Diesel engine emissions also include 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, carbon monoxide, nitrogen oxides and particulate matter. 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 after treatment devices such as NOx reduction catalysts and particulate filters.
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. 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. A possibility of new emissions must be considered whenever additives 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. Diesel engine 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.
All diesel engines for highway applications and some for off-road use are subject to the "tailpipe" emission regulations. These regulations specify the maximum amount of pollutants allowed in exhaust gasses from a diesel engine. The diesel engine emissions are measured over an engine test cycle which is also specified in the regulations. The duty to comply is on the equipment manufacturer. All equipment has to be emission certified before they can be released to the market. Many applications of diesel engines in confined spaces are regulated through ambient air quality standards rather than by tailpipe regulations. The ambient air quality standards specify the maximum concentrations of air contaminants which are allowed in the workplace. These regulations are set and enforced by occupational health and safety authorities.