How (often) are jet engines cleaned?
It used to be done launching crushed walnut shells into the running engine and the blades are polished by the process.  Known as Coke Blasting.  The process is uncommon these days.  

Similarly they use a walnut shelll coated emery cloth to polish crankshafts after machining 

http://www.dennisdawson.com/jetblast.htm

It is still commonly done on industrial turbines apparently