Temperature sensors (cooling water temperature sensors THW, THA, air temperature sensors);
Flow sensors (air flow sensor, fuel flow sensor);
Intake air pressure sensor diagram
Throttle position sensor TPS
Engine speed sensor
Speed sensor SPD
Crankshaft position sensor (ignition timing sensor)
λ sensor
Knock sensor (KNK)
Second, the air flow sensor
In order to form a mixture that meets the requirements and make the air-fuel ratio reach the optimal value, the intake flow of the engine must be accurately controlled. Here are several commonly used air flow sensors.
1, Carmen vortex street air flowmeter
Vortex street air flow sensor is a kind of sensor which uses ultrasonic or photoelectric signals to measure air flow by detecting vortex street frequency.
As we all know, when the electric wire overhead in the field is blown by the wind, it will make a "buzzing, buzzing" sound, and the higher the wind speed, the higher the sound frequency, which is caused by the vortex (that is, vortex) formed after the gas flows through the electric wire. This phenomenon can occur in liquids, gases and other fluids.
Similarly, if we place a vortex generator at the air inlet, such as a cylinder, when the air flows, two rows of vortices with opposite rotation directions will be generated behind the vortex generator as shown in the figure, which appear alternately. This vortex is called Carmen vortex.
Carmen vortex street air flowmeter uses the principle of vortex formation to measure the gas flow, and directly reflects the air flow through the flow measurement.
For the specific Carmen vortex street air flowmeter, there is the following relationship: qv=kf, where qv is the volume flow, f is the frequency of single row vortex street, and k is the proportional constant, which is related to the pipe diameter and column diameter. According to this relationship, the volume flow is proportional to the output frequency of Carmen eddy current sensor. By using this principle, the air flow can be calculated only by detecting the frequency f of Carmen vortex street.
According to the different detection methods of eddy current frequency, the eddy current air flow sensor for automobile can be divided into ultrasonic detection and optical detection. For example, Toyota Lexus LS400 imported from Chinese mainland and Crown 3.0 imported from Taiwan Province Province use photoelectric detection vortex street air flowmeter; Japan's Mitsubishi Jeep, China's Changfeng Cheetah Jeep, and South Korea's Hyundai Motor all adopted vortex air flow sensors for ultrasonic detection.
(1) optical Karman vortex air flowmeter
The particle theory of light in modern physics holds that light is a particle flow with energy. When an object is irradiated by light, the effect produced by absorbing photon energy is called photoelectric effect. Photosensitive transistor is a kind of semi-transistor.
The characteristic of conductor device is that when it is irradiated by light, it will produce photovoltaic phenomenon with internal photoelectric effect, thus generating current.
Working principle: In the process of producing Carmen vortex, the air pressure on both sides of the vortex generator will change and act on the metal foil through the guide hole, thus making it vibrate. When the light of LED shines on the vibrating metal foil, the reflected light on the metal foil received by the phototransistor is eddy current modulated light, and then the phototransistor outputs a modulated frequency signal, which represents the air flow signal.
(2) Ultrasonic Carmen vortex street air flowmeter
Ultrasonic wave refers to the mechanical wave whose frequency is higher than 20HZ and the human ear can't hear it. Its characteristics are good directionality and strong penetration, and it will produce obvious reflection when encountering impurities or object interfaces. For example, bats, whales and other animals in nature are located by ultrasound. Using this physical property, we can convert some non-electric quantities into acoustic parameters and convert them into electric quantities through piezoelectric elements.
The working principle of ultrasonic Karman vortex flowmeter is basically the same as that of optical Karman vortex flowmeter, except that the optical element is replaced by acoustic element.
In our daily life, we often encounter such a phenomenon, that is, when calling people along the wind, the other party can easily hear them; When you shout against the wind, it is not easy for the other party to hear. This is because the air flow direction of the former is the same as the sound wave direction, and the sound wave accelerates, while the latter is the result of the sound wave being blocked and slowing down. This phenomenon also exists in ultrasonic flow sensors.
Its working principle is that an ultrasonic transmitting probe and an ultrasonic receiving probe are relatively installed on both sides of the downstream pipeline of the vortex generator, and the ultrasonic transmitting probe continuously emits ultrasonic waves with a certain frequency (generally 40KHZ) to the ultrasonic receiving probe. When the ultrasonic wave reaches the ultrasonic receiver through the intake airflow, the phase (time interval) and phase difference (time interval difference) of the received ultrasonic signal will change due to the influence of the moving speed and pressure of the airflow, and the integrated control circuit measures the frequency of the eddy current according to the change of the phase or phase difference. After the vortex frequency signal is input into ECU, ECU can calculate the intake air volume.
