Diesel engine, like other internal combustion engines, converts 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 vapor (H2O). Indeed, diesel exhaust gases are primarily composed of CO2, H2O and the unused portion of engine charge air.
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 include also 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 sulfur compounds and fuel additives. Common pollutants include unburned hydrocarbons (HC), carbon monoxide (CO), nitrogen oxides (NOx) or particulate matter (PM). Total concentration of pollutants in diesel exhaust gases typically amounts to some tenths of one percent. Much lower, “near-zero” levels of pollutants are emitted from modern diesel engines equipped with emission aftertreatment devices such as NOx reduction catalysts and particulate filters.
Carbon and hydrogen construct the origin of diesel fuel like most fossil fuels. For ideal thermodynamic equilibrium, the complete combustion of diesel fuel would only generate CO2 and H2O in combustion chambers of engine (Prasad and Bella 2010). However, many reasons (the air–fuel ratio, ignition timing, turbulence in the combustion chamber, combustion form, air–fuel concentration, combustion temperature, etc.) make this out of question, and a number of harmful products are generated during combustion. The most significant harmful products are CO, HC, NOx, and PM.
Figure 1 shows the approximate composition of diesel exhaust gas (Khair and Majewski 2006). Pollutant emissions have a rate of less than 1 % in the diesel exhaust gas. NOx has the highest proportion of diesel pollutant emissions with a rate of more than 50 %. After NOx emissions, PM has the second highest proportion in pollutant emissions. Because diesel engines are lean combustion engines, and the concentration of CO and HC is minimal. Besides, pollutant emissions include a modicum of SO2 depending the specifications and quality of fuel. It is produced by the sulfates contained in diesel fuel. For the present, there is not any aftertreatment system like a catalytic converter to eliminate SO2. Nowadays, most of oil distributors and customers prefer ultra low sulfur diesel (ULSD) for diesel engines to prevent harmful effect of SO2.
There are other sources that can contribute to pollutant emissions from internal combustion engines—usually in small concentrations, but in some cases containing material of high toxicity. These additional emissions can include metals and other compounds from engine wear or compounds emitted from emission control catalysts (via catalyst attrition or volatilization of solid compounds at high exhaust temperatures). Formation of new species—normally not present in engine exhaust—can also be facilitated by catalysts. This seems to be especially the case when catalysts are introduced into the combustion chamber. For example, some fuel additives—so-called “fuel-borne catalysts”—used to support the regeneration of diesel particulate filters have been linked to emissions of highly toxic dioxins and furans [Heeb 2011]. A possibility of new emissions must be considered whenever additives (catalytic or not) are introduced into the fuel or lube oil and when fluids are introduced into the exhaust gas. A well known example is urea used as a NOx reductant in SCR catalyst systems—emissions from SCR engines can include ammonia, as well as a number of products from incomplete decomposition of urea. Low quality fuels can be still another source of emissions—for instance, residual fuels used in large marine engines contain heavy metals and other compounds known for their adverse health and environmental effects.