Expire Session When Not Active

I would like to ask on how to automatically expired user session when it's not active?
I find out that CI never expire it's session  and when I tried to  make $config['sess_time_to_update']  lower than $config['sess_expiration']
it expired even if the user is actively using the web application.

Thanks in advance.

Or maybe something like this in MY_Controller if yu like to kick out people who are inactive (=makes no requests):
http://stackoverflow.com/questions/91245...or-15-mins
Depends what you mean with "not active".