A/F Ratios, Boost Pressure, Combustion

Ok, I’m bored so I’m going to try to give a very detailed answer to this plaguing question.
First we'll start with a glossary:

AFR - Air/Fuel Ratio, given in standard ratio format, i.e. 14.7:1 means 14.7 parts air to 1 part fuel

Predetonation – Or “knock” is the condition when an out-of-proportion AFR, too much boost pressure, or incorrect timing result in combustion prior to the spark plug’s sparking.

Stoichiometric - Stoich for short. This is in reference to a "perfect AFR" An AFR of 14.7:1 is said to be "stoich"

Pump gas - Pump gas is just that, gasoline that you can buy at a regular pump. For this discussion, pump gas will refer to 92 octane fuel.

Boost pressure - Measured in psi (pounds per square inch) of force applied through a fluid in motion. [At least in terms of this discussion] Note: a fluid by definition is a gas or a liquid; air is a fluid

Ok, in order to fully understand this concept, we must first understand some of the basic physics behind the issue. First, we’ll discuss the basic principles of combustion. Then, we’ll discuss the principles of fluid pressure. Finally, we’ll discuss different fuel types and draw them all together.

Combustion requires four separate but equal parts to occur. In fire science, this is referred to as the Fire Tetrahedron (see image below). In the tetrahedron, there are four basic components of combustion: air, fuel, heat, and chain reaction. To create combustion, all four of these components are necessary, and if any one of the four is removed, combustion will not occur. This is a remarkable fragile principle because, not only do all four components need to exist, but they must also exist in correct ratios to one another. This is where your AFR comes into play. If you have too much air or too much fuel, combustion will not exist. However, we’re going to ignore this for this discussion because in an internal combustion engine, there are other factors at play that will cause combustion to occur, just not efficiently. This lack of effiency can rob you of horsepower or destroy your engine internals through predetonation.

Note: this section applies only to forced-induction

Fluid pressure is a surprisingly complex issue. There are two factors at play here. First is the literal pressure of the fluid (the “psi” reading). There is also the volume of the fluid, which is measured in cfm (cubic-feet per minute). Now, to simply say that a given amount of literal pressure is impossible to run in any certain application if completely false. For instance, in my job of fire fighting, holding a fire hose at 100 psi and 200 gpm (gallons per minute) of water is possible for just one person to do. However, at 50 psi, 350 gpm is impossible for even two people to hold. So what am I saying here? Well, the two factors must be viewed in aspect to each other. In a turbo-charged application, a very large turbo at a much lower pressure will cause damage sooner than a much smaller turbo at a much higher pressure. For instance (keep in mind that this in a “perfect world” where these figures actually possible) a T-25 stock 2G turbo on an eclipse GS-T at 30psi will cause less harm than a big 16G will at 20 psi.

Fuel types vary widely. Just on pump gas, for instance, you can purchase 87, 89, or 92 (93) octane gasoline. There are also fuel types like methanol, race gas (pro-gas), and other alternative fuels that can be used in different applications. In this discussion, I’m dealing with purely pump gas at various octane ratings from 92 up to 96. The higher the octane rating, the more difficult it is to ignite the fuel (see the fire tetrahedron section). So, at 110 octane, for instance, it takes either more heat or more air to create combustion since the fuel isn’t as readily ignitable.

To tie these three principles together, there is yet again a delicate balance that needs to be met. This ties in with the principles of combustion and the Fire Tetrahedron. Again, there are more than any one factor that must be viewed equally important when trying to create horsepower from internal combustion. Now, AFRs need to be calculated carefully, depending on the use. For instance, stoich is “perfect” but only in a perfect world. Realistically, a good AFR for forced induction is closer to 11.5:1 or 12:1. A naturally aspirated engine can go slightly leaner than that (say 13:1). The leaner you go, the more efficiently you can create combustion and horsepower until you go too lean and induce predetonation. Diesel engines actually intentionally induce predetonation because they use high compression levels to create the heat necessary for combustion to occur. However, the internals of a gasoline engine are not strong enough to hold predetonation under the control to keep your engine from ripping apart. The more rich your AFR, the less efficient your combustion will be, this results in un-burned fuel being wasted by being injected into the cylinder, and not ignited completely. It is not possible to make a completely efficient engine, though and engineers attempt to make more efficient engines, rather than completely efficient engines.

