Search Results

A vehicle investigation programme was initiated to evaluate the influence of diesel fuel sulfur content on the performance of a DeNOx catalyst for NOx control. The programme was conducted with a passive DeNOx catalyst, selected for its good NOx reduction performance and two specially prepared fuels with different sulfur contents. Regulated emissions were measured and analysed during the course of the programme. The NOx conversion efficiency of the DeNOx catalyst increased from 14 to 26% over the new European test cycle when the sulfur content of the diesel fuel was reduced from 49 to 6 wt.-ppm. In addition the number and size of particles produced using 6 wt.-ppm sulfur fuel were measured by two different techniques: mobility diameter by SMPS and aerodynamic diameter by impactor. The influence of the assumed density of the particulate on the apparent diameters measured by the two techniques is discussed.

Currently, throughout the world combustion engine development is influenced by two primary concerns. First is the increasing concern for global warming, and second is the concern over particulate and oxides of nitrogen emissions, each of which affect the environment and human health because of the particles' toxicity and ground level ozone production, respectively. To address the global warming issue, in late 1997, various nations approved the Kyoto Protocol to reduce CO2 emissions because of its identified contribution to the greenhouse effect. The Diesel engine is the most efficient power plant for mobile and stationary purposes and, thus, Diesel engines are considered to be one alternative to gasoline engines to reduce fuel consumption and, thus, CO2 emissions. To address the emission concerns, the European Community and the U.S. Environmental Protection Agency (EPA) proposed emissions standards prescribing substantial reductions of NOX and PM emissions [1,2]1.

Particle size distribution and particle number emissions rather than legislated particulate mass emissions from diesel engines are subject of rising concern especially in the US and several Western European countries [1]*. Recently also particle number emissions from gasoline engines attracted much attention since these engines are supposed to emit very high numbers of ultrafine particles or even nano-sized particles [2]. The work described in this paper focused on the impact of modern diesel exhaust gas aftertreatment systems like diesel oxidation catalysts (DOC) and diesel particulate filter (DPF) on particle number emission. Especially the effect that these aftertreatment systems are supposed to significantly increase ultrafine particle numbers (because they may act like “reactors” actively producing ultrafine solid particles) gave reason for investigating the effects and mechanisms more in detail.

The reduction of particulate emissions from diesel engines is one of the most challenging problems associated with exhaust pollution control, second only to the control of NOx from any “lean burn” application. Particulate emissions can be controlled by adjustments to the combustion parameters of a diesel engine but these measures normally result in increased emissions of oxides of nitrogen. Diesel particulate filters (DPFs) hold out the prospect of substantially reducing regulated particulate emissions and the task of actually removing the particles from the exhaust gas has been solved by the development of effective filtration materials. The question of the reliable regeneration of these filters in situ, however, remains a difficult hurdle. Many of the solutions proposed to date suffer from high engineering complexity and/or high energy demand. In addition some have special disadvantages under certain operating conditions.

In the last years the development of diesel particulate traps and trap regeneration systems has led to some very promising concepts. In parallel the development of diesel engine technology for passenger cars, as well as for light and heavy duty vehicles, has resulted in remarkable improvements especially regarding particulate and NOx emissions, engine performance and fuel economy. Unfortunately, for some aspects the development and application of particulate trap systems on the one hand and diesel engine technology on the other have led to conflicting solutions. For example, exhaust gas temperatures of at least 500 °C to 600 °C are necessary to burn off the soot that has been emitted by the engine and collected in a particulate trap. However, the increased fuel efficiency of modern TDI diesel engines and the trend to reduced average traffic speed very often cause exhaust temperatures below 200 °C at urban driving conditions.

For the reason of environmental and health protection, future Diesel emission legislation will become increasingly severe. Ceramic honeycomb traps are well suited to efficiently reduce particulate emissions. Several of such filtering devices have been introduced, but the major problem of regenerating the trap regarding reliability and failsafe operation is not yet sufficiently solved. This paper describes a particulate trap system with a self supporting, sequential regeneration system. The regeneration of the collected soot inside the channels of the honeycomb trap during vehicle opertion is initiated by ignition of the soot layer using meander shaped heat wires installed in the front end of the trap inlet channels.