The piston inertia force is independent of the type of engine, two stroke, four stroke or steam....

Omega is the angular velocity in radians per second and Omega (t) or your (Wt) is an instantaneous crank shaft position from T.D.C. To figure the force at T.D.C. substitute zero radians or degrees for (Wt) and for other positions insert your crank angle. The formula is often written with the Greek letter Theta for the angle.

The formula you show is derived by first describing the piston position in terms of the crank angle and the successive derivatives for velocity and acceleration are obtained using the binomial theorem. Your parenthesis show the first two cosine terms of the rapidly converging series.