DIESEL EMISSIONS / REF / 42 / 2014

Diesel exhaust (sometimes known in Britain as clag when emitted by diesel locomotives, or diesel engine emissions in scientific papers) is the exhaust gas of a diesel engine.
(The word "clag" originally meant the coal-smoky exhaust of steam locomotives, and nowadays means black exhaust emissions of many older British diesel locomotives.)
Inside diesel engines, conditions differ from spark-ignition engine, since power is directly controlled by the fuel supply, not by controlling the air supply as in conventional gasoline engines. When the engine runs at idle, enough oxygen is present to burn the fuel. Diesel engines only make significant amounts of smoke when running without enough oxygen. This is usually mitigated in a turbocharged diesel engine.
Diesel exhaust is known for its characteristic smell, but this has largely disappeared in recent years following reductions in sulfur content.
Diesel exhaust contains toxic air contaminants. It is listed as a carcinogen for humans by the IARC in group 1.^[1] Diesel fuel also contains fine particles associated with negative health effects. Diesel exhaust pollution was thought to account for around one quarter of the pollution in the air in previous decades, and a high share of sickness caused by automotive pollution.^[2] Diesel engine exhaust has become far cleaner since 2000.
The lean-burning nature of diesel engines and the high temperatures and pressures of the combustion process result in significant production of nitrogen oxides, and provides a unique challenge in reducing of these compounds. Modern on-road diesel engines typically must use selective catalytic reduction to meet emissions laws, as other methods such as exhaust gas recirculation cannot adequately reduce NO_x to meet newer standards in many jurisdictions. However, the fine particulate matter (sometimes visible as opaque dark-colored smoke) have traditionally been of greater concern in the realm of diesel exhaust, as they present different health concerns and are rarely produced in significant quantities by spark-ignition engines.
Diesel engines produce very little carbon monoxide as they burn the fuel in excess air even at full load, at which point the quantity of fuel injected per cycle is still about 50 percent lean of stoichiometric.


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