Control of Diesel Soot, Hydrocarbon and NOx Emissions with a Particulate Trap and EGR

This study demonstrated the concept of using exhaust gas recirculation (EGR), coupled with a high-collection efficiency particulate trap to simultaneously control smoke, unburned hydrocarbon and NOx emissions from diesel engines. Although EGR technology has been extensively used in gasoline engines, its application to diesel engines has been hindered by the particulate content of the recirculated exhaust gas. Even with the use of conventional ceramic monolith filters, with soot collection efficiencies in the range of 50-80%, the exhaust stream is not adequately clean for recirculation to the engine. This investigation used a high soot collection efficiency Ceramem filter to make EGR possible. This ceramic filter is coated with a thin microporous ceramic membrane to provide soot removal efficiencies in the order of 99%. Aerodynamic regeneration of this filter not only provided low-temperature failure-free operation but, also drastically reduced the unburned hydrocarbon emissions by promoting their condensation on the soot, as was demonstrated earlier (1).¹ This feature is important since EGR often increases unburned volatile hydrocarbon emissions, since it reduces both the in-cylinder oxygen concentration and temperature.