For caked on grease like the picture you posted, I wire brush off the outer crust. The inner crud also gets roughened up to help hold the degreaser to do its work. I use Gunk heavy duty engine degreaser (spray), let it sit for a while, then spray it again and let it sit again. After it loosens up the crud I spray it off with my pressure washer. The pressure washer is a 1700PSI electric cheapo I bought at some big box store years ago. From my experience the Gunk engine degreaser works faster than the purple stuff or Simple Green. It also works much better than the cheap stuff from Wal Mart. The process is messy and stinky and may not be environmentally correct but, it works for me pretty well.
Mark.