Low Intrinsic Torsional Vibration
Path to 300 hp Class Aviation Diesel Engines
Torsional vibration is a major issue for aviation diesel engines. In spite of several decades of intense effort there are no FAA certified 4-S geared engines greater than 4-cylinders and 180 hp and no direct drive engines greater than 4-cylinders and 228 hp. Issues with resonant torsional vibration for the 4-cylinder geared engines have been addressed with the use of dual mass flywheels but with trade-offs of reliability and maintenance issues. Non resonant torsional vibration for the large displacement 4 cylinder direct drive engines have still not been adequately addressed. Simulation studies (see Events & Publications page for links to full publications and also the graphs in the Model Powertrain Torsional Stress Section below) show that vibratory torque doubles going from a 4 to a 6-cylinder and up to nearly 1.5 times going from a 4 to an 8-cylinder engine with standard gear reduction. Thus torsional vibration is likely to be even a greater issue with ongoing efforts to develop 300 hp class 6 and 8 cylinder diesel engines with standard gear reduction. A geared 6-cylinder 300 hp aviation diesel engine has been certified by EASA. It is uncertain whether the viscous damper and dual mass flywheel employed on this engine will be successful in ameliorating the very high torsional vibration in this powertrain structure.
Simulation studies show that power transmission through the camshaft or a dedicated internal driveshaft reduces the first mode natural frequency of the powertrain such that its convergence with dominant excitation frequencies occur at lower rpm and significantly reduces power train torsional stresses in 4-S, 6-cylinder engines. Comparisons of the graphs below illustrate that going from a 6-cylinder engine with standard gear reduction to a 6-cylinder engine with power transmission through a cam drive shaft or an analogous dedicated internal drive shaft goes from a configuration with the highest torsional stress to one with the lowest torsional stress. There was no change in the powertrain stresses for 8, 10 and 12 cylinder engines but there was greater than 6 fold worsening of the 4-cylinder powertrain stress. Engines with 2-S cycles do not benefit from this powertrain configuration (data not shown below but can be found in the SAE publication).
The combination of a Cam Drive Shaft or analogous Dedicated Internal Drive Shaft with a 4-Stroke, 6-cylinder reciprocating engine achieves the right combination of first mode natural frequency and excitatory harmonics that result in the lowest torsional stresses of any combination spanning 4 to 12 cylinders with either a standard gear reduction or power transmission through a camshaft or dedicated internal driveshaft.
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