Principle and application of secondary air injection for automobile engine? . function

Since the implementation of the first automobile exhaust pollution control standard in the world, the secondary air injection system has been widely used in automobiles. Reducing HC and CO emissions in exhaust gas is actually a practical exhaust emission control technology. Moreover, practice has proved that the air injection system can achieve good results whether it is a gasoline vehicle or a diesel vehicle. Its working principle is that the air pump sends fresh air into the exhaust pipe of the engine, so that HC and CO in the exhaust gas are further oxidized and burned, that is, the oxygen introduced into the air is further combined with HC and CO in the exhaust pipe to form water vapor and carbon dioxide, thereby reducing the emission of HC and CO in the exhaust gas. The classification and working principle of the system can be divided into two categories according to the location of air injection: the first category, fresh air is injected into the base of the exhaust manifold, that is, the location where the exhaust manifold is connected with the cylinder block, so HC and CO in the exhaust can only be oxidized from the exhaust manifold; Secondly, fresh air is injected into the exhaust passage in the cylinder head behind the exhaust valve through a special pipe on the cylinder head, and the oxidation of HC and CO in the exhaust gas is carried out in advance. Secondary air injection system can be divided into two types according to the different structure and working principle: air pump type and aspirator type. According to the different control forms, it can be divided into: air pump secondary air injection system. The secondary air injection system of air pump is mainly composed of air pump, diverter valve, connecting pipe and air injection manifold. The working principle is: when the engine is working, the air pump is driven by the crankshaft transmission belt, and the airflow with high pumping capacity and low pressure enters the diverter valve through the hose. Under normal circumstances, the valve on the diverter valve is opened, and air flows into the air injection manifold through the diverter valve and the check valve. The air injection manifold injects the airflow into the engine exhaust hole or exhaust manifold, which reacts with HC and CO in the exhaust gas and further converts it into CO2 and water vapor, thus reducing the exhaust pollution. Once the air pressure pumped by the air pump is too high, the pressure reducing valve will work to instantly cut off the air supply to the air injection manifold to prevent the engine from backfiring. After a few seconds, the double-acting valve fell down, and the air supply to the air injection manifold resumed, and the secondary air injection system worked normally. Detailed analysis of secondary air injection system with air pump The structure of the air pump is installed at the front end of the engine and consists of a centrifugal air filter and a vane pump. The air pump is driven by the crankshaft pulley of the engine through the transmission belt, which provides a large amount of low-pressure air to the injection system. The centrifugal air filter is installed at one end of the pump rotor shaft and rotates at the same speed as the pump. The function of centrifugal air filter is to purify the air entering the air pump. The filtering principle of centrifugal air filter is that when the impeller rotates at high speed, the dust particles in the air are heavier than air and separated from the airflow entering the air pump under the action of centrifugal force. Vane pump consists of pump shell, rotor, blades, blade sealing groove, air inlet and air outlet. In order to form cavities with different sizes between the blades and the inner hole of the pump casing, the rotation center line of the rotor does not coincide with the center line of the inner hole of the pump casing. Driven by a pulley, the rotor rotates on an axis that is not coincident with the inner hole of the pump housing. The two blades are arranged in the groove of the rotor at an included angle of180, and slide in the groove, and there is a sealing groove between the blades and the groove of the rotor. Working principle of air pump A When the pump rotates, the 1 th blade sweeps the air inlet, which gradually increases the volume of the air inlet chamber formed by the rotor, blades and the inner hole of the pump housing, thus creating a certain degree of vacuum. Under this vacuum, the air filtered by the centrifugal air filter enters the air inlet chamber. B the rotor continues to rotate, and the second blade sweeps the air inlet again. At this time, the rotation of the 1 th blade makes the sucked air confined in a larger space surrounded by the two blades, the rotor and the inner hole of the pump casing. When the rotor continues to rotate, this part of the air is swept into a smaller space and compressed. C. the rotor continues to rotate. Once the 1 blade begins to sweep the exhaust hole of the pump, this part of compressed air is pumped into the injection system from the exhaust