Piston Engines - Performance and Power Augmentation — Page 143, Lesson 177

Let's start with the turbocharger itself. An externally driven supercharger is powered by the energy of the engine exhaust gases, and these are generally known as turbosuperchargers or, more commonly, turbochargers. Here's the key construction. A turbocharger consists of a turbine wheel and an impeller fitted on a common rotor shaft. The bearings are lubricated by oil from the engine. The turbine is connected to the exhaust system, and the compressor is connected to the intake system. So you have one shaft spinning both a turbine and a compressor, with the turbine driven by exhaust and the compressor feeding the intake. Now, the turbocharger is not necessarily an integral part of the engine. It may be mounted on the engine or on the fire-proof bulkhead, and it's shielded from combustible fluid lines in the engine bay. That shielding is a safety measure. Let's trace the gas flow. Exhaust gases pass through nozzles and are guided onto vanes on the turbine wheel, causing it to rotate. The gases then pass between the vanes and are exhausted overboard. So the exhaust spins the turbine, and the turbine spins the impeller on the same shaft. Here's the relationship to remember: the more exhaust gases that are diverted over the turbine, the faster it will go, and therefore the faster the impeller will go, and the greater will be the pressure ratio of the compressor. So more exhaust flow means higher compressor pressure ratio. Now, the wastegate. For any particular power output, the turbocharger must deliver a constant mass of air to the engine in a given time. Since the density of air decreases with altitude, the impeller rotates faster as the aircraft climbs to compensate for the reduction in density and maintain a selected manifold pressure. So as you climb, the air gets thinner, and the turbocharger spins faster to keep the manifold pressure where you set it. Some form of control over compressor output must be provided, and this is done by varying the quantity of exhaust gas passing to the turbine to vary its speed and that of the compressor. A turbine bypass, in the form of an alternative exhaust duct, is fitted with a valve known as a wastegate, which regulates the degree of opening through the bypass. So the wastegate is a valve in a bypass duct that controls how much exhaust gas goes around the turbine instead of through it. That's how you control the turbine speed and therefore the compressor output.