is 0F. At 50F it has a pressure of 590 PSI. Nitrous oxide flow through a jet is compressible and sonic, and mass flow rate under these conditions varies directly with pressure modified slightly by the absolute temperature. In other words, if you double the pressure, the flow approximately doubles. The chart shows the effect of nitrous temperature on nitrous pressure and its approximate effect on flow (relative to the flow at 0F). Actual tests on a fixed nitrous jet (.029”) showed a delivery rate at 320 PSI (+7F) of 7.6 grams/second, 8.9 grams/second at 520 PSI (40F), 10.8 grams/second at 660 PSI (58F), and 13.1 grams/second at 800 PSI (73F). In other words, the nitrous delivery rate increased 72% from 7F to 73F.
This inconsistency presents a real problem when using nitrous oxide on a snowmobile. On a cold 0F morning, you start out with a 0F bottle having a pressure of 283 PSI. As the day goes on, air temps increase and, coupled with under-hood temperature rise, the bottle temperature can easily increase to 50F, giving a bottle pressure of 590 PSI. In a typical snowmobile application, nitrous oxide mass flow rate can double between a cold bottle temperature and a warmer bottle temperature.
Does this mean that using nitrous oxide on a snowmobile is hopeless? How can you live with a system with this much inconsistency? If the nitrous oxide delivery rate changes significantly, the power will change significantly, and if nothing is done to the supplemental fuel, it can either be too rich or too lean. Several methods are used to get around these problems.
One method is to just let the nitrous pressure and mass flow rate change, but adjust fuel flow accordingly to prevent a rich condition when the bottle pressure is low or a possible burn-down when the pressure is too high. There are several systems out that do this by various methods. I cannot speak to how well they work, but the main drawback is that the supplemental power delivered by the nitrous oxide system will vary with bottle temperature. Going back to our specific example of 0F and 50F bottle temperatures, if the nitrous oxide system is delivering 30 hp at 50F, it will only deliver about 15 hp at 0F.
I have developed a method which uses a patent pending nitrous nozzle to automatically maintain an essentially constant nitrous oxide delivery rate from 283 PSI (0F nitrous temperature) to 865 PSI (80F nitrous temperature). Our 30 hp nozzle is designed to deliver about 10 to 11 grams/second of nitrous oxide. Tests on one of our 30 hp nozzles delivered 10.1 grams/second at 300 PSI, 11 grams/second at 510 PSI, 10.1 grams/second at 740 PSI, and 10.3 grams/second at 840 PSI. We intentionally allow the nitrous to fall off about 20% as the temperature approaches -40 to protect the engine. Now, since nitrous flow rate is constant, supplemental fuel flow just needs to be kept constant (which is relatively easy to do), and supplemental nitrous power is consistent.
"the game of life of is not so much in holding a good hand as playing a poor hand well"