hole, thus completing an intake-compression-exhaust cycle of the air pump. The rotor completes the above two cycles every 1 revolution. When the rotor of the pump runs at a high speed, the above-mentioned cycle continues to provide fresh air for the injection system. The diverter valve is usually bolted to the air pump as a single assembly, and the pipeline is connected to the air pump and the air injection manifold through hoses. The purpose of setting the diverter valve is to prevent the exhaust system from "backfiring" the air pump when the engine suddenly decelerates. When the throttle valve is suddenly closed and the engine slows down suddenly, a high vacuum will appear in the intake pipe, which will cause the combustible mixture entering the cylinder to be too rich to burn completely in the power stroke. When exhausting, more unburned mixture is discharged to the exhaust pipe through the exhaust valve. If the secondary air injection system injects fresh air into the exhaust manifold or the exhaust hole near the exhaust valve at this time, the fresh air will aggravate the combustion of unburned mixture in the exhaust pipe, resulting in "backfire". The function of the diverter valve is to discharge the air pumped by the air into the atmosphere at the initial moment when the engine suddenly decelerates, so that fresh air cannot be sprayed into the exhaust pipe, thus preventing the occurrence of "backfire". When the throttle opening suddenly decreases and the engine suddenly decelerates, a great degree of vacuum is generated in the intake pipe, which is transmitted to the diaphragm surface of the diverter valve through the pipeline. Under the action of this vacuum degree, the diaphragm moves upward against the spring force, driving the lower valve of the double-acting valve to open the lower valve port, and the lower valve port is communicated with the exhaust hole (made of silencing material) through the lower valve port, so that the air flow from the air pump can be silently and instantly discharged into the atmosphere. However, the air flow from the air pump can only be instantly discharged into the atmosphere through the diverter valve, because there are orifice holes on the diaphragm, which can quickly balance the air pressure on both sides of the diaphragm. Therefore, under the action of the spring force, the diaphragm and the double-acting valve return to the lower position within a few seconds. The double-acting valve closes the lower valve port again, and the air pump starts to supply fresh air to the exhaust manifold or exhaust valve area again. Pressure reducing valve (pressure limiting valve) is mainly composed of valve body, spring, valve and valve seat. Its function is that when the engine is running at high speed and the air pressure pumped by the air pump exceeds the preset elastic force of the spring of the pressure reducing valve, the air pressure overcomes the elastic force of the spring and urges the valve to leave the valve seat, and the air with excessive pressure is discharged into the atmosphere through the vent hole between the valve and the valve seat, so that the air pressure entering the air injection manifold is basically kept constant; When the air pressure sent by the air pump is lower than the preset elastic force of the spring, the spring presses the valve to return, thus cutting off the passage to the atmosphere. It can be seen that the preset elastic force of the pressure reducing valve spring determines the air pressure pumped by the air pump to the whole secondary air injection system under various working conditions. Check valve The check valve is installed on the air injection pipe. It allows air with a certain pressure from the air pump to enter the air injection manifold, while preventing high-temperature engine exhaust gas from entering the connecting hose and the air pump. That is to say, if the belt of the air pump breaks or the transmission slips, the air pump stops running or the speed drops, and the air connecting hose leaks, the check valve can protect the secondary air injection system from the damage of high-temperature exhaust gas. Air injection manifold The air injection manifold is usually welded by stainless steel pipes, and its shape and number of branches depend on the structure of the engine and the number of cylinders. The function of the air injection manifold is to inject the fresh air pumped by the air pump into the exhaust hole or exhaust manifold near the exhaust valve of the engine respectively. Pulse secondary air injection system Pulse secondary air injection system is also called getter secondary air injection system. The system uses exhaust pressure pulses to draw fresh air into the exhaust system, instead of using an air pump to pump air into the injection manifold. It is found that every time the exhaust valve is closed, there will be such a short period of time, during which the air pressure in the exhaust hole and exhaust manifold is lower than atmospheric pressure, that is to say, a negative pressure (vacuum) pulse is generated. Using this vacuum pulse, a certain amount of air is sucked into the exhaust