Atmospheric Pressure
5-02 Interpret how the environment affects engine operation.
Air contains weight, which causes atmospheric pressure. At sea level, it has an average value of 1 atmosphere and gradually decreases as altitude increases. Atmospheric pressure, also known as barometric (or baro) pressure, is a significant input into calculations for engines controlled by a powertrain control module (PCM). Air, being a gas, has both mass and weight and exerts this force on everything on earth. The atmosphere blankets the earth, gently squeezing down on everything on the planet. The subtle variations in this atmospheric pressure greatly affect engine performance and the weather.
Air consists of 78% nitrogen, 21% oxygen and 1% of a mixture of other gases. In an automotive engine, oxygen is necessary for combustion to occur and to create power. While the percentage of oxygen does not change with air density, the number of oxygen molecules present in a given volume of air does.
TECHNICIAN TIPThe standard atmosphere (symbol: atm) is a unit of pressure; 1 atm is 101,325 Pa (1.01325 bar), equivalent to 760 mmHg (torr), 29.92 inHg, and 14.696 psi.
Atmospheric pressure is 14.7 pounds (lb) or 29.92 (inHg) inches of vacuum per square inch at sea level. When using the metric system to display atmospheric pressure, it is 110 kPa. Another form of measurement used to measure pressure is a bar. One bar equals 100 kPa or 14.5 psi.
Effects of Temperature and Humidity on Atmospheric Pressure
The air’s density (pressure) depends on certain variables: temperature, pressure, and how much water vapor (humidity) is in the air. All play a part in determining the amount of oxygen present in the air (density). As air temperature increases, the molecules become active, creating a significant amount of space between them. As the air expands, it becomes less dense (lighter) than cooler air. As air temperature decreases, the molecules become less active. Packing the air molecules more tightly increases the air’s density. Dense air has more oxygen molecules in a given space, so the oxygen content in air density increases—which is why intercoolers are needed on forced induction engines.
Pressure and altitude have an inversely proportional relationship: As altitude increases, pressure decreases. A column of air 1 square inch (6.45 cm) in cross-section would have a weight of about 14.7 lb or about 65.4 N (newtons) at sea level. For every 1,000 ft that elevation increases, a drop of 1 inHg is present. For example, Denver, Colorado, is about 1.6 km (1 mile) above sea level. The atmospheric pressure of Denver drops to 0.85 kg per square centimeter (12 pounds per square inch) at this altitude. Therefore, there is proportionately less air density, due to a lower air pressure, containing less oxygen per unit of measure compared to the air at sea level (FIGURE 5-10).
FIGURE 5-10 Depending on where the vehicle is operated, the needs of the engine will change. With the change in air density, it will need to change the amount of fuel that is introduced into the engine.
The final variable in air pressure is humidity, or water vapor, in the air. Compared to the differences made by temperature and air pressure, humidity has a small effect on the air’s density. However, humid air is lighter than dry air at the same temperature and pressure. As the amount of vaporized water decreases (less humidity), the oxygen content increases and the air becomes denser.
Vacuum
Vacuum is the opposite of pressure. Any pressure less than atmospheric is a vacuum. When air has a pressure that is higher than atmospheric, a positive pressure is present. When air has a pressure lower than atmospheric, a negative pressure (vacuum) exists. The two forms of vacuum are called partial and absolute. A partial vacuum has small amounts of air molecules, whereas a perfect (absolute) vacuum has no air molecules. A perfect vacuum does not exist because it is almost impossible to remove all the air molecules from anything.
An engine in its simplest form is a vacuum pump. Therefore, a thorough understanding of the relationship between the vacuum and pressure is critical to engine performance diagnosis. Pressure always moves from a positive (high) pressure area to a less positive (low) pressure area. An automotive engine uses this theory to fill the cylinders. Atmospheric pressure is always available outside of the intake manifold. With the engine running, a vacuum is present in the intake manifold. The restriction caused by the throttle plate (if present) and the vacuum (lower pressure) created in the cylinder with the intake valve open and pulling the piston down creates a pressure difference known as manifold vacuum, which atmospheric pressure fills (FIGURE 5-11).
FIGURE 5-11 A manometer is a tool to precisely measure pressure, used mainly for stationary situations.
An example of the vacuum pressure relationship is evident when drinking from a glass with a straw. A glass that appears empty before it’s been filled with liquid is in fact full. It is full of gas from atmospheric pressure; remember that this pressure is present in everything on earth. If a straw is added to the liquid to drink with it, the liquid rises in the straw, to the same height as the liquid in the glass. The liquid rising is due to the atmospheric pressure exerting force down on the liquid in the glass and up into the straw. Atmospheric pressure is also pushing down on the liquid in the straw. So the liquid continues to rise until the pressure of the atmosphere on the liquid is equal inside the glass and straw. For the liquid to rise in the straw, it must be inhaled, reducing the pressure in the straw, creating an imbalance of high and low pressure. Recall that high pressure invariably moves toward low pressure as they attempt to reach a balance. The higher pressure that is now present on the liquid in the glass forces it up so that it can be drunk. A manometer (U-tube) or slack tube operates on this principle (FIGURE 5-12).
