(4) Electronic control system of exhaust gas recirculation (EGR)
Early research abroad found that a small amount of exhaust gas (5% ~ 20%) can be recycled into the cylinder and burned after mixing with fresh combustible mixture, which can effectively inhibit the production of nitrogen oxides. In fact, in addition to the internal EGR mode in which valve overlap produces exhaust backflow, the common measure is to use a special pipeline to guide part of exhaust gas to the intake pipe, and ECU controls the EGR valve to change the flow cross section to adjust the exhaust volume, so as to realize the change of recirculated exhaust volume. Generally, EGR control is not needed under engine warm-up, idling, low load, high load and other working conditions.
The general process of EGR control is: ECU calculates the optimal recirculation exhaust volume according to the engine speed, throttle opening and cooling water temperature, and then realizes EGR control by controlling the opening of EGR valve. The control of EGR valve by ECU is essentially realized by controlling vacuum regulating valve. The vacuum regulating valve is generally electromagnetic, which is used to convert the electric signal output by ECU into air pressure change, so as to control the pneumatic EGR valve. In addition, the ECU also measures the recirculation exhaust gas rate signal through the pressure sensor for feedback control. When implementing feedback control, independent pressure or differential pressure sensor was used at first, and now EGR position sensor with integrated EGR valve appears, which improves the control accuracy.
(5) Boost electronic control system
Turbocharging systems are increasingly installed in engines to improve intake efficiency. The development of electronically controlled supercharging system makes supercharging technology step up to a new level. At present, electronically controlled exhaust gas turbocharging system is widely used. Usually, the supercharger is designed to match the low-speed and low-load working conditions of the engine. When the engine is running at high load, it is easy to damage the supercharger due to overspeed. Therefore, the electronically controlled exhaust gas turbocharging system specially adds a bypass duct at the exhaust gas turbine chamber in the exhaust pipe, which is adjusted by ECU controlling the opening of the on-off valve. Under normal circumstances, the on-off valve is closed and the exhaust gas is discharged through the turbine chamber; Once the engine is running under high load, the exhaust gas turbine speed will increase. When the intake pressure exceeds the limit, the ECU will open the switch valve through the corresponding mechanism to make the bypass channel conductive, and the exhaust gas will be directly discharged from the bypass channel without passing through the turbine chamber, and the supercharger will stop working.
(6) Fault self-diagnosis system
In the electronic control system of modern automobile engine, ECU usually has a fault self-diagnosis system to monitor and diagnose the faults of various parts of the engine control system. For the sensor, it can be directly judged by detecting whether its signal exceeds the specified range; For the actuator, a special circuit is added to its initial circuit to realize monitoring, and for the ECU itself, there is also a special program for diagnosis.
The fault self-diagnosis system always monitors the working conditions of each control system. When a fault occurs, the fault indicator light on the dashboard of a general automobile can flash to give an alarm, and the fault information can be stored in the memory of a microcomputer in the form of codes. During maintenance, it can be displayed not only by intermittent flashing of the fault indicator lamp, but also by digital display through special testing instruments, so as to further find out the cause of the fault through the manual. The self-diagnosis system solves the problem that it is difficult to judge the fault of complex electronic control system.
(7) Fault insurance system and fault backup control system
When the self-diagnosis system detects the fault of the sensor and its circuit, the fault-proof system in ECU automatically comes into play. The automatic fail-safe system replaces the abnormal signal input by the fault part with the directly controlled programming data. Fail-safe systems are generally implemented by software programming.
When the microcomputer or the main sensor (such as the intake manifold pressure sensor) fails, the ECU immediately switches the main control right from the microcomputer to the fault standby system, and the fault standby system replaces the microcomputer. As an integrated circuit module of ECU, the fault backup system can only determine the simplest control scheme to keep the car running according to the starting signal and the idle contact state signal, so as to ensure the car to "go home slowly" for maintenance, but it can not achieve the best performance under the control of microcomputer.
(8) Other electronic control systems.
1. intake vortex electronic control system intake vortex can promote gasoline evaporation and even mix with air, and improve combustion efficiency. Electronically controlled intake swirl is widely used in some cars (especially those with lean-burn technology). Its structure is to add a vortex control valve near the air inlet, collect signals such as rotational speed, throttle opening and cooling water temperature through ECU, then control the rotation angle, deflect the steering flow to generate vortex, and adjust the vortex ratio to realize vortex control.
2. Variable intake control system Variable intake control system can improve the dynamic performance of the engine from the perspective of increasing intake air volume and improving intake efficiency. There are two types of the system: one is the variable flow area control mode, in which the ECU controls the rotation angle of the control valve installed in the intake pipeline to change the intake flow cross section to meet the requirements of intake air volume under different working conditions; The other is variable flow length control mode, in which ECU controls the control valve in the intake pipe to adjust the length of the intake pipe. Practice has proved that the variable intake control system can improve the power and economy of the engine.
3. Intake air temperature preheating control system The intake air temperature preheating control system promotes gasoline evaporation and improves emission performance by adjusting the intake air temperature at low temperature starting. There are three preheating methods: exhaust pipe preheating, water temperature preheating and PTC preheating.
4. Electronic control system of fuel evaporation The electronic control system of fuel evaporation is used to reduce the pollution caused by the emission of gasoline vapor in the fuel tank into the atmosphere. At present, the electronic control device for evaporation of activated carbon tank has been widely used. When parking, use activated carbon canister to absorb gasoline vapor to prevent it from spreading into the atmosphere; After the engine is running, ECU controls the conduction between the activated carbon canister and the intake pipe, and uses the intake vacuum to suck the gasoline vapor adsorbed in the activated carbon canister into the intake pipe, which can effectively prevent the gasoline vapor from escaping and reduce the HC emission pollution.
5. Crankcase Forced Ventilation Electronic Control System The purpose of the electronic control system for crankcase forced ventilation is to recycle the gas that seeps into the crankcase through the piston ring gap in the cylinder into the intake pipe, so as to reduce the pollution caused by the direct discharge of this gas into the atmosphere. In modern electronic control system, ECU controls the forced ventilation valve according to the throttle position signal and speed signal, so as to realize the communication between the gas in the crankcase and the intake pipe, and then use the gas in the crankcase.
6. As one of the early measures to control pollutant emission, secondary air injection system is currently used in combination with catalytic converter. It also controls the conduction of secondary air injection channel through ECU to introduce air into catalytic converter to realize the conversion of nitrogen oxides, carbon monoxide and hydrocarbons. In the way of introducing air into the exhaust pipe, besides the control of air pump, it can also be realized by exhaust pulse wave. In addition, with the deepening of research, many new technologies have emerged. For example, cylinder stop control can stop the fuel supply and ignition control of some cylinders according to different load requirements, reduce waste and improve engine efficiency; Another example is the electronic control system of the accelerator pedal, which can avoid the error caused by the wear of the mechanical accelerator pedal and increase the control accuracy.
Three. Concluding remarks
With the improvement of microcomputer and electronic technology and the vigorous development of material technology, coupled with the continuous maturity and perfection of control theory, engine electronic control technology is expected to make a greater breakthrough.