Diesel exhaust is the gaseous exhaust produced by a diesel type of internal
combustion engine, regardless of the fuel type or rate of consumption, or
speed of engine operation, and regardless of whether the engine is in
an on-road vehicle, farm vehicle, locomotive, marine vessel, or stationary
generator or other application. 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 not
controlled by the air/fuel mixture, but rather it is directly controlled by the
fuel supply. For instance, diesel engines generally produce 28 times less
carbon monoxide than gasoline engines, as diesels 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, an air
pollutant that constitutes a unique challenge with regard to their
reduction. Total nitrogen oxides from petrol cars have decreased by around 96%
through adoption of exhaust catalytic converters as of 2012, while diesel cars
still produce nitrogen oxides at a similar level to those bought a decade
and a half ago 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 systems to meet emissions laws, as other
methods such as exhaust gas recirculation cannot adequately reduce
NOx to meet the newer standards applicable in many jurisdictions.
Moreover, the fine particles in diesel exhaust has traditionally been of
greater concern, as it presents different health concerns and is rarely
produced in significant quantities by spark-ignition engines. Diesel engines
produce significant amounts of especially harmful particulate
contaminants when running without enough oxygen to fully combust the fuel; when a
diesel engine runs at idle, enough oxygen is usually present to burn the
fuel completely. As a result of the particulate emissions, exhaust from
diesel vehicles has been reported to be significantly more harmful than those
from petrol vehicles. Diesel exhausts have been known for
their characteristic odors, which changed when the sulfur content of
diesel fuel was reduced, and again when catalytic converters were introduced in
exhaust systems. Even so, diesel exhausts have always contained an array
of inorganic and organic contaminants, varying in degree and concentration
depending on fuel composition and engine running conditions. Moreover, diesel
exhaust contaminants include substances listed as human carcinogens by the
International Agency for Research on Cancer of the U.N.'s World Health
Organization. Diesel exhaust pollution is 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.
Definition and composition Diesel exhaust is the exhaust produced
by a diesel type of internal combustion engine, regardless of the fuel or fuel
use, or speed of engine operation, and regardless of whether the engine is in
an on-road vehicle, farm vehicle, locomotive, marine vessel, or stationary
generator or other application. The physical and chemical conditions that
exist inside any such diesel engines under any conditions differ considerably
from spark-ignition engines; 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, but more often quantitative. For instance, very
little carbon monoxide is produced, in general, in diesel engines, as they burn
their fuel in excess air even at full load.
The lean-burning nature of diesel engines and the high temperatures and
pressures of the combustion process result in significant production of
gaseous nitrogen oxide air pollutants. While total nitrogen oxides from petrol
cars have decreased by around 96% through adoption of exhaust catalytic
converters, while diesel cars still produce nitrogen oxides at a similar
level to those bought a decade and a half ago under real world tests; hence,
resulting in diesel cars emit around 20 times more nitrogen oxides than petrol
cars. Auxiliary diesel systems designed to remediate the nitrogen oxide
pollutants are described in a separate section below.
More critically, diesel exhaust contains fine particles, and this is of greater
concern as it is rarely produced in significant quantities by spark-ignition
engines, and the particulates present significant, distinct health concerns.
These especially harmful particulate contaminants are at their peak when such
engines are run without sufficient oxygen to fully combust the fuel; when a
diesel engine runs at idle, enough oxygen is usually present to burn the
fuel completely. Diesel exhausts, long known for their
characteristic smells, changed significantly with the reduction of
sulfur content of diesel fuel, and again when catalytic converters were
introduces in exhaust systems. Even so, diesel exhausts continue to contain an
array of inorganic and organic pollutants, in various classes, and in
varying concentrations. Some components of diesel exhaust are listed as
carcinogenic for humans by the IARC, as present in their List of IARC Group 1
carcinogens. = Chemical classes =
The following are classes of chemical compounds that have been found in diesel
exhaust. = Specific chemicals =
The following are classes of specific chemicals that have been found in diesel
exhaust. §Includes all regioisomers of this
aromatic compound. See ortho-, meta-, and para-isomer descriptions at each
compound's article. Regulation
= International and federal = = Miscellaneous =
To rapidly reduce particulate matter from heavy-duty diesel engines in
California, the California Air Resources Board created the Carl Moyer Program to
provide funding for upgrading engines ahead of emissions regulations. In 2008
the California Air Resources Board also implemented the 2008 California
Statewide Truck and Bus Rule which requires all heavy-duty diesel trucks
and buses, with a few exceptions, that operate in California to either retrofit
or replace engines in order to reduce diesel particulate matter. The US Mine
Safety and Health Administration issued a health standard in January 2001
designed to reduce diesel exhaust exposure in underground metal and
nonmetal mines; on September 7, 2005, MSHA published a notice in the Federal
Register proposing to postpone the effective date from January 2006 until
January 2011. Health concerns
= General concerns = Emissions from diesel vehicles have been
reported to be significantly more harmful than those from petrol ones.
Diesel combustion exhaust is a source of atmospheric soot and fine particles,
which is a component of the air pollution implicated in human cancer,
heart and lung damage, and mental functioning. Moreover, diesel exhaust
contains contaminants listed as carcinogenic for humans by the IARC, as
present in their List of IARC Group 1 carcinogens.