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When the car engine is running, the various systems will be in different working states, such as water temperature, oil temperature, intake pressure, throttle position, etc. This information cannot be directly read by the car's computer. They must be converted into a computer that can recognize the electrical signals. The car sensors convert the light, electricity, temperature, pressure, time, and other information in the car's operation into electrical signals, which are then input into the onboard computer system, and then calculated and analyzed by the pre-stored program in the computer.
The engine sensor control system is the core of the entire automotive sensor. It contains many types, such as temperature sensors, pressure sensors, position and speed sensors, flow sensors, oxygen sensors, and knock sensors. These sensors provide information about the engine's working condition to the engine's electronic control unit (ECU), allowing the ECU to accurately calculate and control the engine's working condition in order to improve engine power, reduce fuel consumption, reduce exhaust emissions, and detect faults.
The temperature sensor is mainly used to detect engine temperature, intake air temperature, cooling water temperature, fuel temperature, and temperature in the catalyst.
There are three main types of temperature sensors: wire-wound resistance, thermistor, and thermocouple resistance. The three types of sensors have their own characteristics, and their applications are also slightly different.
Intake air temperature sensor
Wirewound resistance temperature sensors have high accuracy, but poor response characteristics. Thermistor temperature sensors have high sensitivity, better response characteristics, but poor linearity, and low-temperature adaptation. Thermocouple resistance temperature sensors have high accuracy and wide range of temperature measurement, but it needs to be used with amplifier and cold-end treatment.
The practical products include thermistor temperature sensor (universal type -50℃~130℃, accuracy 1.5%, response time 10ms; high-temperature type 600℃~1000℃, accuracy 5%, response time 10ms), ferrite type temperature sensor (ON/OFF type, -40℃~120℃, accuracy 2.0%), metal or semiconductor film air temperature sensor (-40℃~150℃, accuracy 2.0%, 5%, response time 20ms), etc.
The engine coolant temperature sensor detects engine coolant temperature, converts it to an electrical signal, and sends it to the engine control module (ECU) as the principal correction signal for gasoline injection, ignition timing, idle speed, and exhaust emission control.
The intake air temperature sensor detects the intake air temperature, converts the signal into an electrical signal, and sends it to the engine control module (ECU) as a gasoline injection and ignition timing adjustment signal.
The exhaust gas temperature sensor measures the temperature of recirculated exhaust gas in order to determine the recirculation flow rate.
If the engine temperature sensor fails, it will be difficult for the car to cold start at very low temperatures. In addition, the car will have poor driving characteristics during the warm-up phase, increased fuel consumption, and increased exhaust emissions.
Pressure sensors are used to measure cylinder negative pressure, atmospheric pressure, turbo engine boost ratio, cylinder internal pressure, and oil pressure, among other things.
The suction negative pressure sensor is mainly used for the detection of suction air pressure, negative pressure, and oil pressure. Capacitive, piezoresistive, differential transformer (LVDT), and surface elastic wave pressure sensors are commonly employed in automobiles (SAW).
Negative pressure, hydraulic pressure, and air pressure are all detected using capacitive pressure sensors. The measuring range is 20~100kPa. It has the characteristics of high input energy, good dynamic response characteristics, and good environmental adaptability.
Piezoresistive pressure sensors are greatly affected by temperature and need to be equipped with temperature compensation circuits, but they are suitable for mass production. LVDT pressure sensors have larger output, easy to digital output, but poor anti-interference.
Small size, lightweight, low power consumption, excellent reliability, high sensitivity, high resolution, and digital output are all features of the SAW pressure sensor. It is used for the pressure detection of automobile suction valves and can work stably under high temperatures.
The function of the manifold absolute pressure sensor (MAP) is to detect the vacuum degree of the intake manifold and convert the pressure signal into an electronic signal to be sent to the engine control computer. It is the main reference signal component for controlling the fuel injection pulse width and ignition timing. There are two types of MAP sensors: semiconductor piezoresistive and capacitive manifold absolute pressure sensors.
The flow sensor is mainly used for the measurement of engine air flow and fuel flow. The function of the air flow sensor is to convert the amount of air drawn into the engine cylinder per unit time into an electrical signal and send it to the engine control module (ECU). It is one of the basic signals that determine the amount of fuel injection and ignition timing and is used by the engine control system to determine combustion conditions, control air-fuel ratio, start-up, ignition, etc.
air flow sensor
The rotary vane type (vane type), Karman vortex type, hot wire type, and hot-film type are the four types of air flow sensors.