So, how much boost can you run? Well that depends on all of the factors I’ve just mentioned. I am more than willing to help any true enthusiasts achieve better performance, but only if the enthusiast is truly willing to learn.




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Nitrous

First, to see what it is exactly that nitrous oxide (N2O) does to your car's engine, you must know what N2O is. As its chemical name alludes to, nitrous oxide consists of two nitrogen molucules and one oxygen molecules bonded together. While it isnt flammable, it can support combustion. At around +570 degrees Farenheit, N2O breaks down into its component parts, nitrogen and oxygen. The oxygen then creates rapid flame expansion, and the nitrogen acts as a cooling agent (since N2 is a completely inert gas). Commonly stored in high-pressure containers, N2O is compressed, liquified, and refrigerated to store as much of the gas as posisble in as little space as possible.

Now, how is it exactly that N2O improves the performance of an internal combustion engine? Well, as previously stated, it breaks into oxygen and nitrogen inside the combustion chamber of your engine. Oxygen supports combustion, and nitrogen has no effect other than cooling. When you add oxygen to a fire, the fire will heat up and increase in intensity instantly. However, to counteract this potentially deadly reaction, the nitrogen is unaffected by the chemical reactions occuring around it, and actually acts as a cooling agent to keep the flame under control. N2O was specifically chosen as an oxidizing agent for internal combustion due to this duality. When heat=energy, more heat=more energy. When you harness this heat engery into something useful (spinning a crankshaft) more engery means quicker acceleration of the crankshaft. This of course means quicker acceleration of the flywheel, clutch, and ultimately, tires.

What about the harmful side effects of nitrous injection that everyone hears about? Well the only way I can answer this question is to compare it to forced induction. In a turbocharged application, if you provide too much air and too little fuel to an engine, you will "lean" out the engine, resulting in exhaust gas temperature that are far too high to be contained, and ultimately result in melting the combustion chamber. In the same way, adding too much N2O without enough fuel to compensate can also starve out the engine. The only other real danger of N2O is that, because it's such an efficient and rapid increase of power to your engine, providing too much N2O and fuel will result in more power than your engine can take and can literally rip the engine apart, just like providing too much power via forced induction. Forged rods/pistons, stronger cranks, sleeves, and stronger heads/block are ways to prevent this problem. (See my article about AFR's to understand this better)

Now, to clarify some common mistakes. First nitrous oxide is abbreviated N2O, not NOS. NOS is a name brand, it's a subsidiary of Holley.
In the fast and the furious, actor Paul Walker's car explodes because it catches fire with N2O in it. False, N2O is not combusitible or flammable of it's own and is not explosive in any capacity.
Adding nitrous to an engine will, over time, damage the internals of the engine. This is possible, but not necessarily a result of N2O injection.

Finally, I'll differentiate between the three major types of N2O injection
Wet shot - This is the most popular, and second easiest horsepower gain of the different systems. A wet shot simply uses two nozzles (also called foggers) that spray into your intake piping. One nozzle injects gaseous N2O, the other nozzle injects pure gasoline. These two chemicals combined, enter the intake manifold and couple with the present air/fuel mix to create combustion when sparked.

Dry shot - The same as a wet shot, but without the gasoline. This system is slightly cheaper the wet shot, slightly more efficient, but requires more expert tuning knowledge to compensate for.

Direct Port - The performance purists and highly experienced tuners use this for maximum efficiency, direct porting, just like direct fuel injection, results in ensuring that all cylindars receive the same amount of N2O. Direct port is the most expensive because it involves the most tubing, wiring and installation.

Dump tube - This is a mandatory (by law and for safety according to every race-sanctioning body in the US). It provides an emergency pressure release system in case the tank experiences over-pressure to prevent a grenade-like BLEVE explosion

Note: All this is assuming that you understand the basics of internal combustion. Please do not take this as my telling you to go ahead and install nitrous injection in your car and not worry about it. Just like any major performance upgrade, N2O must be carefully calculated so as not to blow up your engine from mis-use. Also, N2O injection is illegal for street use in every state that i know about. And, for safety concerns, every major race-sanctioning body in the USA requires the use of a dump tube for track use.