Researchers Develop Innovative Process for Imaging Pollutants in Engine Exhausts

Researchers from Sandia National Laboratories have developed an innovative optical device that assists in imaging pollutants in combusting fuel sprays. The device may pave the way for clearer skies ahead.

The optical system, created by researchers from Sandia’s Combustion Research Facility as well as the Technical University of Denmark, has the ability to quantify the creation of soot (i.e. particulate matter predominantly comprising carbon) as a function of time and space for different combustion processes. At first, the scientists paid attention to the combustion of liquid fuel sprays in engines, in which very high temperatures and pressures lead to an optically demanding environment.

In order to satisfy future particulate matter emissions without compromising on fuel savings, engine developers require a state-of-the-art combustion approach to decrease soot formation in spray flames.

The acquired data provides important insights into the fuel spray motion as well as the timing and quantity of soot formed under a wide range of conditions, engine developers can use this information to validate computer models and design advanced engine combustion strategies that will improve fuel economy for consumers while also lowering tailpipe pollutant emissions.”

Scott Skeen, Sandia researcher

The study, titled “Diffuse back-illumination setup for high temporally resolved extinction imaging,” was reported in a paper published in the journal Applied Optics. The paper has been selected as a “Spotlight on Optics” in July by the Optical Society. Fredrik Westlye and Anders Ivarsson from the Technical University of Denmark; Sandia researchers Keith Penney, Lyle Pickett, and Skeen; and former Sandia researcher Julien Manin were the authors of the paper. The Department of Energy’s Vehicle Technologies Office funded the study.

High-speed diagnostics will shed light on future research

The optical system was created to quantify the formation of soot in high-pressure spray flames generated in Sandia’s optically accessible, constant-volume, pre-burn combustion chamber.

Imaging flames, at pressures and temperatures similar to that in engines, can be challenging due to a phenomenon known as “beam steering,” which takes place when light goes through a medium with differing refractive indices and is normally appears like a “mirage” seen on the highway during summer. The surrounding air is heated up by the hot pavement, thereby altering its refractive index. The direction of sunlight gets altered when it passes from cooler air to hotter air. It is due to the steered light rays that we observe the mirage-like formation. Similarly, a flame induces beam steering due to adjacent low- and high-temperature regions. The beam steering magnitude considerably increases in an engine due to the higher pressures. However, by adopting optimized imaging and lighting optics, the impacts of beam steering can be avoided.

The special lighting was achieved by a customized diffuser adequately huge to occupy the area of Sandia’s spray combustion chamber window, measuring 4 inches (or 100 mm). The customized diffuser was particularly designed to discharge light rays of uniform brightness over a particular angular range. Consequently, a light ray that gets steered when it passes through the flame will be substituted by another ray of same intensity.

The customized diffuser’s angle is optimized depending on the magnitude of the anticipated beam steering, the physical dimensions of the experimental facility, and the collection angle of the imaging system.

In fact, without such a specific optical arrangement, quantifying soot via light attenuation in high-pressure spray flames, where beam steering is more severe, would not be possible.

Julien Manin, Sandia researcher

Making cleaner engines

According to Skeen, even though present-day diesel vehicles are much cleaner than earlier engines, some ultra-modern gasoline engines discharge as much particulate matter as early diesel engines. The increased emission of particulate matter can be due to the use of a gasoline direct-injection fuel system, leading to better fuel economy and hence a lower emission of carbon dioxide per mile.

During gasoline direct injection, high-pressure liquid gasoline is directly sprayed into the engine cylinder, in contrast to mixing and vaporizing the fuel in the intake port outside the cylinder. This technique minimizes heat loss and enables free flow of air. Yet, priority is given to consumer savings at the pump and not to decreasing the emissions of particulate matter. In contrast to the much-maligned black smoke discharged from older diesel engines, soot discharged from gasoline direct injection engines cannot be viewed through the naked eye as the particles are very minute.

The diagnosis reported in the paper enables researchers to quantify the particulate matter formation in combusting sprays with unmatched spatial and temporal resolution. The knowledge gained and data obtained on using this diagnosis will assist scientists and automotive manufacturers in developing designs that further enhance fuel efficiency and considerably reduce dangerous tailpipe emissions.

A standardized method

The study stands as an important contribution to the Engine Combustion Network formed by Pickett in 2010. The network induces partnership between engine scientists across the globe. Despite the fact that participation is voluntary and that the network does not offer financial support, over 15 institutions have provided experimental data.

“The network represents the power of a grass-roots movement,” stated Pickett. “We have accomplished 20 years’ worth of research in one-fifth of the time.”

A difficulty faced by wide ranging collaborative efforts of the network is the standardization of experimental diagnostics. “With so many researchers eager to participate, it is important to ensure that everyone contributes high-quality data acquired in a technically sound manner,” stated Pickett.

The optical process formulated in this study is dependent on light attenuation or extinction to quantify the quantity of soot in a flame. When light enters the combustion chamber, the soot particles either absorb or scatter the light. The absorbed light and some light that is scattered do not reach the camera sensor. This decrease in the measured light intensity, in relation to a clear optical path, can be linked to the quantity of soot that exists. In order to easily put the diagnostic tool to good and convenient use for network participants, the paper offers elaborate guidance on the obligatory equipment and instruction for the recommended size of the collection optics and illumination source. As a Sandia-developed LED light source is used instead of a high-speed laser, the challenges faced and cost are considerably reduced.

“This work aims to establish a standardized experimental method of extinction imaging that will increase the reliability and reproducibility of experimental measurements submitted to the network,

Fredrik Westlye, Technical University of Denmark