The sensor that measures the intake air volume detects the amount of air entering the engine and converts this information into an electrical signal, which is then input into the electronic control unit (ECU) to calculate and determine the amount of fuel to be injected.
Generally, sensors used to measure air volume can be divided into two types: intake manifold pressure sensors (i.e., negative pressure type) and airflow sensors. Pressure sensors detect the absolute pressure inside the intake manifold, and the measurement method is an indirect measurement method.
Because air pressure fluctuates as it flows through the intake manifold, pressure sensors offer limited accuracy, but the control system is less expensive to manufacture. Many electronic fuel injection engines use manifold pressure sensors instead of flow meters, which, along with crankshaft position sensors, determine the basic fuel supply for engine operation.
Basic working principle of intake pressure sensor
An intake pressure sensor measures the absolute pressure in the intake manifold to determine air density, and then calculates the amount of air entering the engine based on the engine speed. It's important to note that in ordinary engines, the manifold between the throttle and intake valves is generally in a vacuum state, except for turbocharged engines. In a conventional gasoline engine, the intake power comes from the piston's suction; that is, the piston is the active end, and the throttle is normally closed. This demand exceeds the supply, creating a vacuum in the manifold. Even when the throttle is fully open, due to airflow resistance and a certain lag, a vacuum still exists, albeit a lower degree, because the active end is the piston. This explains why turbocharged engines produce more power. The main reason is that air is not passively drawn in but actively forced in; a larger air volume allows for higher compression ratios, resulting in greater power output.
When the engine is running, the manifold pressure sensor measures the difference between the absolute pressure inside the intake manifold and the ambient atmospheric pressure, converting it into a voltage signal. The ECU calculates the precise intake air volume based on this signal, which in turn causes the fuel injectors to output injection pulse signals of a certain width, ensuring the air-fuel mixture reaches the optimal air-fuel ratio.
The intake pressure sensor detects the absolute pressure in the intake manifold behind the throttle valve. It detects changes in the absolute pressure in the manifold based on engine speed and load, and then converts this into a signal voltage, which is sent to the ECU. The ECU controls the amount of basic fuel injection based on the magnitude of the signal voltage.
In the diagram, strain gauges R1, R2, R3, and R4 form a Wheatstone bridge and are bonded to a silicon diaphragm. The silicon diaphragm can deform under the absolute pressure inside the manifold, causing a change in the value of the strain gauge R. The greater the vacuum level inside the manifold, the greater the deformation of the silicon diaphragm, and thus the greater the change in resistance R. In other words, the mechanical change of the silicon diaphragm is converted into an electrical signal, which is then amplified by an integrated circuit and output to the ECU.
When the engine is running, the vacuum, absolute pressure, and output signal characteristic curves of the intake manifold all change with the throttle opening.
When the engine is running, the smaller the throttle opening, the greater the vacuum in the intake manifold, the lower the absolute pressure in the manifold, and the lower the output voltage. The larger the throttle opening, the lower the vacuum in the intake manifold, the greater the absolute pressure in the manifold, and the stronger the output signal. The output signal is directly proportional to the magnitude of the absolute pressure in the manifold.
Function of intake pressure sensor
1. Detect the intake pressure in the intake manifold behind the throttle valve, calculate the intake volume, and determine the basic fuel injection quantity and basic ignition advance angle. The higher the intake pressure, the more intake volume, the more fuel injected, and the smaller the ignition advance angle.
2. Monitor the amount of exhaust gas recirculation and the amount of steam recovered from the oil tank.
3. It is used in conjunction with the intake air flow sensor to improve detection accuracy.
Types of intake pressure sensors
In modern engine electronic control systems, pressure sensors are widely used and classified into two types based on their output signal: voltage type and frequency type. Voltage type pressure sensors can be further divided into semiconductor piezoresistive type and vacuum diaphragm actuated type. Frequency type pressure sensors include capacitive type and surface elastic wave type.
1. Voltage-type pressure sensor
Semiconductor piezoresistive intake manifold absolute pressure sensor
The semiconductor piezoresistive intake manifold absolute pressure sensor consists of a pressure conversion element (silicon diaphragm) and a hybrid integrated circuit that amplifies the output signal of the conversion element.
The pressure conversion element is a silicon diaphragm made using the piezoresistive effect of semiconductors. One side of the silicon diaphragm is a vacuum chamber, and the other side receives the intake manifold pressure. Therefore, the higher the absolute pressure inside the intake manifold, the greater the deformation of the silicon diaphragm, and the amount of deformation is directly proportional to the pressure. The resistance of the strain gauge attached to the diaphragm changes proportionally to the amount of deformation. Using this principle, the pressure change inside the intake manifold can be converted into an electrical signal.
This type of sensor is typically connected to the ECU via three wires: the power supply wire from the ECU to the sensor (typically 4.8-5.1V), the sensor's signal output wire, and the sensor's ground wire. When the engine is idling, the intake manifold vacuum is high (low absolute pressure), the sensor's resistance is high, and the sensor outputs a low voltage signal of 1.5-2.1V. When the throttle is fully open, the manifold vacuum is low (high absolute pressure), the sensor's resistance is low, and the sensor outputs a high voltage signal of 3.9-4.8V.
Vacuum diaphragm driven variable inductive intake manifold absolute pressure sensor
The vacuum diaphragm-driven variable inductive intake manifold absolute pressure sensor mainly consists of a diaphragm, an iron core, an induction coil, and electronic circuitry.
