The classic 1943 paper by F.P. Porter on IC engine torque harmonics is best known for its data tables related to the gas pressure torques. However, it also presents Porter’s work on the inclusion of inertial and gravitational torques as excitation functions for torsional vibration. This latter part is relatively obscure because (1) it is presented without any supporting analysis, (2) a number of the quantities involved are not clearly defined, and (3) Porter’s presentation is entirely tied to the US Customary Units system. The present paper explains Porter’s results, with clear definitions of all terms and an appropriate system sketch, and provides a physical motivation for each of his terms in a framework suitable for use with any consistent system of units.

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