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The Thermodynamics of Exhaust Gas Condensation
Abstract:
Water vapor is, aside from carbon dioxide, the major fossil fuel combustion by-product. Depending on its concentration in the exhaust gas mixture as well as on the exhaust gas pressure, its condensation temperature can be derived. For typical gasoline engine stoichiometric operating conditions, the water vapor dew point lies at about 53 °C. The exhaust gas mixture does however contain some pollutants coming from the fuel, engine oil, and charge air, which can react with the water vapor and affect the condensation process. For instance, sulfur trioxide present in the exhaust, reacts with water vapor forming sulfuric acid. This acid builds a binary system with water vapor, which presents a dew point often above 100 °C.
Exhaust composition after leaving the combustion chamber strongly depends on fuel type, engine concept and operation point. Furthermore, the exhaust undergoes several chemical after treatments. The transformation of pollutants in these components drastically affects the composition of the possible exhaust condensate, which can for instance shift its pH-value from acidic to alkaline.
Detailed analysis of these condensable exhaust components and understanding of condensate composition becomes of increasing importance with the growing relevance of exhaust gas recirculation (EGR) cooling and when thinking of exhaust gas as a potential water source. In spite of this, the complexity of the exhaust condensation process is, especially with regard to gasoline engines, a field of relatively low focus up until now. The goal of this study is to present a theoretical analysis of the thermodynamics of gasoline engine exhaust condensation including an experimental data based validation. Through investigation of the influence of the air/fuel ratio, pollutant concentrations, and temperature conditions, an improved understanding of the condensation process is gained.