The diaphragm capsule is made of welded thin metal sheets, its interior evacuated, and its exterior connected to the intake manifold. Changes in external pressure cause the diaphragm capsule to expand and contract. An iron core inside the induction coil moves in tandem with the diaphragm capsule. The induction coil consists of two windings: one connected to an oscillating circuit to generate an alternating voltage, creating a magnetic field around the coil; the other is the induction winding, generating a signal voltage. When the intake manifold pressure changes, the diaphragm capsule moves the iron core within the magnetic field, causing the signal voltage generated by the induction coil to change accordingly. This signal voltage is detected, shaped, and amplified by electronic circuitry before being sent as the sensor's output signal to...
Electronic control unit (ECU).
Because this sensor uses a 12V power supply for voltage transformation, unplugging the socket will prevent you from checking its functionality. During testing, insert the multimeter probes (in voltage mode) into the connector and connect them to the two terminals to measure the output voltage. The measurement method is as follows: With the socket in place, close the ignition switch (ON) and connect the multimeter probes to the Vs and E terminals. With the vacuum line open and atmospheric pressure applied, the voltage will be approximately 1.5V. When drawing air into the vacuum line, the voltage should decrease from 1.5V. At engine idle, the voltage will be approximately 0.4V, and this voltage will increase as engine speed increases.
2. Frequency-type pressure sensor
3. Capacitive intake pressure sensor
A capacitive intake pressure sensor uses an alumina diaphragm and a base plate arranged close together to form a capacitor. Utilizing the property that the pressure difference across the diaphragm changes, a capacitance signal proportional to the pressure is obtained. Connecting the capacitor to the oscillation circuit of the sensor's hybrid integrated circuit, the sensor generates a variable frequency signal. The output frequency is proportional to the intake pressure and varies between 80 and 120 Hz.
4. Surface elastic wave type intake pressure sensor
A surface elastic wave (SAW) intake pressure sensor is constructed by ultrasonically fabricating a thin-film sensitive area on a piezoelectric substrate. A transducer (piezoresistive SAW delay line) is then etched onto this area, and the transducer and circuitry combine to form an oscillator. The transducer consists of two interdigitated metal fingers on a polished piezoelectric substrate. When an electrical signal is applied to the input interdigitator finger T1, the inverse piezoelectric effect excites an elastic surface wave on the substrate surface. This wave propagates to the input interdigitator finger T2, is converted into an electrical signal, amplified, and fed back to T1 to maintain the oscillation. The propagation time of the SAW between the two interdigitators is the obtained delay time, the magnitude of which depends on the distance between the interdigitators. Because the intake manifold pressure acts on the piezoelectric substrate, pressure changes will cause strain in the thin-film sensitive area, thus altering the distance between the interdigitators. Consequently, the propagation delay time of the surface elastic wave changes accordingly. Thus, the pressure signal can be output based on the oscillation frequency, which is inversely proportional to the delay time.
Frequency-type pressure sensors typically have a signal voltage of 5V or 12V. While the voltage remains constant as airflow changes, the output pulse frequency changes; therefore, flow rate changes cannot be determined solely by measuring the voltage level. To test, use a multimeter to locate the pressure sensor's frequency signal output line. Set the automotive multimeter to the "DC" setting and press the SELECT function key to switch to the "DC+Hz" setting. Start the engine and gradually accelerate, observing whether the DC voltage on the main display and the frequency on the secondary display change with engine speed. Generally, frequency-type pressure sensors change frequency as the intake air volume increases.
Fault symptoms and detection of engine intake air pressure sensor
The engine is not working properly.
Unstable idling speed
Frequent engine shutdown
Unable to start
The fuel consumption is astonishing.
2. Detection Method
2.1 Detection of sensor power supply voltage
A. With the ignition switch in the "OFF" position, disconnect the connector for the intake manifold absolute pressure sensor.
B. With the ignition switch in the "ON" position (but without starting the engine), use a multimeter in voltage mode to measure the voltage between the power terminal VCC and the ground terminal E2 in the wiring connector. The voltage value should be 4.5-5.5V. If there is an abnormality, check the continuity of the wiring between the intake manifold absolute pressure sensor and the ECU. If there is an open circuit, replace or repair the wiring harness.
2.2 Detection of sensor output voltage
A. Turn on the ignition switch.
B. Disconnect the vacuum hose on the side of the air intake chamber.
C. Use a multimeter in voltage mode to measure the output voltage between the terminals of the intake air pressure sensor PIM-E2 on the ECU connector side under atmospheric pressure, and record this voltage value.
D. Apply a vacuum to the intake pressure sensor using a portable vacuum pump, starting at 13.3 kPa (100 mmHg) and increasing by 13.3 kPa (100 mmHg) each time, until reaching 66.7 kPa (500 mmHg). Measure the output voltage between the PIM-E2 terminals of the sensor at different vacuum levels. This voltage should increase continuously with the increase in vacuum level. Compare the voltage drop at different vacuum levels with the standard value. If they do not match, the intake manifold pressure sensor should be replaced.
3. Intake pressure sensor troubleshooting steps
3.1 Read the documentation, interpret the diagrams, and identify the circuit.
3.3 Detection Methods
Fault diagnostic instrument: reads fault codes and reads data streams.
Multimeter: Flow signal, ground wire, power cord
Oscilloscope: Measuring flow signal waveform
34,000 meters for testing
Signal line: The dynamic signal voltage between pins PIM and E2 increases with the increase of intake pressure, 3.3V~3.9V at atmospheric pressure.
Grounding wire: The grounding resistance between plug E2 and the ground wire should be 0Ω.
Power cord: The power supply voltage between the plug ACC and ground should be 4.5-5.5V.
