HC Transport to Exhaust

Four quench zones basically exist in the engine cylinder.  These zones are

• cylinder head quench layer wall including spark plug crevice
• cylinder wall quench layer and head gasket crevice
• the quench layer on piston head, and
• the quench zone around the piston above the top land crevice.

Depending upon the gas-dynamic flow during exhaust, the flow from these regions is expected to be different. At the end of combustion, hydrocarbons are present in high concentrations along the combustion chamber walls trapped in the deposits, oil films and crevices. During expansion stroke, hydrocarbons flow out of crevices. Some of the hydrocarbons flowing out of crevices and desorbed from oil film and deposits diffuse into the bulk gas, but most of these remain close to the walls. When the exhaust valve opens, gases blow out of the cylinder at a high velocity. During the exhaust blow down process, in the beginning the exhaust gas takes along HC  from the quench zones (1) and (2), spark plug and head gasket crevices which  are closer to the exhaust valve and part of the hydrocarbons released from crevices, oil films and deposits. Thus, the initial exhaust blow down gas is rich in unburned hydrocarbons and about half of the total  unburned HC are emitted.
During expansion, hydrocarbon -rich boundary layer is present along the cylinder walls as the fuel vapours expand out of piston-cylinder crevice and desorbed from the oil film. Upward motion of the piston   during exhaust stroke   scraps   this hydrocarbon –rich    boundary layer   and gases from quench zone (3) rolling these into a vortex along the periphery of piston crown.. The piston motion pushes it towards cylinder head and this vortex may be detached from the piston crown and is partly exhausted. This vortex mechanism leads to high HC concentration in the exhaust gases towards the end of the exhaust stroke.
Typical variation in exhaust HC concentration and mass flow rate exiting the cylinder is shown in Fig. 2.15.  Hydrocarbon concentration peaks during blow down and towards the end of the exhaust stroke. The peak in HC concentration at the end of exhaust stroke is very high as it contains HC mostly originating from the piston ring crevice, but the exhaust flow rate is small. Mass HC emission rates peak during the exhaust blow down process as the exhaust flow rates are high. About half of the mass of HC is exhausted during blow down itself and the remaining half later in the exhaust stroke. Another peak in the HC mass flow rate occurs towards the end of exhaust stroke when the HC from the piston-cylinder crevice region are flowing out as HC concentration is very high, although the exhaust flow rates are small.
it has been estimated that under wide open throttle operation, about 2/3rd of the total unburned HC in the cylinder are exhausted, the balance remaining in the residual gases. Under part load conditions, about half of the unburned HC exit the cylinder.