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ABSTRACT
The post-treatment of polluting components of combustion engines varies according to combustion types. Compression ignition engines involve specific devices. Indeed, unlike in the case of positive-ignition engines, polluting emissions cannot be treated simultaneously due to significant stoichiometry variance and heterogeneity between gaseous and solid pollutants. The devices of compression ignition engines must treat unburnt hydrocarbons and carbon monoxide, reduce nitrogen oxides emissions and filter solid particles. Each post-treatment device must be integrated within a complex system without impairing the performances of combustion engines and present satisfactory sustainability and an attractive cost.
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Read the articleAUTHORS
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Alain SASSI: Doctor of Chemistry - Head of the "Chemistry of pollution control systems" department at PSA Peugeot Citroën
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Emmanuel ROHART: Doctorate in Chemical Engineering - Head of materials development for automotive catalysis, Rhodia Rare Earths Systems
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Gérard BELOT: Doctor of Physical Sciences - Independent consultant in automotive technology, specializing in Energy, Combustion, Emissions
INTRODUCTION
Spark-ignition engines are based on a homogeneous combustion process in which the quantities of air and fuel (gasoline) involved are close to oxidation-reduction stoichiometry, under most engine operating conditions. The result is that, at the end of combustion, residual gases are close to redox equilibrium, containing as many oxidizing species (oxygen and nitrogen oxides) as reducing species (hydrocarbons and carbon monoxide).
The post-treatment of these gaseous compounds is described in the article
In the case of compression-ignition (diesel) engines, the fuel (diesel) combustion process is heterogeneous and takes place in excess of air; under such conditions, in addition to the gaseous pollutants of unburnt hydrocarbons, carbon monoxide and nitrogen oxides, solid carbonaceous particles are formed, inherent to the heterogeneity of combustion; these also have a polluting effect. In addition, the excess air admitted to the combustion chamber results in the presence of significant quantities of oxygen in the exhaust. Under such conditions, pollutant emissions cannot be treated simultaneously (major deviation from gas stoichiometry) and the pollutants (gaseous and solid) to be treated are heterogeneous.
The after-treatment of pollutant emissions from compression-ignition engines will require specific, complementary systems:
oxidation catalyst to treat unburnt hydrocarbons and carbon monoxide;
DeNOx catalysts to reduce nitrogen oxides in the presence of oxygen;
filters to remove solid particles.
These devices are described in detail under organic and functional aspects in this article which is the continuation of
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After-treatment of pollutant emissions from internal combustion engines
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