Reduction of catalyst purge air mass flow at overrun fuel cut-off as an alternative to torque neutral combustions

The following paper introduces a new approach of reducing the purge air slip during overrun phases with fuel cut off to keep the three-way-catalyst near its operating range for the following tip-in, as an alternative to late combustions at zero load with high specific fuel consumption. Therefore there are investigations at a gasoline engine with electromechanical camshaft phase actuation in a wide range presented, that show a significant reduction of the air mass flow at overrun without engine stopping. Using the camshaft actuation the flow and the oxygen concentration over time in the exhaust system are controlled via the measured throttle pressure ratio and the exhaust oxygen sensors. To achieve a noticeable and reproducible reduction in nitrous oxide and carbon monoxide tailpipe emission concentrations at tip-in, the flow has to be reduced quickly towards zero to keep the oxygen concentration low in the exhaust aftertreatment system, while the hydrocarbon tailpipe emission concentration remains at the reference level. The presented observations are pointing out key points for a derived cascaded controller structure. The presented investigation is part of the author’s doctoral thesis program at Mercedes- Benz AG in cooperation with the ILS at TU Berlin.