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.
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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.
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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.
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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.
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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.”
Låter ju verkligen intressant, speciellt om man kunde byta spridarna på en äldre dieselmotor och få ökad effektivitet. Det är oklart om tekniken kräver tryck på samma nivå som i ett common rail system eller om det går att applicera på äldre dieslar också.