Diesel particulate matter

Diesel exhaust is the gaseous exhaust produced by a diesel type of internal combustion engine, plus any contained particulates. Its composition may vary with the fuel type or rate of consumption, or speed of engine operation (e.g., idling or at speed or under load), and whether the engine is in an on-road vehicle, farm vehicle, locomotive, marine vessel, or stationary generator or other application.

Diesel exhaust is a Group 1 carcinogen, which causes lung cancer and has a positive association with bladder cancer. It contains several substances that are also listed individually as human carcinogens by the IARC.

Methods exist to reduce nitrogen oxides (NO_x) and particulate matter (PM) in the exhaust.

The primary products of petroleum fuel combustion are carbon dioxide, water, and nitrogen. The other components exist primarily from incomplete combustion and pyrosynthesis.

The physical and chemical conditions that exist inside any such diesel engines under any conditions differ considerably from spark-ignition engines, because, by design, diesel engine power is directly controlled by the fuel supply, not by control of the air/fuel mixture, as in conventional gasoline engines. As a result of these differences, diesel engines generally produce a different array of pollutants than spark-driven engines, differences that are sometimes qualitative (what pollutants are there, and what are not), but more often quantitative (how much of particular pollutants or pollutant classes are present in each). For instance, diesel engines produce one-twenty-eighth the carbon monoxide that gasoline engines do, as they burn their fuel in excess air even at full load.

However, the lean-burning nature of diesel engines and the high temperatures and pressures of the combustion process result in significant production of gaseous nitrogen oxides (NO_x), an air pollutant that constitutes a unique challenge with regard to their reduction. While total nitrogen oxides from petrol cars have decreased by around 96% through adoption of exhaust catalytic converters as of 2012, diesel cars still produce nitrogen oxides at a similar level to those bought 15 years prior under real world tests; hence, diesel cars emit around 20 times more nitrogen oxides than petrol cars. Modern on-road diesel engines typically use selective non-catalytic reduction (SNCR) systems to meet emissions laws, as other methods such as exhaust gas recirculation (EGR) cannot adequately reduce NO_x to meet the newer standards applicable in many jurisdictions. Auxiliary diesel systems designed to remediate the nitrogen oxide pollutants are described in a separate section below.

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