On aeroplanes it's quite common to use degreaser or sometimes even just a solvent like Shellite or stuff called B1 (which looks/feels just like shellite) - or sometimes even just Kero - on the engines to clean them. In the case of the solvents or Kero, you're rinsing all the crap off with the spray gun and with degreaser then of course you're applying it then rinsing off with water. Best practice with direct solvent cleaning is still to rinse off with water.
Anyway, what I'm comming to is the story of some guys that cleaned off an engine on a Metroliner (small turboprop airliner - about 20 seats I think) with solvent then closed up the engine cowls and went to run it - when they hit the start switch there was a loud "BANG" and cowling doors hanging off at odd angles. du-OH!!
There is of course also a lesson in that for anyone degreasing or otherwise doing work on our cars which results in residual flamable material being left behind - even on a cold engine - and then the machinery being operated.