How Air Flows

Understanding why air behaves the way it does.

This diagram indicates laminar flow through a conduit. The length of the arrows represents the relative speed of the fluid in the conduit. The fastest flow is at the very center of the conduit, and the slowest flow is adjacent to the conduit walls. If the conduit is round, you can picture this as concentric “layers” of flow, gradually increasing in speed from the outside toward the center. The thicker the fluid (high density), the greater the variation in speed of the layers will be. If the density of the fluid is high enough, tumbling between the layers will occur, significantly reducing flow through the conduit.

Air is a fluid. Yes, it is. A fluid is defined as: having particles that easily move and change their relative position without a separation of the mass and that easily yield to pressure: capable of flowing. Being a fluid does not necessarily mean something is a liquid, but liquids are also fluids. If it is capable of flowing, it’s a fluid. Both gases and liquids are fluids. With air, the particles in question are the molecules of the various gases that comprise the air around us. Ordinary sea level air is comprised of approximately 78% nitrogen, 21% oxygen, and the remaining 1% is made of many trace gases, including argon, carbon dioxide, neon, methane, helium, krypton, hydrogen, xenon, etc.

All of the above may seem meaningless, except that most people have a basic understanding of how liquids flow. As Forrest Gump might have put it, “Fluids is as fluids does.” Thinking of liquids helps us understand how air flows. For all general purposes, air flows just like water, but we have to be careful with this comparison. Do not try to envision the air around us as water flowing in a river. Instead, we must envision the air around us as being like water around us if we were at the bottom of an ocean, for in effect, we live at the bottom of an ocean of air. Just like there is pressure at the bottom of the ocean from the weight of the water above, there is pressure at the bottom of our ocean of air from the weight of all the air above us. At sea level, this pressure is about 14.7 pounds per square inch. This is absolute pressure. A vacuum would have zero absolute pressure. Then, just to add some confusion, there is “gauge” pressure. Gauge pressure measures pressure above atmospheric pressure. A good example of this is a common car tire and a tire pressure gauge. A flat tire would have zero gauge pressure. To inflate the tire, we have to add air pressurized above normal atmospheric pressure. Let’s say we inflate the tire to 30 PSI according to our tire gauge. The tire’s pressure would then be 30 PSI of gauge pressure and 44.7 pounds of absolute pressure. Absolute pressure is just gauge pressure plus atmospheric pressure.

The only reason we want to understand absolute pressure is that it will make understanding airflow easier. This is especially true when we start relating airflow to internal combustion engines. What we want to try to envision is how air flows through conduits and within confined spaces, such as the intake or exhaust system, or inside the cylinders. Most basic to flow is the concept that, given the opportunity to do so, fluids will always flow from areas of high pressure to areas of low pressure. The greater the pressure differential between the high pressure and low pressure areas, the faster the flow will be. This is very clear if everything is measured in absolute pressure. Otherwise, if we think in terms of gauge pressure, then we have both positive and negative (vacuum) gauge pressure. With absolute pressure, there is no such thing as vacuum. A vacuum is only the absence of pressure. Therefore, when a piston descends on the intake stroke, it doesn’t create a vacuum, it simply lowers the absolute pressure in the cylinder (and intake system). The outside air, still being at 14.7 PSI absolute pressure, flows into the lower pressure intake system and cylinder. Similarly, as the piston ascends on the exhaust stroke, it pressurizes the contents of the cylinder above atmospheric pressure and the exhaust flows through the exhaust system toward the lower atmospheric pressure outside.

Like water, air does not like to change direction quickly. Of course, air has less density than water, so air will change direction more easily than water. However, abrupt changes in direction generate turbulence, which restricts flow volume. Laminar flow (parallel, layered, non-turbulent flow) that does not make abrupt changes in direction results in maximum flow volume through a conduit. Such flow is not always possible, but effort should be made to keep flow as laminar as possible. Here’s another example, comparing air to a liquid: NASCAR Winston Cup teams spend a lot of effort on their refueling cans to get the fuel flow through the neck of the can to be as laminar as possible. If the fuel sloshed and swirled, formed vortices, etc, there’d be no way to get that 11-gallon can to empty into the car in only 5 or 6 seconds. The same holds true for airflow through an intake port into a cylinder. The smoother and less turbulent the flow, the more air that will get into the cylinder during the short time the intake valve is open. The same is true for the exhaust.

