Nitrous Oxide Injection Systems

Nitrous oxide injection is the easiest and most cost effective performance modification, yet it is also one of the most misunderstood. And these misunderstandings usually result  in very expensive engine damage. Vendors selling nitrous systems tend to gloss over the details and some of the systems lack basic safety features. 

 

My personal involvement with nitrous technology started when I was doing consulting work for Jacobs Electronics in Los Angeles back in 1990. Dr. Jacobs (now retired), had a Camaro equipped with a system by NOS. He recognized a need for an electronic control system. I wound up designing the Jacobs Nitrous Mastermind (shown at left) that they still sell today. Soon after, Jacobs moved to Midland, Texas and we parted company. About the same time, another client, Crane Cams, moved their electronics operation from LA and I followed them to Daytona Beach, Florida.

Several years ago, I purchased a Mitsubishi 3000GT. The performance of the normally aspirated V6 was somewhat lacking, so I decided to install an NOS model 5124 dry manifold nitrous oxide injection system. NOS was the first company to introduce dry manifold systems. Dry manifold refers to the fact that the required supplemental fuel is supplied by increasing the fuel pressure to the OE port injectors. This eliminates potentially dangerous fuel puddles in the manifold runners that can result with older systems using separate fuel injection nozzles for the supplemental fuel. 

While the NOS dry manifold system solved the fuel distribution problem, it created a new problem that quickly became apparent. The NOS system injects nitrous oxide at a constant flow (lbs/hour), determined by bottle pressure and the nitrous jet size. However, merely raising the fuel pressure to the OE injectors does not result in constant supplemental fuel flow matched to the nitrous flow. The lbs/hour supplemental fuel varies with RPM. The engine runs lean at low RPM and rich at high RPM.  

 

Correctly matching the dry manifold system nitrous flow to the supplemental fuel requires an electronic control. Experiments with the 3000GT led to the design of the new Crane Digital Nitrous Control. In the process of developing and testing the new Crane control, a lot of interesting background information was uncovered about nitrous oxide.

 

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The Secrets of Nitrous Oxide Revealed

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Nitrous oxide (N2O) is a colorless and almost odorless gas. Joseph Priestly, an English scientist and clergyman, discovered nitrous oxide in 1793.  Following Priestley's discovery, Humphrey Davy of the Pneumatic Institute in Bristol, England, experimented with nitrous oxide and noted its anesthetic effects. Today nitrous oxide is widely used as a dental anesthetic, an engine performance enhancer, and for processing in the semiconductor industry.

Only limited information is available on nitrous oxide as relates to automotive performance. Recommended books include: David Vizard, The Complete Do-It-Yourself Guide To: Nitrous Oxide Injection (North Branch, MN: S-A Design/CarTech, 1988) and Joe Pettitt, How To Install Nitrous Oxide Injection (North Branch, MN: S-A Design/CarTech, 1998). Dave Vizard, an old acquaintance from my California days, is a very knowledgeable automotive engineer. His book includes some background information on nitrous oxide chemistry. Pettitt is a journalist. His applications are more up to date, but some of the math and technical details are flawed. 

Nitrous oxide was first used as an engine performance enhancer on World War II aircraft. The original NACA (predecessor of NASA) wartime report is now in the public domain and available for download: Huppert et al., Nitrous Oxide Supercharging of an Aircraft-Engine Cylinder (NACA MR E5F26, 1945). More interesting information is contained in another NACA technical note available for download: Sabol and Evans, Investigation of the Use of Thermal Decomposition of Nitrous Oxide to Produce Hypersonic Flow of  a Gas Closely Resembling Air (NACA TN 3624, 1956).

Interest in nitrous oxide enhancement of aircraft performance didn't end with World War II. How about a nitrous oxide enhanced SR-71 Blackbird with a flow rate of 57,600 lbs/hr? During the course of our research, we came across Conners T., Predicted Performance of a Thrust-Enhanced SR-71 Aircraft  with an External Payload (NASA Technical Memorandum 104330, 1997).     

The original NACA report requires very careful reading. This report was done under wartime security conditions and never subjected to peer review.  Tests were done on a single cylinder from a V12 Allison. Nitrous oxide was injected as a gas in all tests, not as a liquid as is the modern practice. Data was extrapolated for low pressure liquid nitrous oxide injection at -128° F, again not directly applicable to modern practice.  

One of the major misconceptions about nitrous oxide relates to whether it is an oxidizer or fuel. Nitrous oxide is an oxidizer. Thermal decomposition of nitrous oxide at combustion temperatures releases significant energy and creates free oxygen. The free oxygen can then react with supplemental fuel and release further energy. As we shall see, injecting the nitrous is generally the easy part - the practical difficulties lie with the supplemental fuel. 

