Aman,

If I had to do this job, i would have fixed a permananet magnet on a pully(main or generator), and place a hall effect sensor/coil near it. This way, one can get RPM of the pully, giving you engine RPM.

For an in-depth knowledge on automotives, have a look at
"Automotive Mechanics" by W.H. Crowse

Good Luck

IA