Delete one cache file with cache driver

I can delete one cache file by

PHP Code:

$this->output->delete_cache('dfgdfg'); 


But why this two doesn't work?

PHP Code:

$this->load->driver('cache');
 if ( $this->cache->delete('dfgdfg') ) echo 1; 


and

PHP Code:

$this->load->driver('cache');
 if ( $this->cache->file->delete('dfgdfg') ) echo 1; 


first one (cache->delete) returns nothing in echo
Second one (cache-file->delete) returns 1 - success? but nothing gets delete, and refreshing page and success success success... but nothing deletes.

$this->cache->file->clean(); works for me
But $this->cache->file->delete doens't, $this->cache->delete doesn't work too.

Why?