2. Hot-line air flowmeter
Composition: Let's look at the structural diagram in the book. Its basic components include platinum hot wire for sensing air flow, temperature compensation resistor (cold wire) for correction according to intake temperature, control circuit for controlling hot wire current, and housing. According to the different installation parts of platinum wire in the shell, it can be divided into the main flow method installed in the main air duct and the side flow method installed in the side air duct.
Hot-wire air flowmeter works by using the cooling effect when air flows over the hot wire. Place a platinum wire hot wire in the intake air flow. When the platinum wire is heated by a constant current, if the air flowing around the platinum wire increases, the temperature of the platinum wire will decrease. If we want to keep the temperature of the platinum wire constant, we should adjust the current of the hot wire according to the air flow. The greater the air flow, the greater the current required. The following figure is the working principle diagram of hot-wire air flowmeter with mainstream measurement mode. Where RH is a thin platinum wire (hot wire) with a diameter of 0.03-0.05, and RK is a cold wire resistance as temperature compensation. RA and RA are precision bridge resistors. Four resistors * * * together form a Wheatstone bridge. In practical work, the heating current representing airflow is converted into voltage output through RA in the bridge. When the air flows at a constant flow rate, the power supply voltage keeps the hot wire at a certain temperature, and the bridge is balanced at this time. When there is air flowing, the resistance value of RH changes, and the bridge loses balance, because the heat of RH is absorbed by the air and becomes cold. At this time, the amplifier increases the current through the platinum wire until the original temperature and resistance value are restored, thus rebalancing the bridge. Due to the increase of electricity, the voltage of RA increases, so a new voltage output representing airflow is obtained on RA.
Any change in intake air temperature will unbalance the bridge. Therefore, there is a compensating resistance wire (cold wire) in the airflow near the hot wire. The temperature of the cold wire compensation resistor is taken as the reference value. In operation, the amplifier will make the hot wire temperature 100 degrees higher than the inlet temperature. The long-term use of hot-wire air flowmeter will accumulate impurities on the hot-wire, so the hot-wire flowmeter adopts burning-out measures to solve this problem. Whenever the engine is turned off, the ECU automatically turns on the electronic circuit in the air flow meter housing, and the hot wire automatically heats up, so that its temperature rises within 1000 degrees 1000 degrees. Because the burn-out temperature must be very accurate, the circuit will not be opened until the engine is turned off after 4 s.
This kind of air flowmeter has no moving parts, reliable operation and good response characteristics. The disadvantage is that the error is large when the wind speed distribution is uneven.
3. Hot film air flowmeter
Although hot-wire air flowmeter can provide accurate intake air flow, its cost is too high, and it is mainly used in luxury cars. In order to meet the requirements of high precision, simple structure and low cost, Bosch Company of Germany developed a hot film air flowmeter by using thick film technology. The working principle of hot-film air flowmeter is similar to that of hot-wire air flowmeter, and both of them work by using Wheatstone bridge. The difference is that the hot film air flowmeter does not use platinum as a hot wire, but concentrates the hot wire resistance, compensation resistance and line bridge resistance on a ceramic chip through thick film technology. This kind of air flowmeter has been widely used in various electronically controlled gasoline injection systems.
Third, the pressure sensor
Function: Convert pressure signal into voltage signal.
Scope of application: There are two main applications in automobiles. First, it is used to detect air pressure, including intake vacuum, atmospheric pressure, in-cylinder air pressure and tire pressure; Second, it is used to detect oil pressure, including gearbox oil pressure, brake valve oil pressure and suspension oil pressure.