The rotary vane type (vane type) air flow meter has a simple structure and low measurement accuracy. The measured air flow requires temperature compensation.
The Karman vortex air flowmeter has no moving parts. It is sensitive and has high accuracy, and it also needs temperature compensation.
The hot-wire air flowmeter has high measurement accuracy and does not require temperature compensation, but it is easily affected by gas pulsation.
The hot film air flow meter has the same measurement principle as the hot wire air flow meter, but the volume is small. It’s suitable for mass production.
The main technical indicators of the air flow sensor are: the working range is 0.11~103 cubic meters/min, the working temperature is -40℃~120℃, and the accuracy is ≤1%.
The fuel flow sensor is used to detect fuel flow, mainly including water wheel type and recirculating ball type. Its dynamic range is 0~60kg/h, working temperature is -40℃~120℃, accuracy is 1%, and response time is less than 10ms.
Position and speed sensors are mainly used to detect crankshaft angle, engine speed, throttle opening, vehicle speed, etc. At present, the position and speed sensors used in automobiles mainly include alternator type, magnetoresistive type, Hall effect type, reed switch type, optical type, semiconductor magnetic transistor type, etc., with a measurement range of 0 to 360 and an accuracy of 0.5. The measured bending angle reaches 0.1.
Position and speed sensor
One of the most significant sensors in the engine's centralized control system is the crankshaft position sensor. It's an essential signal source for validating the crankshaft angle and engine speed. The engine control module (ECU) uses this signal to control fuel injection volume, fuel injection timing, and Ignition timing (ignition advance angle), ignition coil charging closing angle, idling speed, and operation of an electric gasoline pump.
According to the classification of the principle of signal formation, the crankshaft position sensor (CKP) can be divided into three categories: electromagnetic type, photoelectric type, and Hall effect type.
The camshaft position sensor is used to detect the rotation angle position of the camshaft. This signal is used by the engine control module (ECU) to calculate the engine's cylinder sequence, which is used to control the injection and ignition sequences. When the camshaft position sensor malfunctions, the engine's output power decreases.
A throttle opening sensor is used to detect the opening degree of the throttle valve and the speed of opening and closing, and convert the signal into a voltage signal and send it to the control computer of the engine, which is used to control the fuel injection pulse width, ignition timing, idling speed, exhaust emission. The main correction signal is also an auxiliary signal for the air flow sensor or intake manifold pressure sensor.
The throttle position sensor is a variable resistor. Most throttle position sensors include a sliding contact arm connected to the throttle shaft, and the contact arm slides on a resistive material provided on the shaft of the movable contact.
The analog throttle position sensor is a three-wire sensor. One line of the 5V voltage from the computer power supply supplies power to the sensor's resistive material, and the other line is connected to the other end of the resistive material to provide (negative) grounding for the sensor. The third wire is connected to the movable contact of the sensor and provides a signal output to the (ECU) computer. The movable contact detects the voltage at each point on the resistive material, which is proportional to the throttle angle.
Two switch contacts make up the switch-type throttle position sensor. A rotary switch and a normally closed contact form an idle switch. When the throttle is in the idle position, it is in a closed state. Ground the idle speed input signal terminal of the engine control computer. After the engine control computer receives this signal, the engine can enter the idle speed closed-loop control. When the throttle opening of the other normally open contact reaches full load, ground the full load input signal terminal of the engine control computer to ground. After the engine control computer receives this signal, it can make the engine enter the full load enrichment control state.
The throttle is a very important sensor because the computer uses its signal to calculate engine load, ignition time, exhaust gas recirculation control, and idle speed control. A bad throttle body position sensor can cause problems such as acceleration lag and idling instability, as well as driving performance problems and emission test failures.
The purifying ability of the three-way catalyst for CO, HC, and NOx will drastically decrease whenever the engine's air-fuel ratio deviates from the predicted air-fuel ratio. Therefore, in order to achieve the best exhaust purification performance of an engine equipped with a three-way catalytic converter, the air-fuel ratio of the mixture must be controlled within a very narrow range near the theoretical air-fuel ratio.
The oxygen sensor is used to detect the state of the exhaust gas entering the three-way catalytic converter, and it is an indispensable sensor on the engine using the three-way catalytic converter. There are two types of oxygen sensors that have been used in automobiles: zirconia type and titanium oxide type.
