The objective of this study was to develop knock criteria for aviation diesel engines that have experienced a number of malfunctions during flight and ground operation. Aviation diesel engines have been vulnerable to knock because they use cylinder wall coating on the aluminum engine block, instead of using steel liners. This has been a trade-off between reliability and lightweighting. An in-line four-cylinder four-stroke direct-injection high-speed turbocharged aviation diesel engine was tested to characterize its combustion at various ground and flight conditions for several specially formulated Jet A fuels. The main fuel property chosen for this study was cetane number, as it significantly impacts the combustion of the aviation diesel engines. The other fuel properties were maintained within the MIL-DTL-83133 specification. The results showed that lower cetane number fuels showed more knock tendency than higher cetane number fuels for the tested aviation diesel engine. In this study, maximum pressure rise rate, or Rmax, was used as a parameter to define knock criteria for aviation diesel engines. Rmax values larger than 1500 kPa/cad require correction to avoid potential mechanical and thermal stresses on the cylinder wall coating. The finite element analysis model using the experimental data showed similarly high mechanical and thermal stresses on the cylinder wall coating. The developed diesel knock criteria are recommended as one of the ways to prevent hard knock for engine developers to consider when they design or calibrate aviation diesel engines.

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Compared to turbodiesel technology for ground vehicles, the increasing application of turbochargers in aircraft diesel engines presents a unique set of structural dynamics and aeroelasticity considerations due to their more extreme operating conditions. In particular, blade vibration and flutter are two related but distinct phenomena that impact the design of these turbochargers and reliable operation over their lifetime. Deformation or fatigue due to blade excitation can reduce efficiency or cause components to fail prematurely. The existing literature on turbomachinery covers many research efforts to analyze these phenomena by investigating the physical mechanisms responsible as well as the relationships between the fluid and solid dynamics. This review paper emphasizes those efforts most relevant to airborne diesel turbochargers, including research focusing on altitude effects on centrifugal compressors. Early work in which the dominant parameters for modeling turbocharger behavior were identified is highlighted as are current efforts to develop higher-fidelity models. An overview of existing and proposed techniques for measuring and controlling blade resonance is also given. Finally, an experimental facility for testing of turbochargers is proposed. The facility will include a nonintrusive stress measurement system and enable measurement of blade deflection/vibration together with blade stress, temperature, pressure, and flow rate across a range of simulated altitudes. The goal will be to characterize the blade bending modes, resonances, and critical speeds for various simulated altitude, pressure, temperature, and flow rate conditions so that designs may be devised that could prevent or avoid the associated failure modes in airborne diesel applications.

To investigate the effect of altitude on vibrations in a turbocharger, an aircraft compression-ignition engine was operated in both a sea level cell and an altitude chamber up to 25,000 ft (7620 m). 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.

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