1, capacitive pressure sensor
First, let's learn about capacitance. The capacitance of a capacitor is directly proportional to the dielectric and its relative effective area between the two plates of the capacitor, but inversely proportional to the distance between the two plates, that is, C=ε A/d, where ε is the dielectric constant of the dielectric, A is the relative effective area between the two metal electrode plates, and D is the distance between the two metal electrode plates. It can be seen from this relationship that when two parameters are constant and the other parameter is variable, the capacitance will change with the change of parameters. The capacitive pressure sensor consists of two moving pieces (elastic metal diaphragm), two stationary pieces (metal coatings on the upper and lower concave glass of the elastic diaphragm), an output terminal and a shell. Two series capacitors are formed between the rotor and the two stators. When the force of air intake acts on the elastic diaphragm, the elastic diaphragm will be displaced, which will inevitably reduce the distance from one stator and increase the distance from the other stator (which can be demonstrated by a piece of paper). From the formula, we can see that the distance between two metal electrode plates is one of the important factors affecting the capacitance. With the increase of distance, the capacitance decreases, while with the decrease of distance, the capacitance increases. This structure is called differential structure, which is caused by the equal and opposite parameter changes of one measured and two sensing elements. If the elastic diaphragm is placed between the lateral pressure and atmospheric pressure (the upper cavity of the elastic diaphragm is communicated with the atmosphere), the gauge pressure is measured; If the elastic diaphragm is placed between pressure and vacuum (the upper cavity of the elastic diaphragm is filled with vacuum), the absolute pressure is measured.
Capacitive sensors use a variety of measuring circuits. Let's take the bridge circuit as an example to explain the working principle of the capacitive differential sensor measuring circuit. As shown in the figure, since the capacitance is an AC parameter, the bridge is excited by AC through the transformer. Two coils of the transformer and two capacitors form a bridge. When there is no intake pressure, the bridge is in equilibrium, and the two capacitors are equal, C0. When there is pressure, one capacitor is C0+△C and the other capacitor is C0-△C (△C is the capacitance change caused by external pressure), then the bridge loses its balance, and the voltage is also high where the capacitance is high, resulting in the appearance between the two capacitors.
2. Differential transformer inlet pressure sensor
Differential pressure sensor is an inductive sensor with open magnetic field and mutual inductance. Because it has two secondary coils connected into a differential structure, it is also called differential transmission.
When the primary winding of differential transformer is excited by AC power supply, the secondary winding will generate induced electromotive force. Because the secondary coil is connected differentially, the total output is the difference of the induced electromotive force of the two coils. When the local core is motionless, its total output is zero; When the iron core moves, the output electromotive force changes linearly with the displacement of the iron core.
The detection and conversion process of differential transformer inlet pressure sensor is: firstly, the change of pressure is converted into the displacement of transformer core, and then the displacement of core is converted into electrical signal through differential transmission. This pressure sensor is mainly composed of vacuum diaphragm box (bellows) and differential transmission. When the air pressure changes, the bellows deforms and drives the iron core of the differential transformer to move. Due to the displacement of the magnetic core, a voltage is generated at the output end of the differential transformer and sent to the input end of ECU after processing. If the injection time is determined according to the voltage level and the injector operates, the basic fuel injection amount can be determined.
3. Semiconductor strain type intake pressure sensor
The working principle of semiconductor pressure intake sensor is strain effect.
The so-called strain effect is a phenomenon that the resistance value changes when the conductor and the semiconductor are strained by external force.
Resistance strain gauge is a kind of chip resistance sensor, and its working principle is the so-called piezoresistive effect principle that the resistivity of semiconductor materials will change when a certain load is applied in its axial direction to produce stress.
The intake pressure sensor composed of resistance strain gauge is mainly composed of semiconductor strain gauge, vacuum chamber and hybrid integrated circuit board. Semiconductor strain gauges are four equivalent resistors made on the diaphragm by semiconductor technology, which are connected into bridge resistors. The strain gauge of the semiconductor resistance bridge is placed in a vacuum chamber. Under the action of inlet pressure, the strain gauge is deformed, the resistance value changes, and the bridge is out of balance, thus changing the inlet pressure into the output voltage of the resistance bridge.
Fourthly, the valve position sensor
The throttle position sensor is installed on the throttle body, which converts the throttle opening into a voltage signal to output, so that the computer can control the fuel injection quantity.
There are two types of throttle position sensors: switch output and linear output.
(1), switch the throttle position sensor.
This throttle position sensor is essentially a changeover switch, also called throttle switch. This throttle position sensor includes moving contact, idling contact and full load contact. Idle contact and full load contact can be used to detect the idle state and heavy load state of the engine. Generally, the moving contact is called TL contact, the unloaded contact is called IDL contact and the fully loaded contact is called PSW contact. It can be seen from the structural diagram that the cam can rotate and the moving contact can move along the groove of the cam under the action of the connecting rod linked with the throttle. The throttle position sensor has a simple structure, but its output is discontinuous.
