Starting from 1957, American companies have introduced an electronically controlled gasoline injection system, which is called EFI for short. EFI technology is the engine, which has created a new era for the development of the whole transportation industry. At first, analog EFI was used, and later it developed into digital EFI. Its basic principle is that the microcomputer (ecu) accurately controls and selects the best ignition and fuel injection timing and fuel injection quantity through analysis, calculation and judgment according to the signals of various sensors. The advantages of electronically controlled gasoline injection are as follows: firstly, the fuel quantity can be optimized most accurately according to specific targets for various working conditions, and the best matching can be achieved in all working conditions; Second, it can realize closing control and prevent the deviation of fuel injection quantity caused by the change of injection density.
In a gasoline engine, the combustible mixture in the cylinder is ignited by electric spark. In the ignition system of automobile engine, the ignition coil is the executive part to provide ignition energy for igniting the air and fuel mixture in the engine cylinder. It is based on the principle of electromagnetic induction. By closing and opening the primary circuit of the ignition coil, the current in the primary circuit increases and then suddenly decreases, so that the high voltage required to ignite the spark plug will be induced in the secondary. The ignition coil can be regarded as a special pulse transformer, which converts the low voltage of 10- 12v into a voltage of 25000v or higher.
Therefore, a spark plug is installed on the cylinder head of the gasoline engine, and the head of the spark plug extends into the combustion chamber. All the equipment that can generate electric spark between spark plug electrodes within a specified time is called ignition system, which usually consists of battery, generator, distributor, ignition coil and spark plug. Used for early mechanical contact circuit breakers (namely platinum ignition) and ignition by mechanical high-voltage distribution cap without distributor transistor.
And later the double spark coil. It belongs to an ignition system controlled by a microcomputer, and is mainly composed of the following components: a sensor for monitoring the running condition of the engine, a microprocessor (ecu) for processing signals, an igniter responding to instructions from the microcomputer, an ignition coil, etc. Because the microcomputer-controlled ignition system is no longer equipped with vacuum centrifugal ignition advance adjustment device, the ignition advance angle is controlled by microcomputer, so that the engine can optimally adjust the ignition time under various working conditions and make the ignition advance to the range where the engine just does not knock. The ignition system controlled by microcomputer has the advantages of low energy loss, good high-speed performance, small electromagnetic interference and high ignition accuracy. At present, it is more and more used in mid-to high-end cars. Distributorless ignition mode is used for simultaneous ignition, that is, two cylinders share one ignition coil, that is, one ignition coil has two high-voltage output terminals.
The ignition system consists of a microprocessor (ecu), an ignition coil, an electronic drive module, a high-voltage ignition wire and a spark plug, as shown in the figure (Note: since there is no schematic diagram of the Liana car, this figure is for reference only).
1. Various sensors 2. Electronic control unit 3 igniter (electronic drive module) 4. Ignition switch 5. 12v battery 6. Ignition coil 7. Spark plug 8. Primary coil 9. Secondary coil.
The following explains the characteristics and working principle of each component:
1, microprocessor (ecu)
The so-called ordinary riders' computer refers to the driving computer, ecu-electronic control unit-electronic control unit, which is responsible for the supervision of vehicles and engines. It consists of an input signal sensor, an electronic control unit (ecu) and an ignition actuator. In other words, the ecu we are talking about is some main sensors, such as engine speed, cooling water temperature, intake air temperature, throttle position, oxygen sensor, intake air pressure and so on. After calculation and processing by ecu, it is sent to the execution unit for correction, so as to realize high-precision air-fuel ratio and optimal ignition timing control. In addition to supplying suitable fuel and adjusting ignition angle and timing according to different driving conditions, ecu must also be responsible for controlling various electronic equipment, such as air conditioning system, cooling system and self-inspection system. For vehicles, ecu is equivalent to the brain of human body, which is responsible for receiving various signals, controlling various systems through built-in basic programs, and maintaining the normal running of vehicles. According to the pre-designed program, Ecu calculates the information sent by various sensors, limits the parameters to the allowable voltage level after processing, and then sends them to relevant actuators to perform various predetermined control functions.
The microprocessor calculates the fuel injection time, fuel injection quantity, fuel injection rate and fuel injection timing based on the input data and the data stored in the map. And these parameters are converted into time-varying electric quantity matching with engine operation. Based on the engine speed and load, after calculation and processing by ecu, the action instruction is sent to the fuel injector, fuel supply pump, etc. , so that each cylinder has the most suitable fuel injection quantity, fuel injection rate and fuel injection timing to ensure the best combustion of each cylinder. Because the work of the engine changes at a high speed, it requires high calculation accuracy and fast processing speed. The performance of ecu should develop with the development of engine technology, and the memory of microprocessor is getting bigger and bigger, and the information processing ability is getting stronger and stronger.