Dave Vizard worked out the overall chemical reactions and optimum (stoichiometric) ratio. While gasoline actually consists of many hydrocarbon fractions and additives, equivalent reactions of iso-octane provide a close approximation for practical purposes. Iso-octane is a single component fuel often used for lab tests. The overall combustion reaction of iso-octane and nitrous oxide is:

C₈H₁₈ + 25N₂0 > 8CO₂ + 9H₂O + 25N₂

From this reaction, one can calculate an ideal ratio (by weight) of 9.649:1 nitrous oxide to iso-octane. In practice, a considerably richer 6:1 mixture is commonly used to prevent detonation. 

The thermochemistry of the nitrous oxide and iso-octane reaction can be better visualized by breaking down Vizard's original reaction into two stages: the thermal decomposition of the nitrous oxide and the combustion of the iso-octane. On a gram mole basis:

 25N₂0 > 25N₂ +12.5O₂ + 2040 kJ heat 

 C₈H₁₈ + 12.5O₂ > 8CO₂ + 9H₂O + 5112 kJ heat

The second reaction is the same as if the iso-octane was combusted using oxygen from air. As we can see, for a given amount of fuel (iso-octane), combustion using nitrous oxide as a source of oxygen yields 40% more energy.

All modern applications inject liquid nitrous oxide into the intake manifold. The latent heat of vaporization of the nitrous oxide significantly cools the intake air. Our analysis shows that for a wide range of nitrous oxide injection flow rates, the cooling effect is such that the mass airflow is not significantly reduced. Tests were conducted on a vehicle with a mass airflow sensor. Nitrous oxide was injected into the air stream after the mass airflow sensor and the sensor monitored with an OTC scan tool. The tests showed that over a range of nitrous oxide flow rates, the mass airflow through the sensor did not appreciably drop. 

Potential power gains can be estimated from the amount of supplemental fuel that reacts with the nitrous oxide. Most gasoline engines have a BSFC (brake specific fuel consumption - brake refers to dyno) in the range of .5 lb/hp-hr when operating wide open throttle with a slightly rich 12.8:1 air to fuel ratio. This means that a typical 200 horsepower engine will burn about 100 pounds of fuel per hour.  

If we inject nitrous oxide and provide an appropriate amount of supplemental fuel, we can calculate the power gain available from the fuel that actually reacts with the nitrous oxide. We can think in terms of a BSFC figure for fuel that reacts with the nitrous oxide. Calculations based on thermochemistry and engine efficiency suggest that the BSFC-nitrous is in the range of .285 to .32 lb/hp-hr for modern engines. Computer analysis of data in the original NACA report suggests a measured BSFC-nitrous in the range of .28 to .29 lb/hp-hr for the Allison V12 engine used in the tests. The Allison V12 was run at a very rich 10.5:1 A/F ratio and low 6:1 compression ratio, so the results may not be indicative of modern engines. Modern dyno tests suggest horsepower gains in the range of 20 horsepower per lb/min of nitrous oxide flow. Converted back to a BSFC-nitrous figure (based on stoichiometric nitrous oxide to fuel ratio), this corresponds to .31 lb/hp-hr. 

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Dry Manifold System Considerations

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Detail of the solenoid valves and nitrous pressure regulator used with NOS dry manifold system. When the #1 solenoid valve is energized at wide open throttle, the nitrous pressure regulator applies about 50 psi to the fuel pressure regulator reference port. A separate fuel pressure switch (not shown)  senses the increased fuel pressure and energizes the #2 solenoid valve. Nitrous oxide is then sprayed into the air pipe. The Crane Digital Nitrous Control pulse width modulates the #2 solenoid valve to reduce nitrous oxide flow at low RPM and better match the nitrous oxide flow to the fuel flow.

Dry manifold systems require some means of boosting fuel delivery to supply the required supplemental fuel. Systems on the market either boost the fuel pressure (NOS and ZEX) or increase the fuel injector pulse width (Venom). Both approaches have limitations. With a constant injector pulse width, fuel flow with the NOS and ZEX systems varies as the square root of the fuel pressure, i.e. a 41% increase in fuel flow requires doubling the fuel pressure. Most OE systems run normal fuel pressures in the 35-45 psi range. Some injectors run into problems above 80 psi, so a 40% increase in fuel flow is about the practical limit. Most OE systems have limited headroom as far as increasing the injector pulse width. If the stock fuel injection normally runs up to 85% duty cycle at wide open throttle, the Venom system can only increase fuel flow by15% by holding the injectors open. In one recent magazine sponsored dyno test, the Venom system gave a power gain of only about 10%. On the other hand, 30-40% power gains are feasible with the NOS and ZEX systems. This is probably about the limit of what an otherwise stock engine can tolerate with major internal modifications. 

