Diesel Exhaust

Diesel exhaust (known as clag when emitted by diesel locomotives, or diesel engine emissions in scientific papers) is the exhaust gas of a diesel engine.

In diesel engines, conditions in the engine differ from the spark-ignition engine, since power is directly controlled by the fuel supply, rather than by controlling the air supply. Thus when the engine runs at low power, there is enough oxygen present to burn the fuel, and diesel engines only make significant amounts of carbon monoxide when running under a load.

Diesel exhaust is well known for its characteristic smell; but in Britain this smell in recent years has become much less (while diesel fuel getting more expensive) because more of the sulfur is now removed from the fuel in the oil refinery, plus the effect of catalytic converters which are able to function thanks to the new low-sulfur fuel.

Diesel exhaust has been found to contain many toxic air contaminants. It is a carcinogen (see List of IARC Group 1 carcinogens) which causes lung cancer and is associated with bladder cancer. Among these pollutants, fine particle pollution is perhaps the most important as a cause of diesel's deleterious health effects. Diesel exhaust pollution accounts for over one quarter of the total hazardous pollution in the air, and a disproportionately high share of the load of sickness and death caused by pollution.

The lean-burning nature of diesel engines combined with the high temperatures and pressures of the combustion process results in significant production of nitrogen oxides, and provides a unique challenge in the reduction of these compounds. Modern on-road diesel engines typically must utilize 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.