Tunable ducted fuel injection to eliminate soot

October 24, 2019 //By Julien Happich
Tunable ducted fuel injection to eliminate soot
Researchers at the Sandia National Laboratories Combustion Research Facility have designed a ducted fuel injection which they say can fine-tune the fuel-air mixture in an engine to the point of eliminating between 50 to 100% of the soot depending on the engine’s instantaneous speed and power level.

Ducted fuel injection was first conceived by Staff Scientist Charles Mueller from a humble Bunsen burner. “If you unscrew the tube on a Bunsen burner and you light the gas jet, you get a tall, sooty orange flame,” Mueller explains. “Turn off the gas, screw the tube back on and re-light the burner. Now you get a nice, short blue flame right at the end of the tube. The flame is blue because there isn’t any soot.”

Mueller thought that concept might be adaptable to combustion engines, so he and his team, intern Christopher Nilsen, post-doctoral appointee Drummond Biles and technologist Nathan Harry, began experiments that have now resulted in an assembly of four to six small tubes, or ducts, directing fuel mixture from the injector to the points of ignition.

According to Mueller, the injectors in a traditional diesel engine create local igniting mixtures that contain two to 10 times more fuel than is needed for complete combustion.

Sandia National Laboratories researchers, from left,
Nathan Harry, Christopher Nilsen, Drummond Biles
and Charles Mueller show off the prototype ducted
fuel injection module. (Photo by Randy Wong).

“When you have that much excess fuel at high temperature, you tend to produce a lot of soot,” he notes. “Installing the ducts enables us to achieve diesel combustion that forms little to no soot, because the local igniting mixtures contain less excess fuel.”

Generations of engine designs have failed to take soot out of emissions because there was a physical limit to the chemistry of fuel combustion. “In the past, there’s always been this problem called the soot-NOx trade-off. That is: when you do something to lower soot, emissions of nitrogen oxides — or NOx — go up, and vice versa” explains Mueller.

Nitrogen oxides are also atmospheric pollutants, and the trade-off meant that truck, car and equipment makers couldn’t meet current legislated limits without adding exhaust-gas after-treatment systems.

“Now that we’ve got soot out of the way, there’s no more soot-NOx trade-off,” the researcher explains. “So, we can add dilution — taking some of the engine exhaust and routing it back to the intake — to get rid of NOx without soot emissions becoming a problem. It’s like a two-for-one deal on reducing pollutants.”

Mueller said that in engine experiments, his team has observed simultaneous, orders-of-magnitude reductions in soot and nitrogen oxides. “This gives us a path to much lower emissions for diesel engines, solving a long-standing problem for this highly efficient technology,” he said.

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