Introduction to turbochargers
A turbocharger is actually an air compressor that compresses air to increase air intake. It uses the inertial momentum of the exhaust gas discharged from the engine to drive the turbine in the turbine chamber. The turbine in turn drives the coaxial impeller. The impeller pressurizes the air sent from the air filter pipe to pressurize it into the cylinder. When the engine speed increases, the exhaust gas discharge speed and the turbine speed also increase simultaneously. The impeller compresses more air into the cylinder. The increased pressure and density of the air can burn more fuel, which increases the amount of fuel and adjusts the engine speed accordingly. You can increase the engine output power.
Structural principles
First, let's talk about the general structural principle of the turbocharger. The exhaust gas turbocharger is mainly composed of a pump wheel and a turbine, and of course there are other control components. The pump impeller and the turbine are connected by a shaft, which is the rotor. The exhaust gas discharged from the engine drives the pump impeller, which drives the turbine to rotate. After the turbine rotates, it pressurizes the intake system. The supercharger is installed on the exhaust side of the engine, so the working temperature of the supercharger is very high, and the rotor speed of the supercharger is very high when it is working, which can reach hundreds of thousands of revolutions per minute. Such high speed and temperature Common mechanical needle rollers or ball bearings cannot work for the rotor, so turbochargers generally use fully floating bearings, which are lubricated by engine oil, and coolant is used to cool the supercharger. In the past, turbochargers were mostly used on diesel engines. Because the combustion methods of gasoline and diesel are different, the form of turbocharger used in the engine is also different.
A gasoline engine is different from a diesel engine in that it is not air that enters the cylinder, but a mixture of gasoline and air. If the pressure is too high, it will easily explode. Therefore, the installation of a turbocharger must avoid knocking. There are two related issues involved here, one is high temperature control and the other is ignition time control.
After forced supercharging, the temperature and pressure during compression and combustion of the gasoline engine will increase, and the tendency of knocking will increase. In addition, the exhaust temperature of gasoline engines is higher than that of diesel engines, and it is not suitable to increase the valve overlap angle (the time when the intake and exhaust valves are opened at the same time) to enhance the cooling of the exhaust. Lowering the compression ratio will cause insufficient combustion. In addition, the rotation speed of the gasoline engine is higher than that of the diesel engine, and the air flow changes greatly, which can easily cause the turbocharger to respond lag. In response to a series of problems that arise when gasoline engines use turbochargers, engineers have made targeted improvements one by one so that gasoline engines can also use exhaust gas turbochargers.
Intercooler
The temperature increases, which not only affects the inflation efficiency, but also easily causes deflagration. Therefore, it is necessary to install a device that reduces the intake air temperature, which is an intercooler. It is installed between the turbocharger outlet and the intake pipe to cool the air entering the cylinder. The intercooler is like a radiator, cooled by wind or water. The heat of the air escapes to the atmosphere through cooling. According to tests, an intercooler with good performance can not only maintain the engine compression ratio at a certain value without causing knocking, but also reduce the temperature and increase the intake pressure, further increasing the effective power of the engine.
impeller
Since the gasoline engine speed range is wide and the air flow changes greatly, the compression impeller shape of the turbocharger is a complex three-dimensional curved ultra-thin wall impeller blade. There are generally 12 to 30 blades arranged in a radial curve. The blade thickness is between Below 0.5 mm, it is made of aluminum using a special casting method. The quality of the blade shape directly affects the performance of the turbocharged engine. The more reasonable the shape and angle of the impeller, the lighter the mass, the more sensitive the impeller's startup, and the smaller the "reaction lag" that is the inherent defect of the turbocharger.
Deflagration sensor
In addition to lowering the temperature to reduce the possibility of deflagration, a deflagration sensor must be used. Its function is that when deflagration occurs, the sensor will immediately feedback the information to the engine ECU (electronic control unit) control system when it senses abnormal vibration, and will ignite the engine. Delay the timing a little, and then resume normal ignition timing when deflagration does not occur.
other
Since the speed of a car gasoline engine is higher than that of a diesel engine, the air flow velocity is fast and the range of changes is large, so its turbocharger has higher requirements. Modern car engines have generally adopted electronic injection systems. With the cooperation of electronic control technology and new materials, the application of turbochargers on gasoline engines will become increasingly common.
Exhaust gas turbochargers used in cars all use a single-inlet turbine housing, which means that only the pressure energy of the exhaust gas is used without using other auxiliary energy. Since the speed range of the car engine is large, the exhaust gas turbocharger must have an adjustment device so that the engine can obtain a relatively constant boost pressure within a certain speed range. In addition, the gasoline engine uses spark-ignition, and its compression ratio is limited to a certain range. If it is too high, it will cause deflagration. Therefore, a deflagration detection and control mechanism is required to adjust the ignition advance angle at any time.
The exhaust gas turbocharger of a car is generally installed near the exhaust pipe. The turbine and impeller are installed in the turbine chamber and the supercharger respectively. The two are coaxially rigidly connected and rotate synchronously.
