How Air "Works"

[Whizbang]

From my experiences with the internet, it's apparent to me that there is a general misnomer about how air "works". Many websites, internet forums, etc tend to haze over the truth of how air acts in both the form of exhaust and induction. It is my intent to correct this and give you, the reader of this long thread, a dumbed down, yet explicit explanation of why air does what it does.

INTRODUCTION

First off, air is a fluid. The textbook definition (literally out of a text book) of a fluid is "a substance that is unable to resist the application of a shear force without undergoing a continuing deformation". (pg.2 of Introduction to Fluid Mechanics). So what does this mean? Its mean that in order for something to be labeled a fluid, it must not resist a force parallel to it. Think of a can of Pepsi sitting on your counter. Without the solid, tin surrounding the liquid contents, the cola would just be a puddle on the counter. Gases behave the exact same way.

So because air is a fluid, it has to follow some rules, the biggest being that air is a compressible fluid. The second most important is the "Conservation of Mass". The "Conservation of Mass" basically means that if you put mass into a system, you should get the equivalent amount of mass out. Granted there are some exceptions and a few of which stem into math that i frankly, don't understand. Now keeping this ideas in mind, I'm going to explain how air works in two of the most important parts of our engines operation, induction and exhaust.


AIR AND INDUCTION

Induction is obviously the intake track leading to the combustion chamber that gives the engine the fresh air it needs to operate. If you didn't already know this, it saddens me. And frankly, i have no idea why the hell your reading this. But getting back on track, induction can take on two methods. These are as follows:

1. Forced Induction - the absolute pressure is limited only by that of the device forcing the air.

2. Natural Induction - the absolute pressure is limited to that of the atmospheric pressure generally (14.7 psi at sea level)

Lets look at natural induction first, even if it comes 2nd on my list.

Natural Induction

Natural induction is probably the most widespread form of induction you will find in the automotive world, regardless of my want to see turbochargers and superchargers (or both) on everything, that is the sad reality. What happens is as your engine draws in air, a vacuum is created due to the pressure change resulting from the air entering the combustion chamber. This is where the name "induction" comes from. Air (or anything under pressure for that matter) will move from the area of higher pressure to lower pressure, meaning it will be drawn in to file up the "void". At wide Open Throttle (WOT) conditions, this vacuum is generally not able to be read on a gauge because the air entering the combustion chamber is easily replaced by air coming in from the outside, which at sea level is at an absolute pressure of about 14.7 psi or 101 kPa for those rocking the metric system. Why your standard everyday gauge will not show "14.7", but instead "0" is because it is reading something conveniently called "gauge pressure"; which is the reading of the pressure about that of the atmosphere. When the throttle plates are closed, the mass volume of air entering the engine becomes significantly limited and the vacuum will build up inside the intake manifold. This is become the mass volume entering the engine is greater than that passing through the throttle body so the vacuum is created.

Forced Induction

Start shoving air into the intake track at pressures above that of the atmosphere, you'll have yourself forced induction. In the WOT condition, the air entering is forced passed the throttle plates. When the air enters the combustion chamber, the pressure instead the chamber equalizes to that of the entering air. In a sense, the air being drawn into the chambers during natural induction is actually being forced in at a set pressure of 14.7 psi. Forced induction is merely the result of these pressure going above the atmospheric pressure. This can be achieved by several methods, most commonly the turbocharger or supercharger. Hell, "ram air" induction will generate a pressure above atmospheric, but the vehicle would have to being moving at a few hundred miles per hour to see any significant benefit. Other methods exist for creating small increases in pressure, but they are generally to complicated to explain here.

So why does forced induction created more power? That is for two reasons, one is that the cylinder pressure increases, and more importantly, more burnable oxygen is forced into the chamber. And now is where I'm going to bring in some math:

Density = (pressure) / (Temp x Gas Constant) [ D = P / RT ] and

Mass Flow = (Density x Velocity x Area) [ M = DVA ]

In order for more oxygen to be in the charge, you need more density. This is why cooler air yields more power, as cool air is denser than heated air. Why? Well look at the formula above for density. If the temperature increases, then the density will decrease. Whereas, if the pressure increases, the density will increase. So if we run about 15psi from a turbocharger, that is roughly double the amount of oxygen coming in IF THE TEMP REMAINED THE SAME. Compressing air will create heat. Turbochargers generally create more heat due to the proximity of the exhaust. This is why intercooling works so well, since it has a minimal loss in pressure but reduces the temperature thus giving us a improved density and the ability to run more pressure without melting holes in pisons, which is a whole other topic.

So what about the "mass flow" formula? Well that comes into play in our next sections: Induction (charge) piping and exhaust.