Tunable ducted fuel injection to eliminate soot
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.
“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.
The near-elimination of soot and nitrogen oxides may also open up the market for renewable fuels. “Ducted fuel injection works great with conventional diesel fuel, but it works even better with oxygenated fuels, which have one or more oxygen atoms bonded into some or all of their molecules. Many renewable, sustainable fuels are oxygenated,” Mueller continued.
“Using oxygenated fuels with ducted fuel injection lowers emissions, perhaps enough to enable a less-expensive engine system because less exhaust after-treatment would be required,” he said. “On a modern on-highway truck, after-treatment amounts to about $12,000 in initial and operating costs over the life of the vehicle. Reducing even a fraction of those costs is a big deal, given the large number of these vehicles and their importance to the economy.”
“Breaking the trade-off between soot and nitrogen oxides is a research area of highest priority for diesel engine development,” said Paul Miles, manager of the engine research program.
What’s more, according to the researchers, ducted fuel injection could conceivably be retrofitted onto existing engines. This means large costly engines such as those found in ships or locomotives could be upgraded with the new injection system for immediate benefits.
Ford and a manufacturer of off-road equipment recently signed a Cooperative Research and Development Agreement with Sandia to help advance the technology. This research was conducted as part of the Co-Optimization of Fuels and Engines program sponsored by the U.S. Department of Energy, Office of Energy Efficiency and Renewable Energy, through the Vehicle Technologies and Bioenergy Technologies offices.
Sandia National Laboratories – www.sandia.gov