Dear Santosh,
There are various departments in an organisation...Production, Sales and Marketing, Finance, Technology, HR, Operations et all. The structure of the departments in an organisation depend on the business it is in.
As you said you are a Mechanical Engineer. And so, you would be more in line with the Technology department which would be looking at R&D, Engineering & IT.
In a manufacturing firm, Engineering & IT are maintenance and support necessary for production, planning while New product R&D activities are executed in the form of a project. In a service organisation, each development or maintenance activity is usually undertaken as a project.
A project is defined as a temporary endeavour to create a unique product, service or a desired result under the constraints of scope, time, cost. The resources to perform the activities are provided by various other departments.
The expertise of each department is different and only a business adminstration course or a long stint of employment working across departments provides you right kind of exposure for a broader view. Working in a project provides you with the best exposure with cross functional interactions and oppurtunities to learn. Hence, i believe it is a best fit for you as a Project engineer.
An employee's growth prospects in terms of postion and salary are dependant on not only organisation, its structure but also what are his aspirations and how well he executes his growth plans to clinch the best oppurturtunities with the people he would be working with.
So good luck.
Hope this helps.