Crankshaft torsional vibration excitation torque FFT

Hi All,

I have been stuck with the problem for more than 3 weeks now. Still could not find the answer. Please help me!

For torsional vibration analysis, I have read this paper http://www.cmnt.lv/upload-files/ns_96art11_CMNT1802-63.pdf where it says the excitation torque in time &crank angle domain needs to be FFT and then different order is used to calculate the vibration response. Here I do not know how to do this FFT, what is the input for my vibration calculation. Should I use Y=fft(X) directly? or should I rewrite my torque based on Amplitude and Phase?

Thanks