Sources of Income for US Engineering Faculties As a PhD student it is interesting to know what are the financial opportunities in pursuing a future academic job. From what I understand, engineering faculty members in US have sources of income other than their 9 month salary. Now, can anyone explain what are the various sources of income for such faculties? I am also interested to know about the "cap"s, if any applies.

Plus, I am curious to know whether faculties can pay themselves from non-industrial projects (Such as NSF projects).
In my experience, here are the common sources of additional income for faculty in the US.  Some of this can be done only in the summer, while other things can be done throughout the year.  


Summer salary from research grants.  This is paid by the university to the faculty member as salary at the same monthly rate as during the academic year.  This summer salary must have written into the budget of the grant proposal, and it's not possible to simply take arbitrary amounts of cash out of the grant.  This goes to your question about "paying yourself from non-industrial projects."  
Summer salary from teaching.  A professor may be paid to teach one or more summer school courses.  The pay rate might vary, but a typical policy is that a faculty member receives one month of summer salary for teaching a course during the summer.  At our institution summer school courses meet for one hour a day, 5 days a week, for 8 weeks (about the same number of hours as meeting for one hour, three days per week, during a 14 week semester.)  We get paid one month of summer salary per course and sometimes have the opportunity to teach two courses.    
Consulting income.  Most universities have policies that allow faculty to work for companies outside of the university, typically with a limit on how much time a faculty member can spend on consulting (e.g. no more than 1 day every two weeks, or perhaps 1 day per week during the academic year.)  Consulting rates are negotiated between faculty and their clients and vary widely, but it's not at all uncommon to charge hundreds of dollars per hour.  This can be very lucrative if you can find enough consulting work to keep busy.  
Patents and other "intellectual property."  Most universities have policies that allow the faculty member to receive a share of the profits the university earns from the faculty member's inventions.  This typically also applies to profits from software that is protected by licensing and copyright.  Some patents can generate millions of dollars, but most never make any money.  
Royalties from books.  This is different from (4) in that most universities allow faculty to receive 100% of the royalties from books that they write.  Research monographs and textbooks for upper division and graduate level courses seldom sell more than a few thousand copies, and the royalties per book are typically less than $5, so it is hard to make any significant income in this way.  Popular textbooks for lower division gen ed courses (Intro Economics, Calculus, etc.) can generate a lot more in royalties.  
Starting a company.  A faculty member can use his/her expertise (and perhaps patents from point 4 above) to start a company.  Most startups fail, but a few have become very successful.


In terms of "caps", the most important one is that you can only earn up to three months of summer salary paid from grants and summer teaching.  There's no cap on how much you can earn from consulting, patents, book royalties, or your own company.   There is also typically a limit on how much time you can spend on consulting during the academic year.