Effect of Altitude on Turbomachinery Vibration in an Aircraft Compression-Ignition Engine

Turbomachines, whether gas turbines or turbochargers, are frequently used in aircraft propulsion systems to compress air for combusting fuel and performing mechanical work. In aircraft compression-ignition engines, turbochargers are used to achieve higher power and efficiency with smaller, lighter configurations. However, these turbochargers are often derived from automotive applications and are not designed for operation at altitude. As a result, the rotor blades are susceptible to fluid–structure interactions such as resonant vibration and flutter leading to fatigue failure. To investigate the effect of altitude on vibrations, an aircraft diesel engine was operated in both a sea level cell and an altitude chamber up to 25,000 ft. The turbocharger was instrumented with a nonintrusive stress measurement system to analyze the frequencies, magnitudes, and critical speeds of the blade bending modes as the ambient pressure, ambient temperature, and engine power varied. The measurements were also compared to data from accelerometers mounted on the compressor housing. At sea level conditions, the largest deflection amplitudes were associated with excitations of the first blade bending mode. These deflections grew in amplitude as the altitude increased and the turbocharger/engine worked harder to produce the required pressure rise and power. There was also evidence of a higher-order mode being excited at elevated altitudes. By understanding the factors contributing to resonance and flutter in aircraft turbomachinery, modeling and prediction tools can be improved to update operating envelopes for current designs and minimize these phenomena in future, aviation-specific designs.