Innovative engine test bench set-up for testing of exhaust gas aftertreatment and detailed gas species analysis for CNG-SI-operation

For an efficient reduction of methane slip, a precise understanding of exhaust gas after treatment under real conditions is essential. Since it is not possible to produce catalytic converters in near-series geometry on a laboratory scale, it is necessary to resort to significantly smaller sample catalysts. Therefore, an engine test bench was designed to ensure real operating conditions for such samples with the help of space velocity and temperature control. A comparison between the actual and reference values of the space velocity results in a small deviation of 0.1% on average. Furthermore, the pressure conditions at the catalyst have been measured showing a propagation of pressure oscillations from the engine outlet which in combination with the space velocity regulation show that real conditions could be applied to the catalyst sample. Subsequently, the exhaust gas concentrations were monitored with a Fourier transform infrared spectrometer. The catalyst material used is PdO on Al₂O₃, common for methane oxidation. The measurements show that the CH₄ conversion is higher under lean conditions, but is below complete conversion. In a final comparison between purely stoichiometric operation and dithering, the course of the CH₄ conversion rate over the test period is examined more closely. In addition to sampling pre- and post-catalyst, the exhaust gas composition is measured spatially resolved within a catalyst channel using special measurement technology. In the temporal course of the CH₄ emissions, a stabilizing effect due to the change of the operating mode can be seen, showing that dithering seems to prevent further deactivation.