The signal is input to the electronic ignition controller, and after being amplified and shaped by the high-power transistor pre-circuit, it controls the charging and discharging process of the high-energy dry ignition coil primary. When the power tube is turned on, the primary of the ignition coil is also turned on, and the ignition coil stores energy. When the signal turns off the power tube of the controller, the primary of the ignition coil is turned off, and the instantaneous high voltage is induced in the secondary of the coil.
The control signal from the microcontroller is amplified by the power transistor in the igniter. (Note: I have never disassembled the ecu of Liana car. Some cars use power module or Darlington, or directly use microcontroller as the core of ignition electronic control unit, which consists of power supply, input signal shaping, driving amplifier circuit, communication circuit and other functional modules. Either way, the principle is the same. Realize the on-off control of the primary circuit. Namely ignition control: including ignition sequence control, ignition timing control and ignition energy control. The ignition system should be ignited according to the working sequence of the engine, that is, the ignition sequence should be consistent with the working sequence of the engine, otherwise the mixture can not be ignited in time and the engine can not work normally. The purpose of ignition timing control is to make the engine have high power output, low fuel consumption, low knock and low emission. The ignition system must be ignited at the most favorable moment, and the above goal needs to be compromised. The ignition time is expressed by the ignition advance angle, and the crank angle from the spark plug to the piston operation to the top dead center of the compression stroke is called the ignition advance angle. The optimal ignition advance angle of the engine under different working conditions is different. In the ignition system controlled by microcomputer, the working state of the engine is determined according to the signals of sensors such as engine speed and load, and the optimal ignition time is calculated. The microcontroller outputs the control signal, turns off the power transistor and cuts off the primary circuit, thus realizing the control.
2. Ignition coil
In the ignition system of automobile engine, the ignition coil is the executive part to provide ignition energy for igniting the mixture in the engine cylinder. It is based on the principle of electromagnetic induction. By closing and opening the primary circuit of the ignition coil, the current in the primary circuit increases and then suddenly decreases, so that the high voltage required to ignite the spark plug will be induced in the secondary. The ignition coil can be regarded as a special pulse transformer, which converts the low voltage of 10- 12v into a voltage of 25000v or higher. The win current mainly passes through that primary coil and is store as a magnetic field. When the current of the primary coil winding is suddenly cut off (the grounding end of the circuit is disconnected by the power transistor), the magnetic field decays, causing the secondary coil winding to generate induced electromotive force, and the voltage of the induced electromotive force is enough to discharge the spark plug, which is called induced discharge ignition. (pictured). There is also a capacitive discharge ignition system, which is usually called cdi ignition mode.
Our Liana car uses a closed magnetic circuit solid ignition coil, which is mainly composed of a low-voltage coil winding, a high-voltage coil winding connected in series with a high-voltage damping resistor and divided into two outputs, a closed magnetic circuit iron core, a shell and a solid filler (its external structure is shown in the figure).
3. Another component in the ignition coil-high voltage line.
As the name implies, the high voltage conductor is responsible for transmitting high voltage electricity from the high voltage coil to the spark plug. High-voltage lines are actually very simple insulated wires. One of the most common metal wires is covered with high-strength insulator. Its main quality index is that it can have good insulation strength at high and low temperatures. The current it passes through is very small, and the requirements for the metal wires inside are very low; The pass voltage is very high (15000v-40000v), so the insulation coefficient of the insulation material is required to be very high. The main problem is the aging of insulation materials and the decrease of insulation strength, which leads to leakage. An excellent set of high-voltage lines must have minimum current loss to avoid electromagnetic interference caused by high-voltage power transmission. Therefore, the high-voltage ignition wire is designed with resistance, and the circuit is scientifically called damping resistance. The resistance of high-voltage lines varies according to the design of various high-voltage output systems, some are only a few hundred ohms, and some are above 10k. Of course, the high-voltage line with damping resistance is only used in EFI vehicles, and the carburetor car of platinum automobile ignition system does not need to be used here.
