Turbocharging Into 2014
While turbocharging systems are used in many applications from automotive to marine, they were first developed for aviation –with experimental installations beginning in the 1920’s. The function of an aircraft turbocharging system is to maintain desired manifold pressure at a given throttle setting, regardless of varying conditions or ambient air pressure. This is obtained by a turbocharging system, generally consisting of a turbocharger, a bypass valve and one or more controllers. As a fail-safe feature, many applications also include a pressure relief valve that can avert a total control failure or cold start overboost.
A turbocharger operates by taking engine exhaust gas and diverting it through a turbine. Exhaust gas pressure and heat energy extracted from the gas will cause the turbine wheel to rotate the compressor wheel. Rotation of the compressor wheel takes ambient air, draws it into the compressor housing where it is compressed and directed to the intake manifold ducts. This compressed air allows a corresponding increase in the amount of fuel that can be utilized, maintaining the proper air to fuel ratio required for efficient combustion at altitude.
An exhaust bypass valve, more commonly known as a wastegate, is the component in a turbocharging system that regulates the flow of exhaust gas to the turbocharger by redirecting exhaust gas around the turbocharger when needed. The flow of exhaust gas directly regulates the speed of the turbine wheel and turbocharger. There are two types of aircraft wastegates, one is regulated with a poppet valve and the other more popular version uses a butterfly valve. Both versions are by default open, allowing most of the engines exhaust gas to exit through the exhaust system, bypassing the turbocharger. When needed the wastegate valve will actuate, rerouting some or all of the exhaust gas into the turbocharger hot section where it drives the turbine. The butterfly valve or poppet is regulated by a controller and can be adjusted to any position between fully open and fully closed.
The aircraft controller is designed to maintain constant manifold or deck pressure by adjusting the wastegate, which controls the flow of exhaust gas through the turbocharger. In most systems when the throttle is advanced the controller will actuate the wastegate, causing an increase in manifold pressure. The controller then senses the increase in pressure and repositions the internal poppet towards the open position. The poppet in the open position will then allow oil pressure upstream to decrease and the wastegate adjusts towards the open position. When the wastegate is open less exhaust gas is routed to the turbocharger, causing the deck and manifold pressure to stabilize or decrease to normally aspirated levels.
The pressure relief valve is a poppet type valve that is attached to either the ducting between the compressor discharge and the engine throttle manifold or to a flange on the turbocharger compressor housing. The valve defaults to a closed position until the deck pressure exceeds the predetermined settings. Once the settings are exceeded the deck pressure acts directly on the valve head, opening it and allowing the pressure to vent until within acceptable limits.
Quality Aircraft Accessories is an FAA Part 145 Class 1 and Class 2 Repair Station. Our EASA approval allows us to meet the needs of the general aviation industry worldwide from our facility in Tulsa, Oklahoma. We have been overhauling turbocharging systems and other aircraft components since 1999 and can also provide factory exchange units. If you have a question about installation or troubleshooting, please give us a call. Our customer service representatives are either A&P mechanics or pilots and we make our bench mechanics available for detailed troubleshooting.
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