Tham khảo tài liệu 'internal combustion engines fundamentals episode 2 part 4', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | 570 INTERNAL COMBUSTION ENGINE FUNDAMENTALS escapes the primary combustion process because the entrance to these crevices ill too narrow for the flame to enter. This gas which leaves these crevices later ifl the expansion and exhaust processes is one source of unbumed hydrocarbon emissions. Another possible source is the combustion chamber walls. A quench layer containing unburned and partially burned fuel-air mixture is left at the Wall when the flame is extinguished as it approaches the wall. While it has been shown that the unbumed HC in these thin mm layers bum up rapidly when the combustion chamber walls are clean it has also been shown that the porous deposits on the walls of engines in actual operation do increase engine HC emis-H sions. A third source of unburned hydrocarbons is believed to be any engine oil-3 left in a thin film on the cylinder wall piston and perhaps on the cylinder head. J These oil layers can absorb and desorb fuel hydrocarbon components before and after combustion respectively thus permitting a fraction of the fuel to escape the primary combustion process unburned. A final source of HC in engines is incom- plete combustion due to bulk quenching of the flame in that fraction of the engine cycles where combustion is especially slow see Sec. . Such conditions are most likely to occur during transient engine operation when the air fuel ratio spark timing and the fraction of the exhaust recycled for emission control may not be properly matched. The unbumed hydrocarbons exit the cylinder by being entrained in the bulk-gas flow during blowdown and at the end of the exhaust stroke as the piston pushes gas scraped off the wall out of the exhaust valve. Substantial oxidation of the hydrocarbons which escape the primary combustion process by any of the above processes can occur during expansion and exhaust. The amount of oxidation depends on the temperature and oxygen concentration time histories of these HC as they mix with the bulk .
