Condensate Formation in Exhaust Systems during Cold Start

In order to minimize tailpipe emissions of vehicles with combustion engines, highest conversion rates of modern exhaust gas aftertreatment systems are indispensable. Especially at low ambient temperatures, gaseous emissions increase due to inhomogeneous mixture formation and incomplete combustion. Simultaneously, formation of condensate on exhaust gas-carrying components is stimulated due to temperatures dropping below the dew point. Depending on exhaust gas composition, ambient and local surface component temperatures along with engine operating conditions, the acidic or alkaline aqueous condensates contain a small fraction of aliphatic and aromatic hydrocarbons. Nevertheless, in an acidic environment, for example, the hydrocarbons of the condensates can be polymerized, forming insoluble deposits that become progressively less reactive with time. These deposits may harm components of exhaust gas duct as exhaust gas recirculation (EGR) valves and/or coolers or exhaust gas aftertreatment systems by fouling. As low temperature conditions are particularly promoting condensate formation, the aim of this study is to investigate condensate formation and composition during cold start and early warm-up phases of a Diesel and a Gasoline engine. Two engine test benches enabling transient driving cycles at cold start conditions down to -20 °C are used to provide realistic exhaust gas and sampling conditions. Samples of condensate, which is formed in the exhaust systems of a turbocharged, direct-injected Gasoline engine and a turbocharged Diesel engine compliant with EURO 6dtemp regulations, are collected under various test conditions and at different positions along the exhaust duct. Their composition and acidity are analyzed using a pH meter, ion chromatography, Karl Fischer titration, and gas chromatography coupled with mass spectrometry (GC-MS). The correlations between condensate composition, exhaust gas composition, sampling position as well as fuel composition are examined. Dew point temperature is calculated as a function of the exhaust gas composition to compare measured and computed condensate composition, enabling potential predictions.