When supercharging is not required, such as when idling or when there is a sign of knocking, part of the exhaust gas will escape through the bypass valve and not enter the turbocharger. When the engine speed reaches 2,000 rpm, the solenoid valve closes the bypass valve to direct the exhaust flow to the side of the turbine, causing the turbine to rotate. There is also a design that adjusts the angle of the turbine blades to adjust the turbine speed through changes in resistance, thereby changing the amount of boost.
Cooling the air can shrink the air and increase its density, allowing more air to be crammed into the same volume and preventing deflagration. Therefore, the turbochargers of cars are equipped with an intercooler. This intercooler is generally air-cooled and installed in front of, next to or in a separate position of the engine radiator, using the car's oncoming airflow or its own fan for cooling.
The key part of the turbocharger is the bearing. This type of bearing, which is named according to its lubrication form, is called a "full floating bearing". It has extremely high operating speed and harsh working environment. Therefore, ensuring lubrication is very important. If the oil supply is slow due to low oil pressure, it can damage the bearings and cause the turbocharger to fail. This type of failure will not occur during normal engine startup, but if the engine is started for the first time after replacing the oil and oil filter, slow oil supply will occur, causing the bearings to lack oil lubrication. In this case, it is necessary to idle for about 3 minutes after starting, and the speed cannot be directly increased to the starting speed of the turbocharger. Similarly, do not stop the engine immediately after driving at high speed or going uphill. Keep the engine running at idling speed for about 1 minute so that the turbocharger bearings that continue to idle will not be short of oil. Therefore, drivers who use turbocharger cars must follow the manufacturer's instructions and pay great attention to the quality of the engine oil. It is not advisable to operate turbocharger cars as ordinary cars.
Supercharger classification
For a car to run fast, it needs strong power. At present, the power system of automobiles can be roughly divided into two categories: natural air intake system and supercharged air intake system. Among European sports cars, except for BMW, which still insists on using naturally aspirated engines, other car companies have adopted supercharging systems in order to improve the power performance of their vehicles. For example, Mercedes-Benz sports cars use supercharging systems, and Shenbao Automobile uses supercharging systems. The originator of turbocharging. In recent years, Japanese cars have also begun to use turbocharging technology extensively. The naturally aspirated system does not install any form of supercharger, but only uses the negative pressure generated by the downward movement of the piston to suck in the mixture. Although the naturally aspirated system can obtain greater horsepower output through the variable valve timing system, the power improvement is very limited. In order to effectively increase the output power of the engine, using a supercharging system can be said to be an effective way.
Common engine supercharging systems include mechanical supercharging and exhaust gas turbocharging.
Supercharged
The engine mechanically drives a supercharger to supercharge, which is called supercharging. When an engine is supercharged, the engine crankshaft usually drives the supercharger through a gear. Superchargers generally use centrifugal or Roots compressors, and some use screw compressors. In recent years, new mechanical scroll superchargers have also begun to be used abroad. Because driving the compressor consumes a certain amount of engine output, the thermal efficiency of a supercharged engine is not necessarily improved, and sometimes is even lower than that of a non-supercharged internal combustion engine. When selecting the boost pressure, first of all, it is necessary to ensure that the required average effective pressure can be achieved, and secondly, to obtain the lowest possible fuel consumption rate. These two requirements are often contradictory for supercharging. If the average effective pressure is pursued, it will inevitably lead to a reduction in mechanical efficiency and an increase in fuel consumption. Therefore, the selection of the boost pressure value should seek a good compromise between power and fuel consumption. Supercharger systems are currently commonly used in European cars. Because the supercharger's supercharger is continuously running driven by the crankshaft, it does not cause turbo lag like a turbocharger. Although supercharging can only increase the power output by about 10% to 20%, the smoothness and continuity are beyond the reach of turbocharged engines.
exhaust gas turbocharging
The use of engine exhaust energy to drive a turbocharger is called exhaust gas turbocharging (referred to as turbocharging). As shown in the figure, the exhaust gas turbocharging system is shown. The characteristic of exhaust gas turbocharging is that there is no mechanical connection between the turbocharger and the engine. They are connected by air path. Because the work consumed by the compressor is part of the energy recovered by the turbine from the exhaust gas, the turbocharged engine can not only increase the power of the engine, but also improve its thermal efficiency and reduce fuel consumption. If you see the Turbo or T logo on the rear of a car, it means that the engine used in the car is turbocharged. A turbocharger is actually an air compressor. It uses the inertia of the exhaust gas discharged from the engine to drive a turbine. The turbine in turn drives a coaxial impeller to compress the air sent from the air filter pipe, so that the air is pressurized and enters the cylinder. When the engine speed increases, the exhaust gas discharge speed and the turbine speed also increase simultaneously. The impeller compresses more air into the cylinder. The increased pressure and density of the air can burn more fuel. Accordingly, increase the oil volume and adjust the engine speed. It can increase the engine output power.
