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 aftertreatment devices such as NOₓ and CO reduction catalysts and particulate filters.
Emissions can be categorized into a particulate- and gas/vapour phase, where each contains both organic and inorganic components. The particulate phase of diesel exhaust includes clusters of respirable particles composed mainly of carbon and is termed “diesel particulate matter” (DPM). A variety of chemicals are contained within or absorbed onto the diesel particulate matter, which has the potential to affect the toxicity of the particulate.
The major constituents of the gas/vapour phase include carbon monoxide (CO), oxides of nitrogen (NOₓ), Sulphur dioxide (SO₂), Aldehydes and many polycyclic aromatic hydrocarbons (PAHs).
Exhaust emissions does not only pollute the air we breathe, but also changes the characteristics of landscapes through soil contamination and water pollution. The effect of non-compliance with health standards and recommendations could also impact the health of humans far outside the demography of such pollutant producing mechanisms.