The basic element of the zirconia oxygen sensor is a special ceramic body, that is, a solid electrolyte of zirconia. The ceramic body is made into a tube (zirconium tube) and fixed in a fixing sleeve with a mounting thread. A gas-permeable platinum electrode is installed on the zirconium tube's surface, along with a protective tube and a wire connector. The inner surface is in communication with the atmosphere, and the outer surface is in communication with the exhaust gas. A protective sleeve is also installed on the outer surface, and a vent groove is opened on the sleeve. The ceramic body of the zirconium tube is porous, allowing oxygen to penetrate into the solid electrolyte. At higher temperatures (above 300°C), oxygen ionizes. If the oxygen concentration measured inside the ceramic body (atmosphere) and outside (exhaust gas) is different, a voltage drop will occur on the surface of the two platinum electrodes, and the side with higher oxygen content is at a higher potential. When the mixture is lean, there is more oxygen in the exhaust gas, and the concentration difference between the two sides is small, and only a small voltage is generated. On the contrary, when the mixture is rich, a high voltage is generated.
According to the measured voltage value, the oxygen content on the outer surface of the oxygen sensor can be measured. The oxygen content in engine exhaust mainly depends on the air-fuel ratio of the mixture. Therefore, ECU analyzes the combustion status of gasoline according to the electrical signal input by the oxygen sensor in order to correct the fuel injection volume in time to make the air-fuel ratio in an ideal condition, that is, λ=1, so this type of sensor is also called a λ sensor.
Oxygen sensors generally have four lead types: single-wire, double-wire, three-wire, and four-wire. The single wire is a zirconia oxygen sensor; the double wire is a titanium oxide oxygen sensor; the three and four wires are zirconia oxygen sensors.
The difference between three-wire and four-wire: the heater negative electrode and signal output negative electrode of the three-wire oxygen sensor share one wire, and the heater negative electrode and signal negative electrode of the four-wire oxygen sensor each use one wire.
Knock sensor refers to the abnormal phenomenon of spontaneous combustion produced by the final mixture in the combustion chamber. Since knocking will produce a high-intensity pressure wave that impacts the combustion chamber, not only the sharp metal sound can be heard. It will also have a greater impact on the components of the engine. Premature ignition time is the main cause of knocking. In order to make the engine run at maximum power. It is best to advance the ignition time to the limit range where the engine just does not knock, so a knock sensor must be added to the ignition system.
The knock sensor detects knock during the engine's combustion process and sends the knock signal to the engine control computer as a critical reference signal for correcting the ignition advance angle.
The magnetostrictive knock sensor and the piezoelectric knock sensor are the two most common knock sensors.
The common knock sensor is mainly piezoelectric, which is installed on the cylinder block of the engine. This sensor uses the piezoelectric effect of crystal or ceramic polycrystal. The piezoelectric resistance effect of doped silicon can also be used. The sensor's enclosure contains piezoelectric elements/counterweights as well as cables. The principle is: when the engine's cylinder block vibration is transmitted to the sensor housing, relative movement occurs between the housing and the counterweight. The squeezing of the sandwiched piezoelectric element and the weight is changed, and the output voltage signal is changed. The control component can only detect the voltage formed by the 7KHZ vibration. According to the magnitude of this voltage to judge the knock intensity. Then delay the ignition time accordingly to avoid knocking.
The appearance and structure of a magnetostrictive knock sensor have a permanent magnet, a ferromagnetic iron core excited by the permanent magnet, and a coil around the iron core. The sensor resonates with the engine at roughly 7kHz as the cylinder block vibrates, and the permeability of the ferromagnetic material core changes, causing the magnetic flux density of the permanent magnet to pass through the core to change as well. The induced electromotive force is generated in the winding around the iron core, and this electric signal is input to the ECU.
Knock sensors are classified as either non-resonant or resonant. They're usually positioned between cylinders 2 and 3, or one in the middle of cylinders 1 and another between cylinders 3 and 4. Shielded wires are wrapped around the connecting wires of general knock sensors.
Of course, there are many more sensors on the engine. They are like our eyes, ears, nose, and skin, which convert all the information we see, hear, smell, and feel into electrical signals and transmit them to the car. The computer system allows the engine to make correct judgments and assist the driver to drive the vehicle better. With the advancement and development of technology, there will be more and more sensors on the engine, and more and more intelligent.