International Journal of Engine Research

This summary covers key and representative developments in diesel emissions regulations, engine developments, and nitrogen oxide (NO _x ), particulate matter (PM), and hydrocarbon (HC) remediation. In California discussions are taking place on another round of criteria pollutant tightening on light duty. Carbon dioxide (CO₂) regulations are being discussed in the developed markets. The US mandates on fuel economy and the proposed standards in Europe and California will result in a paradigm shift, wherein technologies previously rejected based on customer value will now be re-evaluated. Heavy-duty regulatory developments are now focused on Europe, where Euro VI regulations have been proposed for about 2013. The regulations are intended to be technology harmonizing with the US and Japanese regulations.

Engines are continuing to make very impressive progress, with clean combustion strategies in active development mainly for US light-duty application. Heavy-duty research engines are more focused on traditional approaches, and will provide numerous engine/aftertreatment options for hitting the tight US 2010 regulations.

NO _x control is focusing on selective catalytic reduction (SCR) for diverse applications. Zeolite catalysts will be the mainstay for Japan, US, and Euro VI applications. The focus is catalyst improvements and system optimization. Lean NO _x traps (LNTs) are effective up to about 70–80 per cent deNO _x efficiency, and are being used for light-duty applications with significant improvements coming on precious metal utilization.

Diesel particulate filter (DPF) technology is in a state of optimization and cost reduction. Very sophisticated management strategies are being utilized, which open up options for new filter materials and alternative system architectures. New catalyst formulations that oxidize soot at the catalyst—soot interface are emerging and are enhancing regeneration in the first commercial applications.

Diesel oxidation catalysts (DOCs) are lighting-off and generating more NO₂ for DPF regeneration at lower temperatures.

For a more comprehensive review of these technologies the reader is directed to Johnson, T. V., Diesel emissions in review, SAE 2008-01-0121.

The impact of different types of diesel particulate matter (PM) and different sampling conditions on the wall deposition and early soot cake build-up within diesel particulate filters has been investigated. The measurements were made possible by a newly developed diesel exhaust filtration analysis system in which in-situ diesel exhaust filtration can be reproduced within small cordierite wafer disks, which are essentially thin sections of a diesel particulate filter wall. The different types of PM were generated from selected engine operating conditions of a single-cylinder heavy-duty diesel engine. Two filtration velocities 4 and 8 cm/s were used to investigate PM deep-bed filtration processes. The loaded wafers were then analysed in a thermal mass analyser that measures the soluble organic fraction as well as soot and sulphate fractions of the PM. In addition, the soot residing in the wall of the wafer was examined under an optical microscope illuminated with ultraviolet light and a variable pressure scanning electron microscope to determine the bulk soot penetration depth for each loading condition.

It was found that a higher filtration velocity results in a higher wall loading with approximately the same penetration depth into the wall. PM characteristics impacted both wall loading and soot cake layer characteristics. Results from imaging analysis indicate that the soot penetration depth into the wall was affected more by PM characteristics (which changes with engine operating conditions) than by filtration velocity.

Particulate matter trapping and regeneration processes in wall-flow diesel particulate filters (DPFs) without catalysts were investigated through micro- and macroscopic visualization experiment. The vertical walls of a small DPF were polished using a lapping process to create a mirror-like surface on each ceramic particle grain. Using an all-in-focus optical microscope, micro-scale flow pores inside the DPF wall could be clearly observed from the polished surface to a depth of 100 μm. Furthermore, a real-time video with a speed of 30 frames per second could be sharply recorded. Through the microscopic cross-sectional view, transition from depth filtration to surface filtration could be observed clearly. Only surface pores opened on the wall surface were related to the filtration depth, i.e. the penetration depth. During regeneration of the DPF without catalyst, after a particulate (soot) cake was burnt out, the particulates trapped inside the surface pores were oxidized. On the other hand, using a half-cylindrical-shaped, wall-flow DPF, the overall trapping phenomena and regeneration process were clarified through a long-distance focusing lens camera. Diesel particulates were trapped almost uniformly over the entire surface of an inflow channel of the DPF in the direction of the channel flow, while the trapped particulates were not necessarily oxidized uniformly since there was a large temperature difference between the inlet and the outlet of the flow channel. The regeneration patterns were strongly dependent on the initial particulate mass and the inlet temperature of the working gas, including the microscopic phenomena in each location. Consequently, microscopic surface pores played a significant role in the regeneration process as well as in the beginning of trapping. Furthermore, at a macroscopic level, a uniform temperature and wall-flow distributions were found to be significant for quick regeneration.

