Aviation gasoline is in limited supply in many parts of the world although jet fuel is widely available. The turbine engine solution, however, is not a viable option for most of general aviation because of cost. Therefore considerable effort has been expended to develop suitable jet fuel burning compression ignition (CI) engines for general aviation. Because of weight and power concerns in aviation, much of the effort has been directed toward the development of geared engines. Many currently certified geared CI engines, however, require frequent gearbox inspections and have relatively low times before replacement or overhaul. The high pressures produced by CI engines generate torsional vibration that can be troublesome for geared engines. The first mode natural frequency of a drive train can be lowered by increasing its length thereby reducing stiffness so that its convergence with dominant engine excitation harmonics occurs at lower rpm below the high power operating range of the engine. This can be accomplished without increasing the length of the engine and with 2:1 gear reduction by using the camshaft in the case of a four-stroke pushrod engine as the power transmission shaft or a dedicated geared internal power transmission shaft at the desired ratio in the case of an engine with overhead camshafts. Simulation studies show that resonance at the lower speed is associated with a marked reduction in torsional vibration.

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