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Diesel and gasoline emissions have become some of the leading concerns regarding greenhouse gases and global climate change. While diesel engines are more efficient than gasoline-powered engines, they have serious emissions problems. A breakthrough in diesel combustion technology may soon lead to cleaner diesel engines.
Researchers at Sandia National Laboratories have developed a new strategy, known as "low-temperature combustion" (LTC). When conventional diesel engines undergo combustion, smoky particulate matter (PM) forms in regions where fuel concentrations are too high. Additionally, nitrogen oxides (NOx) arise from a high-temperature flame inside the engine. NOx emissions are both toxic and, once released into the atmosphere and exposed to sunlight, react with other pollutants to create ground-level smog, known as ozone. LTC works to reduce NOx emissions by recirculating some of the exhaust gases back inside the diesel engine, where they absorb the heat from combustion.
Another part of the LTC strategy is to spray fuel into the engine cycle earlier to give the fuel more time to mix with air before it burns. This avoids the high temperatures that lead to NOx and the fuel-rich regions that lead to PM.
Despite its reductions in NOx and PM emissions, the LTC strategy still releases carbon monoxide (CO) and unburned hydrocarbons (UHC), which are both toxic and result in a loss of fuel efficiency. However, the Combustion Research Facility (CRF) research team in California was able to use a new optical diagnostic technique to identify the sources of these emissions.
Using a laser-induced fluorescence of other markers of combustion, such as formaldehyde and hydroxyl, researchers were able to observe and understand the chemical processes that result in UHC. The measurements showed that the fuel wasn’t burned to completion due to an over-mixing of fuel near the fuel injector, leading to CO and UHC in the exhaust.
This finding allowed combustion researcher, Mark Musculus, and Sandia post-doctoral researcher Jaqueline O'Connor to discover a way to increase fuel concentration. By adding post-injections, which added more fuel in the right area, the zone of complete combustion extends over a larger region. This results in lower UHC and CO emissions while increasing efficiency by making sure less fuel is wasted.
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"When long-haul truck drivers are burning thousands of gallons per year...or when consumers are faced with high fuel prices, a more efficient engine becomes very important," according to Musculus. "This is the kind of scientific research and data that engine designers, who help to guide our research, tell us they need so that they can build the kind of fuel-efficient diesel engines that consumers will want," he said.
The Sandia work was completed for the U.S. Department of Energy’s Office of Energy Efficiency and Renewable Energy (EERE).
Read more at Sandia National Laboratories.
By. Debra Goldberg
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