How to write a sponsership letter?

im a student of mechanical engineering and few of my friends have made a society by the name DEVELOPERS which is official university society wihich encourages students to come up with projects relating to mechanical engineering. This society will also give funds to the students to fabricate their projects and in august an exhibition will be held to showcase their projects.As a head of a marketing team its my job to appraoch companies both engineering and consumer/general for sponsership and ask them to fund us to we can fund the students and for that i have to write a letter. I need your help.i have to write a letter in such a way that would convince them to give us fund or if not satisfied atleast give us time to give them presentation.