Dear sara58sara, I agree with above answers but if you are interested in a specific case, please provide more details and perhaps I could help you (i.e. project characteristics, type of engineering to be done -basic or detailed?, country).
By the way, you may find interesting the following thread:

[link Point to another website Only the registered members can access] There are also some books to estimate man hours of certain disciplines like "ESTIMATOR'S PIPING MAN-HOUR MANUAL" or "ESTIMATOR'S ELECTRICAL MAN-HOUR MANUAL".
At the end of the day, the better approach to estimate the engineering work depends on the amount of information you have regarding the job. If you share some information, perhaps I could give you some specific advices.
Regards and good luck.