To use CodeIgniter via the commmand line, you need to call the index.php
file and pass in the controller and method as arguments, plus any other arguments. So at the minimum the cron job call would be:
~/public_html/sitefolder/index.php controller method
Or using the path to your application index file. But, you also need to use PHP compiled for the command line, not just PHP for CGI-FCGI. So your call might be something like:
/ramdisk/bin/php5-cli ~/public_html/sitefolder/index.php controller method
Depending on where your PHP CLI is located.