Can Diesel Finally Come Clean?

Volkswagen’s infamous “Dieselgate” emissions scandal did much to support the notion that “clean diesel” may be a delusion. Top executives at one of the world’s leading carmakers were accused of cheating on tailpipe emissions tests to hide the fact that some models’ diesel engines released up to 40 times as much pollution as U.S. Environmental Protection Agency standards allow.

Diesel engine exhaust contains several harmful pollutants, such as nitrogen oxides (NOx) and soot particles. But despite the grime, diesel is not going away anytime soon. Green replacements, based on electrochemical batteries and hydrogen fuel cells, for example, do not yet have the juice to replace diesel as a critical power source in the global economy. Diesel engines are robust, durable, fuel-efficient and, crucially, can provide the big torque needed to move big things. Most of the hundreds of millions of medium and large long-haul trucks on highways today run on diesel—as do majorities of the world’s trains, ships, off-road vehicles and heavy machinery, not to mention many electricity generators, domestic pickup trucks and European passenger cars.

What if diesel engines really could be fundamentally cleaner from the fuel burn onward, without the extra cost and bother of exhaust-aftertreatment systems that need regular refilling? Charles Mueller, a combustion scientist at Sandia National Laboratories, thinks he has found a way: place what amounts to a tiny version of a Bunsen burner—the lab-bench heater familiar to students in high-school science classrooms—in the diesel combustion chamber to promote better burning.

Diesel Engines Now

Understanding Mueller’s invention requires some knowledge of how internal combustion engines work. In gasoline engines, an electric spark plug ignites fuel in a cylinder to push a piston. But diesel engines can create ignition without a spark. First, injectors spray diesel fuel at pressures as high as 200 megapascals—roughly half of the pressure produced by a water-jet cutter—into a cylinder. There, the emerging fuel droplets break down to bacterium-size as they travel at 600 meters per second—about the cruise speed of the supersonic Concorde airliner—and mix with air to form a “fuel-air charge.” Immediately thereafter, a plunging piston squeezes the charge to generate high pressure and thus heat, causing the fuel to self-ignite.

The diesel combustion process delivers greater energy efficiency than its gasoline counterpart, but also releases toxic NOx emissions. In a typical diesel engine, these emissions are minimized by a technique called dilution, in which spent, low-oxygen combustion gases from the previous engine cycle are routed back into the air intake. This procedure reduces the temperature and oxygen concentrations in the fuel-air mixture, which cuts down on the production of nitrogen oxides. But at the lower temperatures that characterize this common NOx-mitigation strategy, not all of the fuel is consumed. What is left invariably yields more particles of partially burned carbon—better known as soot. This long-standing diesel engineering dilemma is called the soot-NOx trade-off. “Breaking the trade-off between soot and nitrogen oxides is a research area of highest priority for diesel-engine development,” says Paul Miles, manager of Sandia’s engine research program.

Credit: George Retseck

Blue Burn

To get around this dilemma, engineers must find a way to burn diesel fuel fully—thus avoiding soot—while keeping temperatures low to avoid excess nitrogen oxide. Several years ago Mueller realized that more thoroughly premixing the fuel with air before ignition could be key to solving the problem, potentially allowing the charge to burn leaner (meaning less fuel-rich) at a lower temperature. But how could one achieve that mixing? The iconic Bunsen burner, with its vertical tube that creates a clean blue flame, came to mind.

“If you unscrew the tube and light the gas jet, you get a tall, sooty orange flame,” Mueller says. “But turn off the gas, screw the tube back on, relight the burner, and you get a nice, short blue flame.” He explains that the orange flame is colored by soot particles heated to incandescence. In contrast, the blue flame has fewer of those particles, because the burner consumes more of the fuel when its tube is in place.

The Bunsen burner owes its more complete combustion to slots near the bottom of the tube. They draw air into the gaseous fuel stream by means of the Venturi effect: a high-speed fluid flow creates regions of low pressure around it, sucking in nearby air. In this case, the Venturi effect ensures that a Bunsen burner will pull more oxygen into the fuel stream when its tube is in place. And with more oxygen mixing into the gas stream, more of the fuel will burn up completely.

Once Mueller made the connection between the science-lab tool and a diesel engine, the rest was relatively straightforward. He saw that by equipping diesel fuel injectors with tiny Bunsen–burner-chimney equivalents—small metal tubes installed a short distance from the injector nozzle hole and aligned with the fuel stream—fuel and air could be more fully premixed to enable that even, soot-free, blue-flame burning. And it could happen at the lower temperatures required for anti-NOx dilution.

Ducted Fuel Injection

Mueller calls his patented technology ducted fuel injection, or DFI. Over the past few years, his team’s DFI research has been funded by the U.S. Department of Energy’s Vehicle Technologies Office. Now Mueller and his colleagues hope to use his concept to try to create the first practical low-soot, low-NOx diesel engines, which, he says, would need less or no exhaust aftertreatment.

The auto industry has taken notice. Ford and Caterpillar just re-signed an existing cooperative-research-and-development agreement whereby they provide support for Sandia’s investigations of Mueller’s invention. Meanwhile, at a recent conference in Japan, Toyota combustion scientists presented a research paper that confirmed DFI technology suppresses soot. Other diesel-engine builders are reportedly also experimenting with the simple-seeming innovation.

“We were pleasantly surprised to see how effective the DFI ducts were in eliminating soot,” recalls Caroline Genzale, an associate professor of mechanical engineering at the Georgia Institute of Technology, who studies combustion in direct-injection engines and collaborates with Mueller to develop the new technology. After demonstrating the effect of the tiny tubes in her lab’s combustion chamber, Genzale and her colleagues now plan to observe how DFI works at the microscopic scale. They plan to zoom in on a heat-resistant, transparent quartz tube with a novel multispectral speed gun that manages to track the ultrabrief passage of the minute fuel droplets. The Georgia Tech group has also used computers to simulate the combustion effects of other spray-modifying devices with different geometries.

“Sandia’s DFI technology is on the cutting edge of new ideas,” says leading diesel expert Rolf Reitz, former director of the Engine Research Center at the University of Wisconsin–Madison. “It represents an alternative to natural mixing phenomena in diesel combustion.” But Reitz also warns that diesel-engine builders are notoriously resistant to adopting new technology because of inherent technical and mass-production challenges, as well as tight market economics. “It takes a lot to move the diesel industry,” he says. But even if DFI does fail to make it into commercial engines, Reitz continues, “it’s a real step, a tool, toward understanding the fundamental mixing process.”

Mueller is optimistic about the new technology, particularly because it does not require the installation of entirely new engines. “DFI could be retrofitted onto existing engines,” he says. One of the initial applications could be “the large, million-dollar engines in ships and locomotives, where converting to electric power is cost-prohibitive. A retrofit would be affordable and offer immediate benefits.”

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