Experience with a New Particulate Trap Technology in Europe 970182

The reduction of particulate emissions is of major interest, especially in urban areas. The technology associated with particulate traps has been under development for more than twenty years, however past experiences have tended to show that regeneration systems are expensive and often suffer from durability problems. In this paper we report on an alternative approach in which a continuous low-temperature chemical reaction mechanism is employed. The system based on this technology consists of a special oxidation catalyst upstream of a particulate trap. Over the catalyst some of the engine-out NO emission is oxidised to NO₂. NO₂ is active for the continuous oxidation of soot collected in the trap above about 250 °C. As a result of this regeneration process the trap is always very lightly loaded with soot and the back-pressure level is therefore very low. The oxidation catalyst naturally also leads to very low emissions of CO and hydrocarbons. The overall conversion rates from raw gas to tailpipe for a vehicle fitted with such a system are typically in excess of 90% for CO, HC and particulate matter. There is a small reduction of NO_x, typically between 3% and 8%.

The successful operation of a system based on this technology requires the use of virtually sulphur free fuel. For this reason the earliest work was done in Sweden where the so-called “Swedish Environmental Class 1 diesel” is the principle transport fuel. This fuel has a maximum sulphur content of 10ppm. About 1800 systems are in operation in Europe to date, a wide variety of engine types and applications having been tested. The maximum recorded mileage on a single vehicle is 390,000 km. In addition to the Swedish experience, the cities of Paderborn, Wiesbaden, London and Oxford have small fleets of vehicles with systems in operation. A large European bus manufacturer has commissioned a durability test on the system involving a fleet of fifty vehicles¹.