The main functions of damping resistor of high-voltage line are:
A, to prevent electromagnetic leakage caused by high voltage, also known as emi protection, to protect the safety of electrical appliances in the car and make the ecu of the driving computer stable, so emi is the most important issue to be considered. In fact, the signals received by ecu are all low-voltage analog signals, including crankshaft position /O2. These signals are only used after a/d conversion in ecu. This kind of radiation /emi leakage from wires is likely to bring essential interference to the automobile sensor signal, which is originally a low-voltage signal, resulting in great signal deviation. After a/d conversion, ecu is easy to alarm by mistake, and even works with protection mode parameters.
B, in order to prevent high-voltage current from accidentally damaging electronic components of high-voltage output system. When the high voltage path is normal, this resistance can be regarded as straight-through, that is, the resistance is zero, because the ignition resistance of automobile spark plugs is usually around 30m, 1m= 1000k. This ignition resistance is very large relative to the resistance of the high-voltage line, so the current-limiting resistance of the high-voltage line can be ignored in the total resistance of the high-voltage output system, but when the circuit behind the high-voltage line is accidentally short-circuited, the current-limiting resistance can limit the current to the range that the high-voltage system can withstand without being damaged. As the ignition system of ecu, if there is a short circuit, the instantaneous high voltage of any coil secondary will be released and the ecu will stop working.
C. In order to make the primary input circuit and the secondary high-voltage output circuit of the high-voltage coil have good impedance matching, damping resistors are arranged in the high-voltage output line and the spark plug. The high voltage output line with damping resistor can effectively match the impedance of electronic components (power tubes) in the primary input circuit. Previous vehicles basically used platinum ignition system, so there is no need to set damping resistors for high-voltage output lines and spark plugs.
4, spark plug working principle:
The high voltage discharge of spark plug ignites the mixture and makes it explode. The working process is short, but there are several processes from ignition to explosion. First of all, when the high-voltage current generated by the ignition coil passes through the gap between the spark plug electrodes, a potential (which can be understood as high voltage) is formed at the positive electrode of the spark plug. Because of the potential difference, it must be discharged to the negative electrode under normal circumstances, because there is a gap in the middle to form an arc, thus generating sparks to ignite the mixture in the combustion chamber of the cylinder. Of course, the performance requirement of spark plug is that the stronger the spark, the more stable it is. Because the insulator between the grounding electrode (shell) of the spark plug and the high-voltage center electrode is resistant to high temperature and has excellent insulation ability, most of them are alumina-based ceramics, but the high-performance spark plug pays much more attention to the electrode material. At present, high-quality spark plugs are mostly made of precious metals such as iridium or platinum (platinum), which can emit stronger and more stable sparks. They are more durable than ordinary models made of copper-nickel alloy, but their prices have also doubled. On the other hand, another important function of the spark plug is to take away the heat in the cylinder to maintain a suitable working temperature (500-850℃). Excessive temperature will damage the insulator and electrode of the spark plug, and the spark plug burned red by high temperature will cause premature combustion and detonation. However, if the temperature is too low, the oil attached to the surface of the spark plug will not be fully burned, which will easily form carbon deposits, weaken the efficiency of the spark plug, or even produce sparks! Therefore, spark plugs with different thermal ranges will be used in engines with different characteristics to maintain the normal operation of spark plugs. Cold spark plugs-that is, spark plugs with higher labels (note that different brands of spark plugs may have different standard labels) are suitable for high-performance engines that often work at high speed (high temperature) because of their rapid heat dissipation, while hot spark plugs with slower heat dissipation (lower labels) are suitable for engines with low speed and low compression ratio. Therefore, when choosing spark plugs, we must think of a suitable calorific value (because different brands have different ways to mark calorific values). The calorific value refers to the heat dissipation capacity of the spark plug. Generally, the higher the compression ratio, the higher the calorific value of the engine, which requires a colder spark plug, that is, a spark plug with good heat dissipation ability, while a lower compression ratio uses a hotter spark plug. The heat dissipation capacity directly affects the combustion of the engine. If the heat dissipation is too strong, the flame will be very small and weak, and the spread speed will be delayed, so the combustion of mixed gas is not ideal. On the other hand, if the heat dissipation is not enough and the flame burns too fast, it may cause knocking, and at the same time, because the temperature between electrodes is too high, it will be easier to ablate the spark plug electrode. What kind of spark plug with calorific value should be selected depends on the standards in the manual. At present, the calorific value of spark plugs in most cars is between 6 and 8, which can reduce the temperature by one degree, but it is not good to use 7. If the heat dissipation is too fast, Mars will be reduced, and the combustion speed of the mixture will be reduced, and it will be discharged before it is completely burned, unless it has been 6000 revolutions.