Air Density - Playing It By Air

Many of us are very in tune with the weather and can tell when the weather changes. What may be slight, nearly unnoticeable changes to some people, others can feel. If it's too hot or too cold, your body lets you know right away. If the humidity rises or falls, your body lets you know right away. But barometric pressure is a bit different. Some of us have a hip or a big toe that hurts when the weather changes. While this is a unique and interesting condition, it does not always work as needed at the track, where you have to rely on instrumentation for your data points.

But why does the weather have an effect on how your engine or car performs? Just what is happening with the air that leads you to change how you adjust your engine? First we need to look at air a bit differently. Air is actually a fluid. It has many of the same characteristics as water, as it flows in much the same way that water would flow through a tube or between the banks of a river. Yes, there are some very distinct differences, but the mixture of gases we call air is a fluid. Unlike water and many other liquids, air is compressible. This is where things start to get a bit more complex-interesting, but complex.

If we stand back and look at the big picture, we see the earth is surrounded by air. All of the air surrounding the earth has mass, no different from your car or you. For the purpose of illustration here, mass indicates weight. We can measure this mass, and we'll call it measure weight.

We all know about weight. In this country, we measure weight in pounds. The weight of air at sea level is about 14.2 psi. Just what does that mean? The air is pressing down on the earth with a weight of 14.2 pounds per square inch.

Let's look at a single slice of air from ground level and go straight up about 15 or 20 miles. Within our imaginary slice of air, we can remove segments and measure the mass of the air in our slice at various levels or altitudes. We can know how much that section of air weighs. The higher we go, the less mass each segment has and the weight is less. The lower we go, the more each segment weighs. Why does it weigh more? It weighs more at the lower levels because the weight of the air in the upper levels are additive to the whole, so we have a greater amount of pressure being applied to the bottom of the pile than the top.

It would be no different if you started to pile rocks on a scale: The higher the stack of rocks, the heavier the total load. The pressure on the bottom rock is greater and greater, while the density of the rock does not increase because it is a solid and is not compressible (at least for this illustration). Air is compressible, and the pile of air compresses the air at the bottom due to the weight of the air above. It is just that simple.

We have established that the air has weight. The weight of air at ground level, where we race, is controlled by the weight of the air above. It is this component of weight that makes us faster. And the weather is what controls the air above. Are you starting to see the connections? We call this atmospheric pressure, but more on that later.

As stated earlier, the air is composed of a mixture of gases: nitrogen, xenon, helium, carbon dioxide, several other trace gases, and the biggie, the one we racers are looking for, oxygen. It is the oxygen component of the air that makes our engines run better. If there is less than 19 percent oxygen, the air will have a difficult time supporting higher life forms, such as people.

We have established that air is a fluid, it has weight, and it is composed of a mixture of gases. So how do we use this information to improve our position on the track and ultimately reach Victory Lane?

We have established that oxygen is required to develop power. It is not just the oxygen but the weight and/or pressure that are good things. Contrary to popular opinion, air is not sucked into our engines. Air is pushed into the engine. As the pistons move down in the cylinder, a low-pressure area is created. The valve opens, the carburetor is opened, and the air of a higher pressure outside the engine is pushed into the engine. The trick is finding a way to get more of this good thing.

It is not all that simple since you do not control how much you get. What you do get is the opportunity to make adjustments around the amount of oxygen you are given. We measure the weight of air in terms of pressure. The most common method for measuring atmospheric pressure is in inches of mercury, ranging, again, from 27 to 31 inches. The higher the pressure, the better, within reason.

Let's go back to our example of slicing air into segments. As we went higher up the column, we found that the weight or pressure was lower. The air is not lighter per se, but rather less of it occupies the same space. Consequently, at lower pressures, we have less air in a given volume. So, as our car is moving down the track, the engine is consuming large amounts of air. If the measured volume of air is at a lower pressure condition, we will have less oxygen to burn within the given volume of air we are pumping through the engine. Less oxygen, less horsepower-simple. It is not that we have less oxygen in an air mixture, as the homogeneity of the mixture stays the same. Dalton's Law of partial pressure states, "The total pressure that a mixture of gases exerts is equal to the sum of the separate pressures which each of the gases would exert if it occupied the whole volume." Translation: The percentage of oxygen in the air is the same at 27 inches of mercury as it is at 31 inches of mercury.