FIGURE 5-12 The engine is a large vacuum pump that creates negative pressure inside the engine so that the positive pressure (atmospheric) can be sucked into the engine with the fuel mixture to support combustion.
Vacuum measurements include inches of mercury (inHg), the most widely used in an automotive repair, as well as inches of water, bar, millibar (mbar) and pounds per square inch (psi). Low vacuum is typically measured in millimeters of mercury (mmHg) or Pascals (Pa) below standard atmospheric pressure.
Absolute versus Gauge Pressure
When checking pressure or vacuum, a discrepancy may appear, depending on the tool used for the measurement. Two pressures are available: gauge and absolute. Manual gauges are zero-referenced against ambient pressure, so it is equal to absolute pressure minus atmospheric pressure. Scan tools may use absolute pressure in generic OBDII (On-Board Diagnostics second generation) mode. Absolute pressure is zero-referenced against a perfect vacuum, so it is equal to gauge pressure plus atmospheric pressure.
The simplest way to explain the difference between the two is that absolute pressure uses absolute zero as its reference (zero) point, whereas gauge pressure uses atmospheric pressure (14.7 psi, 1 bar) as its zero-reference point. Due to varying atmospheric pressures, gauge pressure measurements are not as precise, while absolute pressure is always definite. Even though 14.7 psi (atmospheric pressure) is present, gauge pressure is noted by a vacuum/pressure gauge that reads zero (0 psi, or 0 bar) when it’s lying on a toolbox or on a vehicle with the engine not running. Built with an offset of 14.7 psi, the gauge will read only pressure/vacuum (less pressure) on pressures, allowing for atmospheric pressure.
Gauge Pressure
For technicians, the most common pressure reference is gauge pressure, signified by a “g” after the pressure unit—e.g., 30 psig. Gauge pressure is a measurement in relation to ambient atmospheric pressure. A gauge pressure higher than ambient pressure is a positive pressure (FIGURE 5-13). When measuring a pressure that is below atmospheric pressure, it is a negative, or vacuum gauge pressure. When displaying vacuum, the negative signs are generally not included. Gauge pressure is pressure measured relative to ambient atmospheric pressure (approximately 14.7 psia) identified as pounds per square inch (gauge) or psig. A gauge or sensor referenced to gauge pressure will measure 0 psi at sea level.
FIGURE 5-13 Understanding what is happening inside the engine will allow the technician the ability to diagnose a mechanical fault inside the engine.
Absolute Pressure
Absolute pressure uses a perfect vacuum as its reference. A perfect vacuum means that no matter is inside a space. Absolute pressure measurements use absolute zero as their reference point. Absolute pressure’s reference is the gauge pressure of the measured substance plus atmospheric pressure. Using an absolute pressure sensor is always more accurate because it eliminates the reference to a varying atmospheric pressure (Denver, CO, vs. Miami, FL), and instead using a definitive pressure range for reference. Therefore, a sensor or gauge referenced to absolute pressure indicates 14.7 psi (1 bar) at sea level.
Remember that gauge pressure is pressure measured relative to ambient atmospheric pressure (approximately 14.7 psi), and identified as pounds per square inch (gauge), or psig. Absolute pressure is measured relative to high vacuum (0 psia) and defined as pounds per square inch (absolute) or psia.
The difference between the two pressures and their relationship is necessary to verify the accuracy of pressure sensors during diagnosis—for example, when viewing a manifold absolute pressure (MAP) sensor proportional-integral-derivative (PID) on a scan tool in Generic OBDII mode and comparing it to an actual vacuum gauge reading. (When diagnosing a performance concern, never assume that the MAP PID is correct). To verify its accuracy, use a mechanical vacuum gauge to measure manifold vacuum. The MAP sensor reference is to absolute pressure. The vacuum gauge references atmospheric pressure. With key-on, engine off (KOEO), the vacuum gauge will display 0, and the MAP sensor PID will display barometric pressure, approximately 29.5 inHg (14.7 psi, or 1 bar, but this varies with altitude). With the vehicle at idle, the vacuum gauge should show around 16–21 inHg depending on the engine design; the MAP sensor PID generally displays 11–12 inHg. To calculate MAP and verify its accuracy, subtract engine vacuum from barometric pressure. In this example, 29 inHg (baro) − 12 inHg (manifold vacuum at idle) = 17 inHg; that should match the vacuum gauge reading. If the readings vary, a fault is present, requiring further diagnosis.
TECHNICIAN TIPPounds per square inch absolute (psia) reinforce that the pressure is relative to a perfect vacuum, not atmospheric pres-sure. Atmospheric pressure at seat level is 14.7 psi (1 bar). Add this pressure to any pressure reading made in air at sea level. Pounds per square inch gauge (psig) specifies that the pressure is relative to atmospheric pressure. For example, a tire inflated to 35 psi (2.4 bar) above local atmospheric pressure (14.7 psia) will have a pressure of 35 + 14.7 = 49.7 psia or 35 psig.
Units of Pressure
When measuring pressure, several units are commonly used. Most of these units of measurement use the international system of units, such as kilo, mega, etc. The following are definitions of these units: