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Air pollution

Internal combustion engines such as reciprocating internal combustion engines produce air pollution emissions, due to incomplete combustion of carbonaceous fuel. The main derivatives of the process are carbon dioxide CO, water and some soot —also called particulate matter (PM). The effects of inhaling particulate matter have been studied in humans and animals and include asthma, lung cancer, cardiovascular issues, and premature death. There are, however, some additional products of the combustion process that include nitrogen oxides and sulfur and some uncombusted hydrocarbons, depending on the operating conditions and the fuel-air ratio.

Carbon dioxide emissions from internal combustion engines (particularly ones using fossil fuels such as gasoline and diesel) contribute to human-induced climate change. Increasing the engine's fuel efficiency can reduce, but not eliminate, the amount of CO emissions as carbon-based fuel combustion produces CO. Since removing CO from engine exhaust is impractical, there is increasing interest in alternatives. Sustainable fuels such as biofuels, synfuels, and electric motors powered by batteries are examples.

Not all of the fuel is completely consumed by the combustion process. A small amount of fuel is present after combustion, and some of it reacts to form oxygenates, such as formaldehyde or acetaldehyde, or hydrocarbons not originally present in the input fuel mixture. Incomplete combustion usually results from insufficient oxygen to achieve the perfect stoichiometric ratio. The flame is "quenched" by the relatively cool cylinder walls, leaving behind unreacted fuel that is expelled with the exhaust. When running at lower speeds, quenching is commonly observed in diesel (compression ignition) engines that run on natural gas. Quenching reduces efficiency and increases knocking, sometimes causing the engine to stall. Incomplete combustion also leads to the production of carbon monoxide (CO). Further chemicals released are benzene and 1,3-butadiene that are also hazardous air pollutants.

Increasing the amount of air in the engine reduces emissions of incomplete combustion products, but also promotes reaction between oxygen and nitrogen in the air to produce nitrogen oxides (NO). NO is hazardous to both plant and animal health, and leads to the production of ozone (O). Ozone is not emitted directly; rather, it is a secondary air pollutant, produced in the atmosphere by the reaction of NO and volatile organic compounds in the presence of sunlight. Ground-level ozone is harmful to human health and the environment. Though the same chemical substance, ground-level ozone should not be confused with stratospheric ozone, or the ozone layer, which protects the earth from harmful ultraviolet rays.

Carbon fuels containing sulfur produce sulfur monoxides (SO) and sulfur dioxide (SO) contributing to acid rain.

In the United States, nitrogen oxides, PM, carbon monoxide, sulfur dioxide, and ozone, are regulated as criteria air pollutants under the Clean Air Act to levels where human health and welfare are protected. Other pollutants, such as benzene and 1,3-butadiene, are regulated as hazardous air pollutants whose emissions must be lowered as much as possible depending on technological and practical considerations.

NO, carbon monoxide and other pollutants are frequently controlled via exhaust gas recirculation which returns some of the exhaust back into the engine intake. Catalytic converters are used to convert exhaust chemicals to CO (a greenhouse gas), H O (water vapour, also a greenhouse gas) and N (nitrogen).