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Nitrous Oxide PrimerNote: Nitrous oxide use on 3rd gen RX-7's is very, very rare. Supposedly Adam Suwatari uses it to help with bottom end power but he's the only example I've come across. For Nitrous Installation see the Nitrous How To.All internal combustion engines require air to operate. More specifically, they require oxygen (O2). Air is composed of approximately 20% oxygen and 80% nitrogen by weight. Simply put, the nitrogen in the air does not take part in the combustion process, but is merely heated by the combustion of O2 with a hydrocarbon fuel such as gasoline (C8H18, ISO-octane). Steam, hot CO2, and hot N2 then expand to push the piston down. Therefore, if we could replace or add additional O2 to the engine and burn that O2 with more fuel, we can make more power. In theory, if we replace all the air with pure O2, we could get 5 times the power out of an engine by being able to burn 5 times as much fuel per power stroke. In practice, we would blow up the motor at startup. Nitrous Oxide allows us to significantly increase the output of an engine without blowing up the motor, up to a point. Past that point, it’s like trying to put 6 pounds of sand in a five pound bag: the bottom falls out. Over-nitroused motors cause piston crowns, ring lands, pin bosses, and rods to collapse. Don’t over-nitrous! N20 primarily increases engine power by supplying more O2 to the engine. N20 is 36% O2 by weight, whereas air is 20%. A motor running on pure gaseous N20 could make 1.8 times more power than if it ran on air.If we inject liquid N20 into the engine, we can make substantially more power than this because the liquid nitrous is many times denser than air. Inject liquid N20 into the engine displaces only a small percentage of the air. The liquid N20 rapidly evaporates upon reaching the cylinders during the compression stroke. With the valves closed and the cylinder sealed, the initial cylinder pressure is increased before the combustion process starts. (This is similar to the process of supercharging an engine.) All we have done is cram in more N2 and O2. We must add more fuel to burn with the additional O2. If we don’t. the additional O2 will react with the piston crown or exhaust valve like a cutting torch.So there you have it: you put more fuel, oxygen, and nitrogen into the cylinder and you make more power by raising the cylinder pressure.Supercharging in a bottle. But wait—there’s more. With the increased oxygen percentage in the cylinder (remember N20 is 36% O2 whereas air is 20% O2 by weight), the combustion temperature and flame propagation rate can increase dramatically. In order to compensate, we just provide excess fuel as coolant and retard the timing or use a slower burning fuel.The theoretical stoichiometric nitrous to fuel ratio is 9.75 lb. or N20 to 1.0 lb. of C8H18 (gasoline) of 4.13lb N20 to 1.0 lb. of CH3OH (methanol). This will result in maximum power but will also produce unacceptably high combustion chamber temperatures. As mentioned above, we must run additional fuel to cool the combustion process. How much? A reasonable starting point is 100% excess fuel (twice as much fuel as the combustion process requires). In other words we would run 4.82 lb. of N20 to 1.0 lb. of gasoline, or 2.07 lb. of N20 to 1 lb. of methanol.The reversed ratios are 0.21lb of gasoline or 0.47 pound of methanol to 1lb of N20. Using less excess fuel will result in more power but higher cylinder pressure and temperature.How much horsepower can we make? A rough rule of thumb is that if we inject 5lb/min of nitrous into the engine with the appropriate additional fuel, we will make an additional 100HP. Another rule of thumb is to not drop below 50% excess fuel unless you know exactly what you are doing. Don’t add more than 0.3HP/cu inch of engine displacement to a stock (cast piston) motor. For example, for a 120 cu inch motor, 120 cu in X 0.3HP/cu in = 36 HP. For a 440 cu in motor, 440 cu in X 0.3HP/cu in = 132 HP. Retard the timing 3 to 4 degrees per 0.3HP/cu in. Don’t use a slower-burning fuel such as alcohol unless you know what you are doing. Don’t engage the system below 3000 rpm. Always flow-test your nitrous system and fuel system to insure proper fuel and N20 flow. Don’t ever run a nitrous system into the engine rev limiter. With ignition cut outs, dangerous levels of N20 and fuel will build up in the exhaust system, and with modern computer-controlled fuel shut offs, the motor will run excessively lean. Back off before you hit the engine rev limiter or install a safety switch switch turns the nitrous system off before the rev limiter engages. |
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