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The science of Mass Airflow (MAF)

Thursday, August 30, 2018 - 06:00
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We are all living at the bottom of an ocean; an ocean made of air instead of salt water. The earth has a vast ocean of nitrogen and oxygen encompassing it. This ocean of air is being pulled toward the earth by gravity. Gravity is the force that is created between two objects with mass that are attracted to one another. This force is proportional to the product of their masses and inversely proportional to the square of the distance between them. On earth, gravity is what gives weight to physical objects. This vast ocean of air surrounding the earth has mass and therefore has weight. The weight of this ocean of air will change depending on the depth of it. Just like an ocean of water; the deeper the water the more pressure is created. This ocean of air exerts 14.7 Pounds Per Square Inch (PSI) at sea level and at 18,000 feet this ocean of air exerts 7.34 PSI, which is only half of the pressure that is created at sea level.

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How that impacts us

The internal combustion engine uses this air weight or air pressure in order to operate. When the piston moves downward, away from the head, the volume increases, thus creating a low-pressure area within the cylinder. This low-pressure area within the cylinder sets up a pressure differential. A pressure differential is the difference in energy between a higher pressure (atmospheric air) and a lower pressure (cylinder air). High pressure, having more force, always moves to a low pressure having less force, thus the surrounding air moves into the cylinder. This inrush of air into the cylinder will be used to operate the engine in several ways. The first is the volume of air (78 percent nitrogen, 21 percent oxygen, 1 percent other) will be compressed, creating heat within the cylinder. The second is the air within the cylinder, being comprised of 21 percent oxygen, will provide an oxidant for the chemical reaction with the hydrocarbon fuel stock. Third, the 78 percent nitrogen and 1 percent other will be heated by the burning fuel which creates the expansion of the nitrogen, thus forcing the piston downward. This, in turn, produces torque on the crankshaft.

Figure 1

The burning of the hydrocarbon fuel stock within the internal combustion engine is essential. This is what powers the engine so that the pumping losses of the engine and energy needed to move the vehicle can be produced. In order to properly burn the hydrocarbon fuel stock, the weight ratio of the air and fuel will be important. The proper air/fuel ratio to completely burn the fuel stock is referred to as stoichiometric. The stoichiometric ratio between the fuel and air is one where the hydrocarbons and oxygen are at a weight ratio that once they react with one another neither chemical will be present. This means that the hydrocarbons break apart becoming hydrogen and carbon. In the presence of oxygen, the hydrogen combines with the oxygen forming a new chemical; dihydrogen monoxide (H2O — water). The carbon attaches to the oxygen forming a new chemical; carbon dioxide (CO2). If the hydrocarbons and oxygen are at a stoichiometric ratio and react with one another then neither of these chemicals will remain present within the combustion gases (Figure 1). The chemical weight will be the same but the new chemicals formed during a complete reaction will be water and carbon dioxide.

In order to obtain a stoichiometric ratio between the fuel and air, the weight of the air must be known. Since the fuel stock to be combusted is known, the fuel weight will also be known. However due to the load of the engine constantly changing the air weight is an unknown factor, therefore, there must be a method to properly weigh the air. With the fuel injected gasoline based engine there will be three basic methods used. First is the Alpha-n method, which is the method where the Throttle Position Sensor (TPS) is the main sensor used. This is where a look up table for the throttle effected area is used to calculate the air weight entering the engine. Second is the Speed Density method, which is the method where the Manifold Absolute Pressure (MAP) sensor is the main sensor used. This is where a look up table for the absolute pressure within the intake manifold is used to calculate the air weight entering the engine. Third is the Mass Air Flow Measurement method, which is the method where the Mass Air Flow (MAF) sensor is the main sensor used. This is where a look up table for the air entering the induction system is used to calculate the air weight entering the engine.

Why MAF?

In each of these methods an accurate air weight can be calculated. Each of these methods have advantages and disadvantages, but perhaps the MAF method has the greatest advantages. When a thermal measurement air flow device is used, there is no altitude error, no significant moisture influence, no pulsation error, fast response time, and no moving parts. The fast response time from these thermal measurement devices will still have significant delay or latency. Additionally, it is hard to measure air flow in unsteady conditions, such as during a transitional event. Therefore, during an acceleration, the engine control program will not use the signal from these thermal measurement devices.

Figure 2

 When using these thermal measurement device, air weight is directly measured by the sensor. This is accomplished using a sensor that is located before the throttle plate (Figure 2). This sensor must be located in front of the throttle plate. If the sensor were located after the throttle plate, the turbulent air flow would have a negative effect on the sensor’s output voltage, causing an erratic output voltage that cannot be used. Turbulent air is such a problem that these sensors are equipped with an inlet screen to allow the air moving through the sensor to be straightened before being measured. Some manufacturers show a MAF sensor located behind the throttle plate. These sensors are called out as MAF sensors in the wiring diagrams and scan tool data streams but these are really MAP sensors because when the sensor is located behind the throttle plate the sensor is based on the speed density method.

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