Air Flow Meter/Sensor: Description and Operation
HOT-FILM MASS AIR FLOW SENSOR:
The Boxster has a new hot-film mass air flow sensor (HFM 5) from Bosch. Its housing shape means that the mass air flow sensor can be installed in one direction only. It is secured in place with M 5 Torx bolts. The plug is new and coded. As with the previous mass air flow sensors, the new HFM 5 is subjected to a laser adjustment process in a "master tube" at the manufacturers, i.e. a particular air mass generates a precisely predetermined voltage signal.
As a result, the tolerance of the HFM characteristics lies below 3% across the entire detection range up to an mass air flow of 640 kg/h. The response time of the hot film is less than 15 ms at a mass air flow of 10 kg/h.
This mass air flow sensor, also described as a micro-mechanical hot film mass air flow sensor on account of its size, is inserted into the air filter housing at the outlet of the air filter.
Like the previous HFM, the new HFM 5 is also immune to dirt. After 100,000 miles, the characteristics deviation for the HFM 5 is thus less than 3% resulting from dirt deposits forming on the hot-film element The operating voltage range is 9 - 17 Volts. The output voltage of the mass air flow sensor (the voltage to the DME control unit) is between 0 - 5 Volts.
Apart from the power which the on-board voltage supply provides for the mass air flow sensor, the actual measuring element is supplied with a regulated voltage of 5 Volts from the control unit so that fluctuations in the on- board voltage supply, e.g. caused by switching on the consumers, do not have an effect on the measuring process.
Deviations from the supply voltage are thus also included in calculation of the mass air flow performed by the DME control unit. As a result, there are no negative effects on the determination of mass air flow since the control unit calculates the relationship between the supplied supply voltage and the output voltage of the mass air flow sensor.
Air Flow Through Measuring Element:
On the side opposite the plug, the measuring element has an air channel with air inlet openings (1). This channel is positioned in the intake air flow of the engine. Behind the air intake opening, there is the actual hot film with temperature resistors in the measuring channel (2). After the hot film, the air flowing through the measuring channel is deflected and guided to the upper part of the housing and back into the intake airflow (3). The arrangement of the air inlet openings relative to the air channel and air discharge opening of the air channel causes a predefined difference in pressure which, together with the muffler between the intake housing and air filter unit, minimizes air fluctuations and therefore pulsation at the hot-film measuring element and thus ensures uniform, linear mass air flow determination.
Measuring Principle:
Image Legend:
C - Heated area
D - Temperature
F - Return flow
G - Air mass
The complete hot-film element is divided into the heated area which, in the case of the HFM 5, is heated to a constant temperature by means of the electronics, and the two temperature resistors T1 and T2. These two temperature resistors are heated by the heated area (C). If there is no mass air flow, T1 and T2 are heated to the same temperature (characteristic curve A). If a mass air flow is now applied, the temperature resistor (T1) positioned at the front relative to the flow direction will be cooled more than the temperature resistor (T2) behind the heated area. A difference in temperature (AT) between T1 and T2 is therefore produced together with a corresponding temperature characteristic curve (characteristic B). The result of the temperature difference evaluation is derived using the following formula:
(delta)T = T2 - T1
The height of the temperature difference gives the voltage supplied to the DME control unit. Using this measuring method, it is also possible to use the mass air flow sensor to detect the flow direction of the intake air.
Image Legend:
A - Air mass m (in kg/h)
B - Return flow
C - Signal voltage U (V)
The diagram above shows that the mass air flow sensor voltage (can be read using the Porsche System Tester 2) must be approx. 1.8 V without air flow. The voltage increases as the air flow becomes greater. Voltages lower than 1.8 V are produced by a mass air return flow.
INTAKE AIR TEMPERATURE SENSOR:
The intake air temperature sensor (arrow) is installed in the mass air flow sensor housing and is therefore situated in the intake air flow of the engine. It is supplied with voltage from the DME control unit via terminal 43 and its ground is connected to the HFM 5. The intake air temperature sensor functions according to the NTC principle.
The signal from the intake air temperature sensor is used by the DME control device to calculate the replacement load signal if the mass air flow sensor fails. It corrects the load which is calculated by the control unit and dependent on the throttle potentiometer. With higher air temperatures, the replacement load signal is downwardly corrected.