In most applications of dry manifold systems such as the NOS or ZEX that boost fuel pressure, the stock fuel pump will prove inadequate. For maximum performance, you have to match the fuel pressure to the nitrous flow rate. Unfortunately, the manufacturers don't tell you this. Neither the NOS nor the ZEX instructions give any information on this subject. The ZEX system also lacks any safety features to keep the unit from operating with inadequate fuel pressure.

Measuring nitrous flow is not particularly difficult. You can set the system up on a bench, or even do the test with the system installed on the vehicle (engine not running and injector nozzle removed from intake). Run the test outside as the sulfur dioxide added to denature the nitrous oxide is moderately irritating. Secure the injector nozzle so that it can't flail around and make sure it won't spray you or anything else that could be damaged by extreme cold. Do this on a warm day when you can stabilize the bottle temperature at about 80° F. You can use a car battery to energize the solenoids. Weigh the bottle before and after the test. Use an accurate electronic scale that reads down to at least .1 lb increments (don't try this with a cheap bathroom scale). Use a stopwatch to measure elapsed time. Spray for 15 second intervals and then allow the bottle temperature to stabilize again for at least 15 minutes. If you spray for a total elapsed time of about 1 minute, the small volume lost in the supply line will be insignificant. You can then weight the bottle again and calculate the flow rate in lbs/minute or lbs/hour. 

You will need to measure the normal fuel pressure and the increased fuel pressure when the nitrous system is activated. For safety reasons, you should only use a electronic remote sensing fuel pressure gauge. Don't ever try to install a "T" fitting and plumb a line to a mechanical gauge inside the car. Autometer 4363 or 4463 gauges read 0-100 psi fuel pressure and are ideally suited for use with dry nitrous systems.          

Dry Manifold System Calculations

The author has prepared an Excel (2000 version) spreadsheet for analysis of dry manifold nitrous oxide injection as applied to modern engines with port fuel injection systems. With initial entries for baseline engine data and lb/hr nitrous flow, the spreadsheet calculates the horsepower gain and required increase in fuel injector pressure. Running the spreadsheet requires the Solver plug-in for Excel. If you have never used Solver, you will need to load it from your Excel installation disk. Clicking on the download will open the spreadsheet in Excel. You can then save the file. Running Solver while on-line sometimes leaves garbage on the screen.  

Download Dry Manifold Calculations Spreadsheet

 

 

Dry Manifold Calculations Spreadsheet - Baseline Engine and Nitrous Oxide Data                       

Required data entries are in bold. You must enter the baseline engine horsepower, air inlet temperature, fuel pressure, nitrous flow in lb/hr, and nitrous bottle temperature. The other values shown under baseline engine and nitrous injection system data are reasonable defaults. If you place the cursor on any value, an explanatory comment appears as shown. Once you enter the required data, Excel calculates base values for air flow, fuel flow, manifold heat input (to vaporize fuel) and important nitrous oxide properties. 

In this screen shot, the comment box partially obscures the nitrous oxide to fuel ratio, supplemental fuel flow and nitrous oxide latent heat of vaporization. Supplemental fuel consists of fuel burned with oxygen from thermal decomposition of nitrous oxide and additional cooling fuel. The original NACA researchers found it convenient to express supplemental cooling fuel as a mass ratio to nitrous oxide flow.  We have followed this approach and you can change the default .06 ratio.  Excel also calculates the ratio of nitrous oxide to total supplemental fuel. Experience has shown that this should be in the range of 6:1 to avoid detonation.

 

Dry Manifold Calculations Spreadsheet - Running Excel Solver  

The next step is to run the Excel Solver from the Tools menu. Solver parameters are predefined as required for the spreadsheet model. All you need to do is click on Solve and then OK after a solution is calculated. Note that you must run Solver every time you change anything. Solver calculates changes in engine parameters (blue) that occur when nitrous oxide and supplemental fuel injection occur. For the technically inclined, Solver finds a solution for two equations with two independent variables. The independent variables are mass airflow and mixture temperature. Nitrous oxide and supplemental fuel displace some of the baseline air flow. The latent heat of vaporization of both the nitrous oxide and fuel cause the mixture temperature to drop. The lower temperature results in denser air and less displacement. The two equations are heat balance in the intake manifold and mixture flow balance. Under steady state conditions, both of these values are zero (click on the values to see the comment boxes for more details).

