Читать книгу Turbo: Real World High-Performance Turbocharger Systems - Jay K Miller - Страница 16

As the turbine wheel rotates from the hot high-pressure exhaust gas expanding through it, the wheel is connected to a shaft that has a compressor isolated at the opposite end. The compressor performs a job that is just the opposite of the turbine. The compressor’s job is to gather fresh air and raise its pressure before it enters the engine.

The compressor also has two primary components: the compressor wheel and the compressor cover. The compressor wheel is a radial-type compressor, meaning that as the air enters the leading edge of the wheel called the inducer, it is accelerated and turns 90 degrees and exits the compressor wheel perpendicular to the turbine shaft that drives the compressor wheel. The energy extracted from the turbine is used for work to spin the compressor wheel that pulls air into the wheel and then compresses it.

As the air leaves the compressor wheel it enters the portion of the compressor called the diffuser. The diffuser converts the air into static pressure and fills the compressor cover. The compressed air leaves the compressor cover and is routed by way of a boost tube directly into the engine or to an aftercooler, and then enters the engine.

The compressor section of a turbocharger consists primarily of the compressor wheel and compressor cover. (Courtesy Diesel Injection Service Company, Inc.)

The compressor has two functions. The first is to raise the intake pressure to aid the engine in its ability to breathe. Higher intake pressures allow for more complete filling of the engine cylinders with air. The other function is to increase air density. An engine reacts favorably to air density more than boost pressure, and air density is the primary objective of higher boost pressures. However, boost pressure is a critical element as well and is directly responsible for the increase in volumetric efficiency, a concept explained in greater detail in Chapter 3.

An engine has a fixed amount of volumetric capacity or displacement. Therefore the air pressure and density must be increased so that the engine will flow a higher mass of air, which allows for a higher mass of fuel to be introduced. The boost pressure level for a given compressor is highly dependent upon that compressor’s ability to efficiently compress the air without adding excess heat. The concept of density is simply stated as follows: Density = mass per unit volume. Boost pressure will rise as a function of two variables: air temperature and air density. It is the air density increase that allows more fuel to be burned and therefore, more horsepower to be developed.

If two sealed cans were both fitted with pressure gauges and one was heated, its pressure gauge would rise like boost pressure. However, there would be no higher air density in the heated can, just more heat and pressure. The excessive heat is not desirable, but is an example of what happens when the incorrect compressor is installed onto an engine. If an engine requires more air than the compressor can efficiently compress, the air gets extremely hot and engine problems, such as detonation, are more likely to occur.

Proper sizing of the compressor is just as critical to maximizing engine performance as it is with the turbine. Each compressor design is trimmed to flow the amount of air compatible to an engine’s air requirement based upon that engine’s cubic inch displacement, volumetric efficiency, and operating RPM. A compressor that’s too small restricts the engine and limits horsepower. Likewise, a compressor that’s too large requires more power to spin than the turbine is capable of developing and thus will not spin fast enough to supply the airflow and pressure that compressor would otherwise be capable of developing. Thus there is a power balance that clearly becomes obvious and important between the compressor and turbine ends of the turbocharger.