Soot formation occurs when fuels do not fully combust. While in the past decade, engine exhaust regulations focused on limiting the total amount of soot, and NOx emissions, future regulations will include particulate matter as well. “The Euro 5+ and Euro 6 standards and proposed U.S. Tier 3+ and Tier4 regulations limit both mass and number of particles,” explained Bernie Rosenthal, CEO of Reaction Design in an interview with SAE Magazines. There is also the California Air Resources Board’s proposed Ambient Air Quality Standards to consider. Rosenthal pointed to various studies that show that soot is unhealthy, especially particulate sizes of less than 2.5 microns. “The human body cannot eliminate such small particles,” he said.
Reducing soot is a challenge, and it is not just for diesel. “Diesel has a bad rap because of the black smoke people see,” he explained. “But that is caused by larger sizes of particulate matter, which the human body can handle.” Small particulates come from both diesel and gasoline engines.
There are two ways to meet that challenge—either design an engine that minimizes the amount of soot produced or use aftertreatment to remove particulate matter from the exhaust. Aftertreatment costs from hundreds of dollars to many thousands, depending on the size of the engine and other factors, according to Rosenthal.
For cost-effective solutions, he believes there is a huge benefit in knowing how to balance engine-out emissions with aftertreatment. Given the many challenges beyond emissions—especially the need to deliver fuel economy—engine manufacturers are looking at many more potential designs than ever. Building great numbers of prototypes is expensive. “Sophisticated equipment is required for detecting emissions particles, and testing provides little insight into the causes [of soot] under varying conditions,” said Rosenthal.
Enter simulation of soot, via the company’s improved CHEMKIN-PRO and FORTÉ software programs. As part of the Model Fuels Consortium (MFC), which Reaction Design leads, a soot model was developed and correlated with experimental data. Not as easy as it sounds, since Rosenthal explained that even experimental soot data was lacking. The MFC contracted with the University of Southern California to obtain needed fundamental measurements. The result is a model composed of 50 fuel components that addresses a wide range of fuels, including those derived from petroleum, biomass, or even coal or natural gas. They uniquely validated the soot model across about 470 sets of conditions.
“The model accurately predicts engine-out trends for soot,” said Rosenthal, including not only mass emission, but particle size distribution as well. This allows engine designers to compare disparate engine designs or combustion schemes relative to one another. “We are continuing to work on improving the accuracy of the absolute values,” he said.
While details of the model are available only to MFC members for the next two years, anyone can use the model as a ‘black box’ via the software offered by Reaction Design. The company also offers general consulting services to produce simulations.