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Unlike spark-ignited engines where the combustible mixture is predominantly homogeneous, diesel combustion is heterogeneous in nature. Diesel fuel is injected into a cylinder filled with high temperature compressed air. Emissions formed as a result of burning this heterogenous air/fuel mixture depend on the prevailing conditions not only during combustion, but also during the expansion and especially prior to the exhaust valve opening. Mixture preparation during the ignition delay, fuel ignition quality, residence time at different combustion temperatures, expansion duration, and general engine design features play a very important role in emission formation. In essence, the concentration of the different emission species in the exhaust is the result of their formation, and their reduction in the exhaust system. Incomplete combustion products formed in the early stages of combustion may be oxidized later during the expansion stroke. Mixing of unburned hydrocarbons with oxidizing gases, high combustion chamber temperature, and adequate residence time for the oxidation process permit more complete combustion. In most cases, once nitric oxide (NO) is formed it is not decomposed, but may increase in concentration during the rest of the combustion process if the temperature remains high .
Figure 1 summarizes the sources of unburned hydrocarbons (HC) and NO in direct-injected diesel engines. Species formed in both the premixed and diffusion (mixing controlled) combustion phases are shown .
Figure 1. Pollutant Formation Mechanisms in DI Combustion System