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Combustion engine exhaust gases

Published on 10/07/2017

Expertise

CONFLICTING OBJECTIVES


CO2 emissions are responsible for the greenhouse effect owing to their negative impact on both our climate and nature,
and the various exhaust gases produced by combustion engines, such as fine particles and nitrogen oxides (NOx), are classed as very harmful to our health.

It’s no secret nowadays that nitrogen oxide emissions produced by diesel vehicles are exceeding the authorised limits under real-life conditions.
Fine particles are no longer something we talk about solely in relation to diesel vehicles; indeed, direct injection petrol engines are also now part of the issue. The high concentrations of both of these emissions that are harmful to our health are, paradoxically, partially attributable to ambitious international CO2 reduction targets; indeed, following efforts on the part of the automotive industry to reduce fuel consumption, CO2 emissions have been reduced. An increase in efficiency, that is the same engine performance at a lower level of fuel consumption, has been achieved with both diesel engines and direct injection petrol engines. Unlike indirect injection engines, whereby a consistent petrol-air mix forms before the combustion chamber, the fuel is injected into the combustion chamber, according to the direct injection principle, where it mixes with the air. It is possible, by this process, for a few droplets of fuel not to completely evaporate and instead form particles, in the case of both diesel and petrol engines.
 
Following this increase in efficiency, the combustion temperature increases in the case of diesel engines, one of the secondary effects being the formation of toxic nitrogen oxides (NOx) - irritant gases that can cause cardiac and pulmonary conditions, making them particularly dangerous where children are concerned.
Nitrogen oxides also contribute to the formation of ozone and smog, as well as the additional creation of secondary fine particles. When it comes to fine particles, ultra-fine particles measure 0.1μm in diameter, which equates to 0.001mm, meaning that they are so minuscule that they can enter the blood through the airways, where they are even absorbed by the skin. Fine particles also cause pulmonary and cardiac conditions and are carcinogenic.

Reducing toxic gases produced by diesel vehicles in order to reduce all of these pollutants is becoming increasingly costly: Recycling exhaust gases cools the gases released by the cylinder, which are redirected in this form to the combustion chamber. This brings the temperature down, thus reducing the formation of nitrogen oxides during the combustion process.

The toxic gas is deposited in the NOx accumulation catalyst and eliminated during the normal fuel injection process. This system is often used for small engines or in conjunction with an SCR catalyst.

The SCR (Selective Catalytic Reduction) catalyst transforms nitrogen oxides into basic nitrogen by adding urea (AdBlue). Unfortunately, since it is important that the temperature of the exhaust gases be high, this system is less efficient in urban environments. Automotive manufacturers are relying increasingly on SCR in order to adhere to the increasingly strict standards governing exhaust gases.

The particulate filter (DPF) must trap the finest of particles and cannot clean itself when the concentration levels are too high.

Of course, despite these measures, the high values determined by the European Union with regards to excessively high levels of atmospheric pollutants are regularly exceeded, particularly when the traffic is heavy, such as in urban environments and on motorways. Direct petrol injectors will soon be fitted with a particulate filter in order to improve air quality.
Switching to hybrid and electric vehicles in urban areas and cities would be one way to improve air quality, even though it would not solve all of our environmental problems.
 


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