There clearly are harmonic distortions in internal combustion engines - whether diesel or gasoline driven. But the amount of distortions depends on many parameters (eg. number of cylinders, total rotating masses, power request from auxiliary devices (eg. injection pump) and so on).

Thus a generally valid answer about the amount of distortion is impossible. You could start with measuring the 'evenness' of the rotary motion with a high-resolution angular measurement device.

Wish you success

Thank You dear U.Dreher for  your valuable information.

 You could have a concern with diagnosis tasks inherent to generators driven by internal combustion engines. Torque in internal combustion engines has a periodic oscillation due to variations in the combustion process between cylinders. A defective combustion in one or more cylinders produces an imbalance in torque oscillation. So, the presence of harmonics (with pulsed nature) is the result of the imbalance in torque oscillation. On the other hand, analyzing the harmonics of the generator helps to detect and identify the cylinder that is producing the imbalance.

Best regards

Thank you dear O. Boudebbouz  for your valuable answer and the time and effort that you have put on my question .  I have checked  the effect of  misfiring and natural torque oscillation in Internal combustion engine but THD are in range of [0-0.2]%. i am searching about another serious problems that cause higher levels of harmonic voltage.  

Higher harmonic contents? Assuming you're talking about a 3-phase generator, start thinking about

Did you consider these influences ? What THD range are you measuring?

Has the non linearity of the copper / iron content of the generator itself been considered?

Iron has a non linear current vs flux value. If the current drawn is near the saturation knee or the low magnetisation knee would this cause the harmonics you are referring to?  Are the harmonics of equal % throughout the load range?  Are they worse at near max current output?  Perhaps iron is approaching saturation and this will certainly cause harmonics.