Laminar flow demonstrates unique properties of fluid flow. For example, flow through a perfectly straight tube or conduit may not be turbulent, but layers of fluid flow at different velocities, depending on the fluid’s position within the tube. The friction of the fluid against the walls of the conduit creates notable slowing of the fluid adjacent to the walls. As we move from the outside wall of the tube toward the center, the velocity of the fluid layers increases. Think of this as concentric layers of fluid with the outside layer moving slowest and the center layers, or core, moving fastest. This in essence reduces the effective flow area of the tube. How much the boundary layer affects flow depends on the thickness of the boundary layer, and that is related to the density, or viscosity, of the fluid. Viscosity is actually defined as a fluid’s resistance to flow. For example, oil won’t flow through a pipe as easily as air. We’ve all seen examples of this when pouring liquids through a funnel or hose. Water or gasoline will flow quickly, but thicker (higher viscosity) liquids, like oil or paint, flow more slowly. The heavier the weight, or viscosity, of the oil, the more slowly if flows. Naturally, the smoothness of the conduit walls affects the boundary layer, and generally speaking, smoother is better. This also applies to any joints or connections in the conduit. Anything that disrupts the flow is detrimental to the volume of flow. Here is where thinking of air as water can help you visualize what is going on. Where a transition must occur, it is usually better for the area of the tube to enlarge rather than neck down suddenly. Gradual transitions, however, allow the flow to change velocity or to turn without significant turbulence.

Actually, depending on the diameter of the conduit, the fluid velocity, and the viscosity of the fluid, laminar flow may become turbulent flow, slowing the velocity of the flow. This occurs when the concentric layers of flow transfer energy from one layer to another, either accelerating or slowing the adjacent layer, which creates tumbling, or turbulent flow.

Flow will inevitably follow the path of least resistance. That means the flow will usually take the shortest, unrestricted path from the point of high pressure to the low pressure area. The reason we say “usually” is that air, like any fluid, has some mass or weight, and once it is moving at a given speed or in a given direction, it takes some outside force to change it. Airflow doesn’t like to make sharp right-angle turns. It doesn’t like to stop suddenly. In addition, it damn sure doesn’t like to reverse directions abruptly – think of shooting a fire hose against a wall.

All of the above considerations are for dry air. Wet flow, such as air mixed with fuel, imposes still other considerations. If fuel droplets are present in the air stream, those droplets have greater weight and mass than the air. This means the droplets will be more resistant to turning, stopping, or accelerating than the air around them. It also means that if the airflow makes abrupt changes in direction or velocity, the fuel droplet may fall out of suspension in the air. Once the fuel becomes fully vaporized (in a gaseous state), it has little affect on flow and reacts just as the air does. Unfortunately, abrupt changes in velocity or direction can cause fuel vapor to change back to a liquid. When that happens, the fuel tends to drop out of suspension in the air.

Not all fluids behave the same during flow, however. There is one characteristic of air that is not shared by liquids. Air, being a gas, can expand and it can be compressed. Liquids cannot do either. Some compression and expansion of air does occur during flow when the air abruptly changes velocity or direction. As it speeds up, the air tends to “stretch out” and when it slows, it tends to “pile up”. This results in density changes. Of course, the pressure of the air (or any fluid) also changes when the velocity changes. This is actually a phenomenon called Bernoulli’s Principle: an increase in the speed of a fluid produces a decrease in pressure and a decrease in the speed produces an increase in pressure. These changes in density and pressure also affect the flow of air. This is why turbulent flow is undesirable. Turbulent air tends to pile up, slow down, and rise in pressure. Think of turbulence as a blockade to smooth, efficient flow. Avoiding sharp turns, sharp edges, and abrupt change in air passage size reduces turbulence, promotes laminar flow, and decreases resistance to flow.

At Banks, we apply these principles of airflow to both intake and exhaust system components. For example, all our intake and exhaust tubing is mandrel bent for smooth, constant diameter bends. When airflow is increased, pumping losses in an engine are reduced and power is gained. Increasing airflow through an engine also permits burning additional fuel to do more work. It all begins with airflow.

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