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How Turbochargers WOrk
Putting the "Turbo" in Turbodiesel
A turbocharger is a component that transforms waste energy from an engine's exhaust into mechanical energy. A standard turbocharger consists of a turbine wheel and a compressor wheel connected via a comon shaft. The movement of the compressor wheel and turbine wheel are dependent of one another - they rotate at the same speed. The turbine is located in the turbine housing, and the compressor in the compressor housing. All modern diesel engines use turbochargers to increase engine efficiency, which translates into more performance, throttle response, and fuel economy for owners.
How Turbochargers Work
The engine's exhaust is routed through the turbine housing, causing the turbine wheel to spin relative to engine load (more throttle/load causes the wheel to spin faster). The turbine wheel and compressor wheel spin in unison. As the compressor wheel spins, it pulls in air through the engine's air filter and compresses it (creating what is typically referred to as "boost"). The compressed air, is sent through the intercooler (on most trucks) and then into the engine, where the pressurized air fills the cylinders on the intake stroke. On a stock truck, maximum turbo boost is usually in the 20-30 psi range, while only 2-8 psi of boost is being created while cruising (this depends on vehicle speed and engine load). A turbocharger increases intake air density by forcing pressurized air into the cylinders. More fuel can consequently be injected than what would be possible at atmospheric pressures. In engineering terms, a turbocharger increases the engine's volumetric efficiency. Oil is fed into the turbocharger shaft to lubricate the bearings, which is necessary as the compressor/turbine wheels can spin over 100,000 rpm under full load on many modern turbocharger models.
A small turbocharger will spool rapidly (create boost quickly) but lack to produce enough volume of air to the engine as rpm increases. Likewise, a large turbocharger will build boost at a slower pace, but will provide large volumes of air to the engine, even at high engine speeds. Therefore, selecting a turbocharger for an application is a delicate process - the turbo needs to provide plenty of airflow to the engine at all engine speeds, but needs to spool rapidly enough to facilitate low-rpm performance and driveability.
Variable Geometry Turbochargers (VGT)
Variable geometry turbochargers (VGT) were invented so that a single turbocharger could provide the characteristics of both a large and a small turbo in one package. A VGT has a series of "vains" in the turbine housing that open and close to change the volume of the housing. When open, the vains reduce the effective volume of the turbine housing, allowing the turbo to build boost quicker. As exhaust flow and turbo pressure increases, the vains close to increase the effective volume of the turbine housing, reducing the chances of overspooling (spinning to fast) while maintaining pressure. The exact position of the vains depends on engine load.
Wastegates prevent turbochargers from spinning to fast. If a turbo is spun to fast, the compressor and/or turbine wheel can literally fly apart from the heat and centrifugal force. The wastegate is a valve that controls exhaust flow in order to regulate turbo pressure. When a preset pressure is achieved, the wastegate diverts exhaust flow around the turbine wheel and out the exhaust system. In other words, exhaust gases exiting through the wastegate do not contribute to the forces causing the turbine to rotate.
Turbocharging vs Supercharging
Turbochargers and superchargers are both forced induction engine components which increase volumetric efficiency by forcing pressurized air into an engine's cylinders. The important difference between superchargers and turbochargers is that while a turbocharger converts engine waste heat energy (uses engine exhaust to operate), a supercharger is run by the engine crankshaft. A supercharger is connected to the engine crankshaft via a drive belt, and thus spins at speeds relative to engine rpm. Where a turbocharger uses waste energy from the engine's exhaust, a supercharger actually uses engine power to rotate. This is the biggest downfall of supercharging, making a turbocharger the more efficient method of forced induction. Both methods are acceptable for diesels, but superchargers are usually added to diesels without ditching the turbocharger.
Journal Bearings vs Ball Bearings
Journal bearings are the most common bearing used in factory turbochargers. They are simple, cost-effective, and work effectively. Ball bearings are typically used in aftermarket turbochargers. The advantages of using ball bearings include increased longevity/durability, crisper throttle response, reduced turbo lag, and reduced oil flow. Most performance engine builds utilize a ball bearing turbocharger.
If 1 turbocharger is good, 2 must be better. Compound turbocharger systems utilize 2 or more turbochargers (yes, 3 & 4 turbo applications due exist on some performance engines) of different sizes. The concept is to have 1 small turbocharger that will spool rapidly feeding into a larger turbocharger that will supply higher volumes of airflow that become necessary as engine speed increases.
Ford DualBoost Turbo
The 6.7L Power Stroke uses a unique turbo design called a Single Sequential Turbocharger (SST), marketed as the "DualBoost" turbo. It is a single turbocharger with 1 turbine housing/wheel and 2 compressor housings/wheels. As the turbine rotates, it spins 2 compressors of different sizes, giving this single turbocharger the characteristics of both a small and large turbo.