Summary of  HC Emission Processes in SI Engines

In a port fuel injection SI engine, typically:

• About 9 percent of the total fuel supplied escapes normal combustion. Of the this Approximately 3 percent escapes as fuel itself through absorption in oil and deposits routes, and as liquid fuel films deposited in the cylinder.
• About 5 percent as fuel-air mixture due to flame quenching on walls and crevices, and leakage through exhaust valves.
• Around 1% of fuel –air mixture escapes as crankcase blow by.
• Of the fuel contained in crevices and quench layers about 2/3rd is oxidized inside the cylinder.
• Of the fuel in oil layers, deposits and liquid fuel film only about 1/3rd is oxidized inside the cylinder.
• About 2.7 % of fuel exits the exhaust port and nearly 1 % is retained in the residual burned gases.
• Of the hydrocarbons exiting the exhaust port about 1/3rd is further oxidized in the exhaust system.
• Under warmed up operation, engine out HC emissions from a PFI are approximately 1.8 percent of the fuel supplied. The balance 7.2 percent fuel that does not leave the cylinder is either oxidized in the cylinder or at exhaust port and manifold or recycled in the residual gases and crankcase blow by gases.

Approximate contribution of different sources to HC emissions for a port fuel injected SI engine is shown in Fig. 2.16