Originally Posted by gathermewool
...... variable valve timing now being common on both intake and exhaust valves. I don't know how it would work, exactly, but there's got to be a way to allow some of the DI fuel to be forced back into the intake just enough to provide at least some minimal level of cleaning. Does this make sense? Is this already a real thing?
Great conversation, btw. I'm enjoying reading the comments!
Real thing, yes it is already being done, as an implementation of the Atkinson Cycle for more MPG, via late intake valve closing. It burps some fuel-air mixture back up into the intake during the compression stroke. Problem is, it's not a high speed flow to really clean the intake valve stems, but at least keeps them sorta wet, which could maybe(?) loosen carbon on the intake valve stems. Lots of engines do Atkinson part time or full time these days.
VW, in all their wisdom, implemented something effectively similar to Atkinson with their "Budack" cycle EA888 2.0L turbo, which is early intake valve closing. Like Atkinson, It still makes a reduced partial charge so that the combustion stroke has less molecules to explode and more expansion. Notice Budack doesn't wash the intake valves though like Atkinson manages to do.
Adding insult to injury, the Euro version of the EA888 uses dual port+DI injectors, ours only DI. Bummer.