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Understanding Diesel Emissions (Part 1)

​Diesel engines, like other internal combustion engines, convert chemical energy contained in the fuel into mechanical power. Diesel fuel is a mixture of hydrocarbons which — during an ideal combustion process— would produce only carbon dioxide (CO₂) and water vapour (H2O).

The concentrations depend on the engine load with the content of CO2 and H2O increasing and that of O2 decreasing with increasing engine load. None of these principal diesel emissions (with the exception of CO2 for its greenhouse gas properties) have adverse health or environmental effects.

Diesel emissions on older equipment, however, do include other pollutants that can have adverse health and/or environmental effects. Most of these pollutants originate from various non-ideal processes during combustion, such as incomplete combustion of fuel, reactions between mixture components under high temperature and pressure, combustion of engine lubricating oil and oil additives, as well as combustion of non-hydrocarbon components of diesel fuel, such as sulphur compounds and fuel additives.

Common pollutants include unburned hydrocarbons (HC), carbon monoxide (CO), nitrogen oxides (NOₓ) or particulate matter (PM). The total concentration of pollutants in diesel exhaust gases typically amounts to some tenths of one percent — this is schematically illustrated in Figure 1. Much lower, “near-zero” levels of pollutants are emitted from modern diesel engines equipped with emission after-treatment devices such as NOₓ and CO reduction catalysts and particulate filters.

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