 

Dry Manifold Calculations Spreadsheet - Performance Results  

After Solver has found a new solution, performance results are updated. You can see the expected horsepower with the nitrous oxide system activated. Most applications will show gains near 20 horsepower per lb/min nitrous flow. Excel displays the required fuel pressure. As explained in the previous section, you should check your fuel pressure with a gauge. If you are planning to obtain more than 10-15% power gains, you will likely require a high output fuel pump. 

The last figure displayed is the nitrous oxide to air mass ratio. The original NACA researchers felt that this ratio best expressed the overall operation of the system. Ratios above .20 will likely cause significant combustion temperature increases.

You can examine some interesting "what if" scenarios with this spreadsheet tool. For a real eye opener, calculate the change in performance with bottle temperature. Assume that you adjust the jet size as required to maintain the same nitrous flow. You will find cooler nitrous oxide gives more performance. The reason is simple if not obvious. The thermodynamic properties of nitrous oxide are such that the latent heat of vaporization rapidly drops with temperature. Near 97° F, nitrous oxide changes to a supercritical fluid and losses its cooling potential. Without the cooling effect, mixture temperature increases and more air flow is displaced. We were surprised by the extent of this effect. We would like to give particular credit to the helpful staff at Puritan-Bennett for providing us with the complete thermodynamic data on nitrous oxide.      

Bottle Heater Controversy

The author would like to offer some thoughts on bottle heaters. Anyone who has surfed the web looking for information on nitrous oxide has probably come across Doyle Schoenberger's website that displays the unfortunate result of a bottle explosion in very graphic detail. This incident has given rise to a lot of discussion and controversy about the necessity and safety of bottle heaters.  

The bottle heater involved in the explosion was manufactured by Nitrous Express and used a pressure sensor that turned off the heater when the pressure reached a set limit. If the bottle valve was closed the heater could always remain on. The unfortunate customer admits to having made several mistakes and an overfilled bottle may have been a contributing factor. NOS sells bottle heaters that use a thermal switch. This appears to be a safer approach and there are no known explosions involving NOS bottle heaters.  

Even the remote possibility of a bottle heater turning into a bomb fuse raises questions. What is the effect of bottle temperature on performance? Is a bottle heater even necessary? The general rationale behind a bottle heater is that the nitrous oxide pressure increases with temperature. Consequently the flow rate should increase with temperature.  If nitrous oxide were a well behaved liquid and no other effects entered in the picture, power should also increase. However, nitrous oxide is not a well behaved liquid at room temperature. At the critical temperature of 97.58° F, nitrous oxide turns into what physicists refer to as a supercritical fluid - with properties in between those of a liquid and a gas. The latent heat of vaporization rapidly drops above 90° F and reaches zero at the critical temperature. The cooling effect is lost and more airflow is displaced. The result is a slight drop in power, even though the nitrous oxide mass flow rate is increasing. The chart below shows  the measured flow rate for a #40 jet and the calculated power gain at various bottle temperatures. 

Over a reasonable range of bottle temperatures, power only changes a few percent. If you can keep the bottle temperature above 70° F, you probably don't need to worry about a bottle heater. The optimum temperature appears to be around 80° F - somewhat less than the typical 85° F set point used for most bottle heaters. You don't want the bottle temperature to go much above 80° F as the power starts to drop off. If you live in a warm climate, you might want to consider cooling your bottle on hot days. 

Analysis of the ZEX Dry Manifold System

 

The new ZEX Innovation dry manifold system was ballyhooed as a great improvement in the state-of-the-art with its patented principle of compensating fuel pressure based on nitrous pressure. We purchased a unit to test but never put it on the vehicle. Downloading and reading the patent would have saved some trouble. The unit certainly looks attractive, but appearances can be deceiving. We were disappointed by what we found when we took the unit apart.

 

 

Here's what they don't want you to see. The ZEX unit uses a single solenoid valve. The small black electronics module contains a throttle position sensor interface. Near wide open throttle (about 4 volts), it energizes the solenoid valve. Nitrous oxide injection immediately starts. Some of the nitrous flow  bleeds through a miniscule orifice hidden inside the brass four port manifold and then  pressurizes the fuel pressure regulator. The unit lacks any safety features to inhibit nitrous injection if the fuel pressure is too low.

The basis of ZEX patent #5967099 (click on the link, then search the US Patent Office using the number). The small threaded brass plug on the left has a .008" bleed orifice that pressurizes the line to the fuel pressure regulator. Incredibly, the ZEX unit lacks any kind of filter. A few large dust particles would block the orifice. The unit also generates a very large pressure boost. After destroying one gauge, we measured 120-150 psi. This high pressure would dead head the fuel pump and defeat their patented principle of fuel pressure compensation.

 

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