When the throttle valve is completely closed, the voltage increases from the TL terminal to the IDL terminal, and then returns to the electronic controller. When the signal is transmitted in this way, the electronic controller knows that the throttle is now completely closed. When the accelerator pedal is depressed and the throttle opening is higher than a certain opening, the voltage is transmitted from TL terminal to PSW terminal and then to the electronic controller. The electronic controller knows that the throttle is now open to a certain angle.
Next, I will explain the influence of idle speed signal and load signal on fuel injection quantity. When the IDL signal is output and the engine speed exceeds the specified speed, the oil supply is interrupted to prevent the catalyst from overheating and save fuel. When the IDL signal is switched from output to no output, the electronic controller judges that the throttle valve is switched from the fully closed state to the open state, and of course, it also judges that the vehicle is in the starting or re-accelerating state, so it will accelerate and enrich according to the warm-up state of the engine, increase the fuel injection and supply the rich mixture needed for acceleration.
When the PSW signal is input to the electronic controller, the output enrichment function is exerted to increase the fuel injection quantity. When driving under heavy load, if there is no PSW signal output, there will be no output enrichment effect, and the output power of the engine will be slightly lower.
(2) Linear throttle position sensor
The linear throttle position sensor is installed on the throttle and can continuously detect the opening of the throttle. It is mainly composed of potentiometers connected with throttle and idle contact. The moving contact of the potentiometer (i.e. the throttle opening output contact) slides on the resistive film with the throttle opening, so that a linear voltage output proportional to the throttle opening is obtained at this contact (TTA terminal). As shown in the figure. When the throttle valve is fully closed, the other moving contact linked with the throttle valve is connected with the IDL contact, and the sensor outputs an idle signal. The linear voltage signal output by throttle position is transmitted to the computer after A/D conversion.
Verb (abbreviation for verb) oxygen sensor
In a gasoline jet engine using a three-way catalytic evolution device, an oxygen sensor is generally installed in the exhaust pipe to detect the oxygen content in the exhaust gas, so as to indirectly judge the concentration of the mixed gas entering the cylinder, and thus control the actual air-fuel ratio in a closed loop. When the oxygen content in the exhaust gas is too high, it means that the mixture is too lean, and the oxygen sensor outputs an electrical signal to the ECU, instructing the injector to increase the fuel injection quantity. When the oxygen content in the exhaust gas is too low, it means that the mixture is too rich. The oxygen sensor immediately transmits this information to the ECU and instructs the injector to reduce the injection quantity. At present, there are two kinds of oxygen sensors used in automobiles: titanium dioxide oxygen sensor and zirconium dioxide oxygen sensor.
Working principle: The oxygen sensor is installed in the exhaust pipe of the engine to measure the oxygen content in the exhaust gas. It is a battery that generates electromotive force according to the difference of oxygen concentration in the atmosphere and exhaust gas. As shown in the figure, the inner surface and the outer surface of the ceramic electrolyte are coated with platinum respectively to form electrodes. When it is inserted into the exhaust pipe, its outer surface is in contact with the exhaust gas, and its inner surface is exposed to the atmosphere. At temperatures above about 300 degrees, ceramic electrolyte will become a conductor of oxygen ions. When the mixture is lean, that is, the excess air coefficient α > 1, the exhaust gas will contain a lot of oxygen, and the oxygen concentration difference between the inner and outer surfaces of the ceramic electrolyte is small, resulting in only a small voltage. However, when the mixture is rich, that is, the excess air coefficient α < 1, the oxygen content in the exhaust gas is less, accompanied by a large number of incomplete combustion products, such as CO and hydrocarbons. These components may react with oxygen under the action of catalyst, consuming the residual oxygen in the waste gas, making the oxygen concentration on the outer surface of ceramic electrolyte tend to zero, making the oxygen concentration difference between inside and outside the electrolyte suddenly increase, and the output voltage of the sensor suddenly increases, and its value tends to 60.
Sixth, the temperature sensor.
Function: Used to measure cooling water temperature, intake air temperature and exhaust air temperature.
Type: There are many kinds of temperature sensors, such as thermistors, semiconductors and thermocouples.
The so-called thermistor is that this resistor is very sensitive to temperature. When the temperature acting on this resistor changes, its resistance will change with the change of temperature. Among them, those that increase with temperature are called positive temperature type thermistors, and those that decrease with temperature increase are called negative temperature coefficient type thermistors.
The measuring circuit of thermistor temperature sensor is relatively simple. As long as the sensor and a precision resistor are connected in series to a stable power supply, the temperature change can be reflected by the voltage division output of the series resistor.