Originally Posted By: ecotourist
Originally Posted By: Ducked
Originally Posted By: ecotourist
But I doubt that this is what happens in an engine. If the carbon were actually incandescent as is apparently required, in the presence of oxygen it would simply burn off - ie it wouldn't be there in the first place. There'd be no need for water.
By that logic a coked-up engine is impossible, so the discussion is er..moot?
The water provides excess oxygen. I suppose oxygen must be in limited supply in an operating engine, otherwise, it wouldn't coke-up in the first place.
Let's consider the case of the valves in a DI engine - one of the most relevant coking concerns today. You can (and sometimes do) get coke on the cool side of the intake valves - valves cooled by incoming combustion air - and not washed by solvents in the fuel - because there is no fuel in the incoming air of most DI engines (some Toyota engines being one exception). There are engine oil residues (a source of carbon) courtesy of the PCV system. The intake valves of some DI engines presumably don't get hot enough to burn off the resulting carbon. There is plenty of oxygen so that's not the issue. And if you injected water upstream of these intake valves I highly doubt it would change into CO and H2. Any cleaning action would be by water blasting +/- solvent action. I'm prepared to be shown I'm wrong but I'd want to see experimental evidence.
If that carbon on intake valves could get hot enough to drive C and H2O to CO + H2 it wouldn't be there because (as you say) it would have burned off long ago.
And that's why there is no unusual problem with coking of the exhaust valves of DI engines. As with most engines, exhaust valves do get hot enough to burn off any carbon.
Designing a DI engine that doesn't coke its intake valves is pretty good engineering. And the designer can be helped out by an owner who gets the engine hot frequently and runs it hard from time to time.
Seems reasonable, but also slightly irrelevant to the situation IN an operating engine, as per your quote of me above.
The back of the valves is outside the combustion chamber and so in excess oxygen, but apparently they don't always get hot enough to burn off gunk, since they're cooled by the incoming air.
I dunno if water cleaning works for DI intake valves, never having had one, or tried it, or heard of anyone that has, though someone must have. So its not most relevant to me today.
If it works it would suggest that water also works as a solvent in this situation, since I'd doubt (though I dont know) that it gets hot enough for the water gas reaction.
Given that blowby gases include water but intake valves exposed to them still coke up, perhaps it doesn't work very well?