There is a lot of misunderstanding of what nitrous oxide is and what it does. For instance, many people think it is a poison and can kill you. It is definitely not a poison (it is in a can of whipped cream), but if you are in a room completely filled with nitrous oxide you will suffocate. Some people think it is an explosive, but it will not burn. Some people think it is a liquid, others think it is a gas. Both are correct; at normal atmospheric conditions, it is a gas, but when stored under sufficient pressure, it becomes a liquid. Why would a snowmobiler be interested in the stuff?
Nitrous Oxide- Go Fast Juice
One supplier of nitrous oxide products answers the question “when should you use nitrous oxide?” with the answer “when you want to go fast”. If nitrous oxide won’t even burn (it is not a fuel), and if you would suffocate in a room full of it (it does not support combustion), what about it would make you “go fast”? The answer is that at sufficiently high temperature, such as that existing in an engine cylinder, the nitrous oxide molecule breaks down into individual nitrogen and oxygen atoms. These additional oxygen atoms allow burning of additional fuel to provide additional power. It is just a little bit more complicated than that, and the following three points will explain this further.
Air contains 23% oxygen by weight; nitrous oxide contains 36%. Therefore, if nitrous oxide is introduced into an engine, the oxygen percentage is increased allowing more fuel to be burned, resulting in higher torque and power.
The manner in which nitrous is stored is another reason you can make more power. Nitrous oxide at normal atmospheric conditions is a gas. Manufacturers of nitrous oxide compress it to a high pressure, changing it from a gas to a liquid, allowing it to be stored in a relatively small volume. When released from its storage bottle, it changes back to a gas and becomes very cold, about -127 F (this is its boiling point at sea level pressure). When introduced into an engine, it cools the incoming charge to the engine and allows more mass to be stored in the crankcase in a two-stroke engine. In fact, thermodynamically speaking, this process is essentially the same as putting liquid nitrous oxide into the crankcase (along with supplemental liquid fuel), and since liquid takes up much less volume than a gas, more mass of fuel and oxygen exists in the engine crankcase and consequently the cylinder when nitrous oxide is used. More fuel and oxygen mass in the cylinder means more torque and power.
The third point is that nitrous oxide improves engine breathing. The use of nitrous oxide causes an increase in cylinder pressure (this is what gives the higher torque and power), and as discussed above there is more mass in the cylinder. Therefore, when the cylinder “blows down” at exhaust port opening, the vacuum wave created by the inertia of the exhaust gases is stronger because of the higher pressure and greater mass. This “negative exhaust pressure supercharging” improves mass flow through the engine, allowing it to “spin” faster. For instance, if you give an engine a 25 Hp “shot” of nitrous oxide and supplemental fuel, you typically get an extra 5 Hp “free” due to the peak torque moving to a higher RPM.
The Achilles Heel of Nitrous Oxide
Nitrous oxide has a bit of a bad name when used to increase engine power. I feel that the primary reason for this problem lies in the basic physics of the substance, specifically its vapor pressure vs. temperature.
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?
The problem is this; 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 uses a thermostatically controlled bottle heater, possibly along with an insulating blanket, to keep the bottle at a temperature of say 80F, giving a constant nitrous pressure of 865 PSI. Several drawbacks to this method are the cost of the heater, the power draw of the heater (which may be more than the electrical system can handle), and the thermal time constant of the bottle/heater combination - the time it takes for the bottle heater to do its job. I have not done any tests on this time constant, but I would think it would be in the order of 1/2 to 1 hour depending on the mass of the bottle and the wattage of the heater. This wait time may be unacceptable in some cases.
Another 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 third 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. Clutching will remain optimal, and you can run your race or climb your hill whenever you want.
We are presently developing nitrous kits around this new nozzle concept. These kits are compatible with our carb compensators (VARI FLOW, TEMPA FLOW, and ATACC), or other compensators such as DPM. These kits are available this season only in limited quantities and only to qualified parties. Our present kit requires the use of an air box and a pulse pump, but are developing kits for air filter application and fuel injection application.
