Q: why does the peer- review process take long time?

Answer: Thank you for your question. It is unclear from your query which journal you are referring to - perhaps this one: Proceedings of the Institution of Mechanical Engineers, Part E: Journal of Process Mechanical Engineering. In any case, different journals have different processes that they follow for peer review. The best way to know the status of your manuscript article would be to email the editor to ask for an update. Ideally, one should allow a month after it has been submitted before following up. Since you said it has been that long, there is not harm in drafting a polite email asking for an update. There could be a few reasons for the long time taken: 1. the research is complex and requires detailed study by the peer reviewers 2. they may have invited an additional peer reviewer to review the paper 3. peer reviewers may have not completed the review on time 4. they may have had difficulty finding a peer reviewer in your area of study. Whatever the reason is, a simple email will help put your mind to rest - and maybe will help remind the editor that they need to push the peer reviewers to complete their reviews soon!