The real-time estimation of soot mass loading on diesel particulate filters (DPF) is essential to assure perfect and fuel efficient regeneration. Computer models based on the pressure drop across the DPF are usually installed in the engine control unit to estimate the soot mass loading during vehicle driving, but this approach is still subject to uncertainty about the lack of resources available. The aim of this study is to develop a portable instrument that can make real-time measurements of soot mass loading under transient mode for calibrating the models. The instrument is based on the light scattering technique: A laser beam illuminates diesel particles in a small exhaust sample and the intensity of scattered light from soot particles is reduced to soot mass concentration using calibration data obtained under steady engine operating conditions. It is demonstrated that the prototype instrument can measure the temporal variations of cumulative soot mass and filtration efficiency of a diesel particulate filter under a transient mode, and that it has a minimum detectable concentration of 5 μg/m³, which is low enough to detect the particle breakthrough in the DPF. The proposed technique has potential to be applied for on-board diagnostics if the device is downsized.

Many heavy-duty commercial vehicles are now equipped with urea-selective catalytic reduction (SCR) systems, which can reduce NO _x emissions sufficiently to meet the requirements of legislation such as Japan's New Long-term Diesel Emissions Regulations. However, in order to meet even stricter exhaust emissions regulations (and fuel consumption standards) due to be imposed in many parts of the world in the near future, urea-SCR systems with greater catalytic efficiency combined with diesel particulate filters (DPFs) will be needed. Therefore, in the study presented here the scope for enhancing the efficiency of a urea-SCR system was explored by optimizing the urea dosing system and injection strategies, and the gas flow in the exhaust pipe. However, since improving the catalysis parameters could have the greatest overall effect on conversion efficiency, work focused on modifying the catalyst materials to increase their adsorption capacity for the NH₃ reducing agent, and thus increase the collision frequency between NO _x and NH₃ absorbed on the surface of the catalyst. In addition, the oxidation parameters of the oxidation catalyst were optimized, which enhanced the NO _x conversion efficiency of the system, not only in a steady cycle but also in a transient cycle. Following these adjustments, a DPF-plus-SCR system with the new catalytic material delivered 90 per cent conversions of NO _x and particulate matter to N₂ and CO₂ respectively, in the JE05 test cycle.

In addition, a new concept, a miniaturized ‘urea-SCR with DPF function system’ was proposed and tested, which delivered 90 per cent NO _x conversion rates and 90 per cent reductions in particulate matter emissions in the JE05 test cycle.

An experimental study was performed to examine the possibility of simultaneous reductions of NO _x , hydrocarbons (HC), CO, and soot emissions for near-stoichiometric diesel combustion using a three-way catalyst and a diesel particulate filter (DPF). The DPF was deactivated in order to ensure the non-regeneration state and was just used as a soot filter trap upstream of the three-way catalyst. The goal of the research was to investigate the efficiency of emission conversion for an automotive diesel engine with a general three-way catalyst normally used for gasoline-powered vehicles under near-stoichiometric combustion operation. A modified single-cylinder engine was used for the experiments that considered throttled diesel combustion at 65–90 kPa intake pressure to achieve near-stoichiometric conditions, and a common rail injection system with 90 MPa injection pressure was used for mixture preparation and to avoid spray wall impingement. The results showed that the three-way catalyst was able to reduce NO _x and CO emissions by up to nearly 98 per cent, and the DPF trapped soot emissions up to 99.6 per cent for near-stoichiometric operation. However, the particular palladium/rhodium catalyst formulation used in the study was less effective for HC emissions, which were only reduced by up to 57 per cent. It is concluded that the use of the present proposed three-way catalyst/DPF aftertreatment system provides significant promise for future efficient, low emission diesel engines.