2001 Mercury Mariner 50-60HP Factory Service Manual, Page 193Get this manual
EMISSIONS
Stratified Charge
A stratified charge engine only pulls air through the transfer systemThe fuel required for combustion is forced into the cylinder through an injector placed in the top of the cylinder (head)The injector sprays fuelair mixture in the form of bubble into the cylinderSurrounding this bubble is air supplied by the transfer systemAs the bubble is ignited and burns, the surrounding air provides almost complete combustion before the exhaust port opensA stratified charge is hard to ignite, the fuelair bubble is not evenly mixed at 14.7:1 and not easily ignited