What are some good ideas for starting a mechanical engineering based company after college practically without any investment in personnel, machinery or material?

You can research and investigate opportunities in clean technologies mainly, as a consultant to help other enterprises reduce their carbon footprint. One of the popular ways to do this is to do carbon audit (or, energy audit) and finding ways to reduce carbon emissions within an organisation. Of course, you would need to grow your expertise in certain niches like HVAC systems, materials, solar energy etc. to be able to be successful as a consultant and distinguish yourself from the competition.

The previous answers are good, but more broadly you can start a engineering consulting firm. Most software such as CAD software can be obtained for free (check out FreeCAD), or for an as needed basis. Then it is a matter of knowing individuals who can pass work on. The simplest is to just offer CAD skills, with simple detailing or reverse engineering items where the drawing data has gone missing. You could also act as a contractor, farming yourself out, if needed.

That is a very interesting question, but the answer depends on how good you are in applying your new education, building on it and learning more and your comfort level with failure and rejection. The best way is to become a consultant and service provider to small and medium enterprises, or larger ones. You will need to find out the biggest problems faced by these firms in your area and come up with solutions.  This can include writing software, troubleshooting technical problems, coming with new strategies and techniques to make them more efficient and increase profits to take them to the next level, evaluating suppliers and evaluating new technologies and spearhead their adoption. Typically, new graduates have brand new skills and the latest knowledge, which is valuable to many people in industry whose staff is lacking in that knowledge. But you have not got much exposure to the real world, where problems are multidimensional and things can get complicated very quickly. You are going to face a steep learning curve and failures along the way as you find your way.  Hopefully this will lead you to new technologies and business models that you can use to expand the business.

Mechanical engg related business always require some capital. One thing u can do is, to develop an automated device (since u have automation in your academics) that can replace laborious task, if not up to industrial level but to solve real world problems. This will always attract venture capitalists to invest in and bring your product into reality. Try to create some products which require integrated domain knowledge (Mechanical and electronics) totally unique of its kind. You never know ....one good product can create many contacts and bring in revenue, which is sufficient enough to start related business. All the best

Well mechanical engineering startups are tough without proper Investment so basically what you can do is: Designing: you can design solutions for various companies. You can design components Like gears, machines, machine parts etc for companies. You can do analysis for them ans predict the life and working of a component. Production: you can also do something related to productivity levels. There are many techniques that have been developed(especially by Japanese people) to increase the productivity and use the plant to its maximum potential. Programming: If you are good at programming you can try your hand on making and development of new softwares. To make any industrial software, knowledge of mechanical engineering is must.  Automation : Automation of machines etc is also very common in Industries today. You can give Automation solitons. A little electronics is involved. Explore your options. Analyse the market. I'm sure you'll do well :)

Bro, we all know what is taught in college , and how much you have actually understood. So firstly in an ideal scenario like yours with no investment, I would  advice you to join a local mechanic ( garage ) car or bike in your area and start learning the actual engineering, practically . So with time develop your link and soon with the capital and toll and man power in garage start making or rightly modifing bikes and car. Modifying or say customising car and bikes has a huge potential , you can soon establish your self if u are creative enough. But my most sincere advice would be do a business in which your interest lies, it might be completely different than what your academic are.

if you are good in designing or have underwent courses in designing field you can start a design solution company, you can either be a consultant for small scale industries or start classes providing courses in cad/cam.