manifold through the air filter, and HC and CO in the exhaust gas are oxidized by oxygen in this part of the air. If the car is also equipped with a catalytic purifier, this air can also be used to supply the oxygen demand of the catalytic purifier. This is the working principle of pulse or aspirator secondary air injection system. The common pulse secondary air injection system consists of steel pipe, one-way aspirator and hose. One end of the steel pipe is connected with the aspirator, and the other end of the steel pipe is communicated with the exhaust manifold of the engine through the connecting disk, so that fresh air passing through the air filter, the hose and the aspirator is introduced into the exhaust manifold. The aspirator is actually a one-way valve, which allows the air from the air cleaner to flow to the exhaust manifold through the steel pipe to prevent the exhaust steel pipe in the exhaust manifold from flowing back to the air cleaner. When the engine equipped with pulse secondary injection system is running at idle speed or low speed, the negative pressure pulse in the exhaust manifold makes the intake valve open. That is to say, in this working condition, every time the exhaust valve is closed, there will be a negative pressure pulse in the exhaust manifold, the one-way valve of the aspirator will be opened once, and the valve will be opened. Under the action of external atmospheric pressure, fresh air will enter the exhaust manifold through air filters, hoses, aspirators and steel pipes, further oxidizing HC and CO in exhaust gas and reducing exhaust pollution. When the engine is running at high speed, the negative pressure pulse period is particularly short because the exhaust valve is frequently closed. Due to inertia, the one-way valve of the aspirator cannot be opened. Therefore, the one-way valve of the aspirator is actually closed. At this time, it only acts as a stop valve to prevent waste gas from being discharged into the air filter. That is to say, when the engine is running at high speed, the pulsed secondary air injection system actually stops working. Electronic control secondary air injection system The air in the electronic control air pump secondary air injection system is guided to the air cleaner, exhaust pipe and catalytic exhaust purifier by the electronic control unit according to the input signal and by controlling the related electromagnetic valves. The system has two sets of main control solenoid valves. The first set of solenoid valves is a diverter valve, which is used to deliver air to the air cleaner. The second set of solenoid valves is a switching solenoid valve, which is used to deliver air to the exhaust pipe or catalytic exhaust purifier. The system has the following working modes. A. When the engine works in a cold open-loop state, because the catalytic purifier is not hot enough to use excess air, the electronic control unit controls the shunt solenoid valve and the on-off solenoid valve, so that the air is sent to the on-off solenoid valve through the shunt solenoid valve, and the on-off solenoid valve leads the air to the exhaust pipe. B, when the engine works in a normal or closed-loop state, the electronic control unit controls the shunt solenoid valve and the on-off solenoid valve, so that air is sent to the on-off solenoid valve through the shunt solenoid valve, and then the on-off solenoid valve sends air between the oxidant and the reductant in the catalytic exhaust purifier, thereby improving the working efficiency of the oxidant. C. When the catalytic purifier is overheated, the added air will pollute the catalyst in the catalytic exhaust purifier. In this case, the electronic control unit controls the shunt solenoid valve to deliver air to the air cleaner. The electronic pulse secondary air injection system is controlled by the electronic control unit to open and close the solenoid valve, which is connected with a one-way valve (also known as a check valve). Because the pressure in the exhaust is a pulse pressure wave alternating positive and negative, when the exhaust pressure is negative, the air from the air cleaner enters the exhaust pipe. When the pressure is positive, the check valve closes and the air cannot return. Secondary air injection system is also often called afterburning system or afterburning system. The reason is that after the combustible mixture is burned for the first time in the cylinder, those parts that are not completely burned are afterburned in the exhaust process due to artificially introducing fresh air, so there is little or no spark in the exhaust gas discharged into the atmosphere through the muffler. In the presence of combustible gas, the presence of Mars in exhaust gas is the main cause of fire. Therefore, the secondary air injection system is also an important technology and facility to prevent fires caused by exhaust gas from internal combustion engines. Not only used in cars, but also widely used in diesel locomotives and special vehicles with high safety requirements, such as liquefied gas transport vehicles, light oil transport vehicles, airport refueling vehicles and so on.