Читать книгу Turbo: Real World High-Performance Turbocharger Systems - Jay K Miller - Страница 24

There are two primary components of the compressor: the compressor wheel and the compressor cover. Within these components there are many critical design types and specific features such as the diffuser, a critical feature that is typically designed into the compressor cover. Understanding turbocharger nomenclature and design features will help in future discussions about turbochargers relative to matching, system design, rebuilding, and failure analysis.

To begin a basic understanding of compressors, it’s important to review (or introduce!) some of the thermodynamic principals involved. While the field of thermodynamics is broad, perhaps one of the most important areas relative to turbochargers comes from within the first law of thermodynamics, the ideal gas law. Simplified, the ideal gas law states that the relationship between volume (V), pressure (P), and temperature (T) can be expressed as:

PV / T = Constant Where P = pressure of the gas V = the volume it occupies T = the temperature of the gas

In simpler terms, if the volume is a constant, an increase in temperature results in a proportional increase in pressure. If pressure is constant, an increase in temperature results in a proportional increase in volume. Inversely, if volume is decreased, and pressure remains constant, temperature must decrease. Pressure and volume are directly proportional to temperature, and inversely proportional to each other.

The inner relationships of these properties in gases are ever present and put to use in everyday life ranging from refrigeration to how a diesel engine operates as a compression ignition engine. When applying the ideal gas law to turbocharger compressors, we can more easily understand how and why boost pressure becomes so hot and why dealing with this heat is important for proper tuning and maximum output.

Turbocharger compressors have design limits relative to how well they do their job of efficiently compressing the intake air. Each compressor has its optimum flow efficiency, maximum flow capacity (choke), and a pressure point where, below that, it will not flow at a given amount of mass or it will stall (surge). When a compressor is operating at its maximum efficiency within its flow range, that efficiency is expressed as a percentage of how close it comes to compressing the gas to meet the mathematical requirements of the ideal gas law. If a compressor was 100 percent efficient, then the compressor’s discharge temperature could virtually be calculated by knowing only the inlet temperature and discharge pressure. Such a compressor would be called adiabatic.

The term adiabatic literally means: occurring without gain or loss of heat. Therefore when a compressor is referenced as having a specific level of efficiency of say 76 percent; that essentially means it has the capability to compress air with a 76 percent adiabatic level of efficiency. The adiabatic efficiency of a compressor will never reach 100 percent however, simply because there are factors that add unwanted, but unavoidable heat. The acceleration of the air causes internal friction among the air molecules, the running contour clearances cause slip and imparts additional internal air friction, the air passing rapidly across the parts of the compressor wheel and cover cause heat from friction, and so on.

Different compressor designs carry different features that are designed around the efficient handling of the air as it’s compressed to allow that compressor to impart as little heat as possible and therefore raise its adiabatic efficiency. At the same time, the compressor has to be designed to efficiently have enough mass flow range to meet the range of airflow its engine requires. As a general rule, compressor mass flow is mapped to show flow ranges above 65 percent. Efficiency levels below that tend to impart too much extra heat into the air causing a variety of problems. If you’re turbo is running below this level, it’s time for a change!