Crankshaft median plane
4-Cylinder Engine and Multi-Cyclinder Engines Torque Fluctuation

The median plane of a crankshaft is a plane perpendicular to the axis of rotation of the crankshaft and passing through the center of the crankshaft. Figure 1 shows that the median plane is the (O, y, z) plane. In addition, on this figure we have represented, for each cylinder, the rotary inertia forces F _i/rot and the reciprocating inertia forces F _i/alt exerted on each crank pin (actions of the connecting rods on the crankshaft).