Originally Posted By: Shannow
Originally Posted By: Silk
Having cleaned both GDI and diesel manifolds - it's the same stuff.
Re the same stuff....
https://energy.gov/sites/prod/files/2014/03/f10/pm009_lance_2012_o.pdf
Basically hydrocarbon molecules that have been partially pyrolised during low temperature combustion and haven't lost all of the hydrogen to become soot...so it stays tarry and sticky.
Could be the stuff boiling off the upper ring lands and bores, or fuel related bottom ends in the quench crevices...note that hot they created it intentionally was to operate at idle, with HIGH EGR, and retarded timing.
Good find! The lacquer-like deposit composition data contained in this report was very interesting...
First they show the deposit to contain poly nuclear aromatics, or 'proto-soot' as they call it. Yes, this is exactly what you would get from poor combustion of diesel (particularly the heaviest part of diesel), but equally it's what you might expect from burning oil. In fact I'd say that there's a degree of boiling range overlap between the heaviest 1% of diesel and the lightest 1% of engine oil.
The deposit was ash-free. This is significant. If, just for the sake of argument, you presume that some of the deposit originated from engine oil (as opposed to diesel fuel), if this stuff was coming from neat engine oil being sucked through worn valve seals or low speed top ring 'throw-off', one might expect to find some ash there; but there is none. On the other hand, if it was light oil strip-out, migrating through the PCV system, this oil would be ash-free (detergents are relatively heavy and stay in the sump) so partial burning of this stuff would yield an ash-free deposit.
I also noted that they found -OH radicals in the deposit. Obviously that's consistent with something being burnt. However one of the things that occurs to me is light-oil strip out not only won't contain detergent (which stays in the sump), but won't contain antioxidant either, so it will be far more prone to oxidative attack once it reaches the combustion chamber.
The target of my original post was gasoline engines fitted with EGR, not diesels, and there's only so much you can stretch a study like this. However I take heart that the analysis done suggest that EGR deposits are consistent with the partial combustion of something relatively heavy. In a gasoline engine, that might be something like oil or gasoline additives!