Basic Properties of Air
The MSA uses 29.92 inches Hg for standard sea-level pressure. As elevation rises, pressure decreases in almost linear fashion. As shown here, it drops to 24 inches Hg at 6,000 feet. (Photo Courtesy Patrick Hale)
The motorsports industry relies on a standard temperature of 60 degrees F and 29.92 inches Hg (mercury) barometric pressure with dry air (no humidity). These conditions correspond to the standard atmospheric pressure of 14.696 psi, commonly expressed as 14.7. (You may also see 14.68 psi, which is calculated by using the aviation standard temperature of 59 degrees F.) This is zero density altitude without the influence of vapor pressure. Any drop in pressure or increase in temperature or vapor pressure raises the density altitude with a corresponding reduction in indicated air density.
All of this affects the oxygen content of the air, which is a big factor in fuel combustion. And as previously mentioned, engines perform their best under conditions of high barometric pressure, low temperature, and minimal water-vapor pressure (low humidity).
Important Terms
The following terms are useful in understanding the various qualities attributed to air. They are variable and influenced by changes in each of them.
Dry Bulb Temperature
Regular “air temperature” measured with a thermometer.
Dew Point
The temperature at which water vapor condenses and separates from air.
Pressure
Air applies pressure to any surface it touches. Pressure is measured as force per specified area. Gravity pulls air toward the earth because air has weight. That causes pressure, resulting in denser, heavier air near the surface exerting a normal sea-level air pressure of 14.7 psi (pounds per square inch, or 1,013 mbar) on everything around it.
The pressure of air moving within a runner varies according to the port velocity, port cross section, and any restrictions where it may encounter abrupt variations in area or direction.
Relative Humidity
This is the percentage of water vapor in the air relative to the maximum amount the air could hold under those temperature and pressure conditions. Relative humidity is 100 percent at the dew point.
We generally discuss relative humidity as it relates to human comfort, with the average person feeling most comfortable at about 50-percent relative humidity. Perspiration increases above that point; lower percentages are often too dry for many people, causing headaches and dry throat, skin, and nasal passages.
Specific Volume
The specific volume of a substance is the ratio of its air volume to its mass. As described in Boyles Law, the given volume of a gas varies depending on its temperature and pressure. For any given temperature, the gas occupies a specific volume. Remember that (mathematically) specific volume is the inverse of density (specific volume = 1 ÷ density).
Temperature
Temperature is usually expressed in degrees Fahrenheit unless conversion to Centigrade, Celsius, or Kelvin is specified. A gas (air) at any given temperature or pressure occupies a specific volume according to its mass, temperature, and pressure.
An absolute temperature scale (Rankine) is used for engineering and thermodynamic calculations. The Rankine scale has the same number of increments as the Fahrenheit scale, but the absolute zero point is equal to –459.67 degrees F (absolute zero is recognized as the coldest temperature in the universe). Outside the fields of meteorology and petroleum engineering, Rankine is used rarely and you will likely not encounter it for the purpose of understanding and working with engine airflow.
Viscosity
Air also has fluid qualities that affect its movement through a passage or port. Air viscosity is a measure of friction and the resistance to efficient flow. It is based on differing velocities near the center of the port and near the port walls (drag).
Because air is compressible, it can gain or lose pressure depending on airspeed and flow area, and the drag varies depending on viscosity. Shear, or stress, determines air pressure depending on viscosity, which varies depending on fuel content, droplet size, and the severity of area and direction change.
Because air is compressible, it can also emulate basic spring characteristics when subjected to sudden starts, stops, and abrupt direction changes along with various shear properties depending on how heavily it is laden with fuel or water vapor.
Water Vapor
Water vapor is the most variable component of the engine’s intake air, and it exerts considerable influence on the combustion process. It constantly changes according to temperature, time of day, weather conditions, and the proximity of water sources, such as lakes, rivers, or clouds. Although water vapor provides a cooling effect it also displaces fuel molecules, thus requiring a leaner mixture to obtain optimal fuel ratios. This is primarily noted as the weather effect for tuning purposes, but it is also experienced when using water injection for charge cooling.
The presence of water vapor actually decreases density and essentially makes the air lighter. That’s because the molecular weight of water is less than the combined molecular weight of the oxygen and nitrogen that make up the bulk of the atmosphere. This is difficult to grasp at first until you recognize that you’re not talking about liquid water, but rather water vapor, which itself is a gas that is lighter than the combined weight of oxygen and nitrogen.
Although temperature and pressure exert the greatest influence on air density, humidity has a lesser but potentially harmful effect. In addition to being lighter, it occupies space that could otherwise be occupied by fuel, thus reducing the oxygen content and the energy content (fuel) available to produce power.
Wet Bulb Temperature
This is equal to dry bulb temperature when air reaches its saturation temperature (dew point), or the lowest temperature water can reach via evaporative cooling. The difference between wet bulb and dry bulb temperatures is a measure of the humidity.
Engine performance is fully dependent on the airflow through the engine. Because there are so many variables and factors that influence airflow and air quality, correction factors were conceived to help engineers make more accurate comparisons. They use the correction factors during product development and dyno testing, whether on an engine dyno or in a running vehicle on a chassis dyno.
The following three laws relate air density to pressure and temperature. They combine to define gas behavior based on a hypothetical ideal gas. They are named for the three men who postulated them and they apply perfectly to the gas you know as air.
• Avogadro’s Law states that at constant pressure and temperature, the volume of a gas is directly proportional to the amount of gas. (Volume is relative to temperature and pressure.)
• Boyle’s Law states that the volume of a gas is inversely proportional