Turbocharger basics

About the basics of turbocharger (supercharger)

The intake / exhaust device of an automobile includes an air cleaner, an inlet manifold, an exhaust manifold, an exhaust pipe, a muffler, and the like. Here, a description will be given of a supercharger that is effective in improving the charging efficiency.

Structure and function of turbocharger

A turbocharger is a device that supplies a large amount of air into a cylinder at a pressure higher than the atmospheric pressure, and can significantly increase the charging efficiency compared to an engine without a turbocharger even with the same displacement. , The output of the engine can be increased.

As a supercharger used for a gasoline engine, there are a turbocharger that uses exhaust gas of the engine as power and a supercharger that mechanically uses the rotational force of a crankshaft.

Although the turbocharger is small and lightweight and has a high degree of freedom in mounting position, when the engine tries to start up from a low-speed rotation range (when the exhaust energy is weak), a delay (time lag) is likely to occur in the rise of the turbocharger itself.

Turbocharger operation

The turbocharger is composed of a turbine housing, a turbine wheel, a compressor housing, a compressor wheel, a center housing and the like as shown in the above figure. The principle of operation is as follows:1~3

  1. The exhaust gas discharged from the cylinder acts on the turbine wheel in the turbine housing as shown in the above figure to rotate the turbine wheel.
  2. When the turbine wheel rotates, the coaxial compressor wheel rotates, and the intake air that has passed through the air cleaner is compressed, discharged from the compressor housing, and supplied as compressed air into the cylinder.
  3. If the engine speed increases, the amount of exhaust gas increases, the turbine wheel speed increases, the boost pressure increases, and the output increases.However, if the boost pressure becomes higher than necessary, the adverse effects of abnormal combustion such as knocking will occur. As a result, the output of the engine is impaired. For this reason, it is necessary to control the supercharging pressure. As a countermeasure, turbochargers are generally equipped with a “waist gate valve”.
Wastegate valve

  • When the boost pressure is within the specified value

The diaphragm of the actuator in the above figure does not operate. Since the waste gate valve is closed by the spring force of the diaphragm spring, all exhaust gas from the cylinder acts on the turbine wheel.

  • When the boost pressure is higher than the specified value

In the above figure, the diaphragm of the actuator is pushed by the supercharging pressure, the wastegate valve opens, and a part of the exhaust gas flows bypassing the turbine wheel, so that the rotation speed of the turbine wheel and the compressor wheel decreases. As a result, the control is performed so that the supercharging pressure does not exceed the specified value when the engine speed is higher than the specified engine speed.

In addition to the control of the wastegate valve, there is a method in which a solenoid valve is turned ON / OFF by a signal from an engine control unit to operate an actuator to open and close the valve.

If the wastegate valve fails and the valve cannot be opened or closed, the engine control unit may control the engine speed so as not to increase the boost pressure to a specified value or more. At this time, the engine may hunt, resulting in a state close to fail safe. Turbo does not work. In the case of hunting, it may be a good idea to suspect that the wastegate valve has failed.

Full floating bearing

For lubrication of the turbocharger, the engine oil is diverted, supplied from the upper part of the center housing via an oil pan, lubricated and cooled to a full floating bearing and the like, and returned to the oil pan from the lower part of the housing.

Turbocharger compressor wheels and turbine wheels rotate at a maximum of about 100,000 revolutions per minute, so they use full floating bearings on the shaft. Since this bearing is completely lifted by oil between the housing and the shaft as shown in the above figure, lubrication and cooling are performed while absorbing vibration at high speed rotation caused by slight imbalance of the shaft. Further, the full floating bearing has excellent durability because the peripheral speed of the bearing is about half of the peripheral speed of the shaft.

A cooling water passage is provided in the center housing to circulate a part of engine cooling water to cool the housing and also cool oil. In addition, when supercharging with a turbocharger or the like, the intake air temperature rises greatly and the compression pressure increases, and knocking is likely to occur.

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