What Are Diesel Emissions

W. Addy Majewski

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Abstract: Regulated emissions from internal combustion engines include NOx, PM, HC and CO. Since the adoption of first emission standards, emissions of pollutants from diesel engines have been reduced by as much as two orders of magnitude. More recent emission regulations also introduce emission limits on CO2 and other greenhouse gases.


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 volumetric concentrations of these gases in diesel exhaust are typically in the following ranges:

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 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 (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 reduction catalysts and particulate filters.

Figure 1. Relative concentration of pollutant emissions in diesel exhaust gas

Representative for diesel engines before the introduction of advanced aftertreatment

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 [2532]. 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.

Non-Exhaust Emissions (NEE). Emissions can also be produced by other vehicle components than internal combustion engines. Of special concern are non-exhaust particle emissions that consist of airborne particulate matter generated by the wearing down of brakes, clutches, tires and road surfaces, as well as by the suspension of road dust [4987][5754]. As tailpipe PM emissions from diesel engines become reduced to very low levels, the relative share of NEE PM becomes higher. In a diesel engine with a particulate filter, tire and brake wear particle emissions can easily exceed exhaust PM emissions.

Non-exhaust particle emissions are also produced by zero tailpipe emission vehicles such as battery electric vehicles. In electric vehicles, brake wear can be reduced compared to combustion engine vehicles through the use of regenerative braking. Tire wear emissions, on the other hand, can by much higher in electric vehicles due to their heavier weight and higher wheel torque gradient.