I\'ve tried the following method in the past:
The simplest way to run a cron via CodeIgniter is to make a cron URL available via your app.
Then call it via wget
wget -O - -q -t 1 http://www.example.com/cron/run
Inside the controller you can then use a log to ensure the cron is not run too often i.e. if the Google robots trigger it by mistake.
A second method would be to use lynx
/usr/local/bin/lynx -source http://www.example.com/cron/run