Simultaneous Reduction of Biodiesel NOx and PM Emissions with Low Temperature Combustion

Jose Michael

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Time integration of spatially integrated flame luminosity (SIFL) and NOx emissions of the 12 cases with different injection strategies and fuel blends. Click to enlarge. Credit: ACS

Simultaneous reduction of NOx and soot in a biodiesel-fueled engine is possible through the use of low-temperature HCCI combustion modes, according to a study by a team of researchers from the US and Taiwan. Their paper was published online 28 October in the ACS journal Environmental Science and Technology.

A number of studies are exploring mechanisms for enabling the use of biodiesel in light- and heavy-duty diesel engines for lower PM, CO, SOx, and HC emissions without incurring a NOx penalty. (Earlier post.) Low-temperature combustion (LTC) is one promising technique to meet these requirements. LTC includes homogeneous-charge compression ignition (HCCI) and several other newer combustion concepts.

This study, by a team from North Carolina State University, National Sun Yat-Sen University, and University of Illinois at Urbana-Champaign, explored the effects of injection timing and different biodiesel blends for low load [2 bar IMEP (indicated mean effective pressure)] conditions. Four test fuels were used (European low-sulfur diesel (B0), B20 (20 vol% soy biodiesel and 80 vol% B0), B50, and B100) in three start-of-injection cases (-25 CAD ATDC, -10 CAD ATDC, +3 CAD ATDC).

The experimental engine was modified from a 300ml single-cylinder DIATA (direct injection aluminum through-bolt assembly) research engine, supplied by Ford.

The results showed that maximum heat release rate can be reduced by retarding fuel injection. Ignition and peak heat release rate are both delayed for fuels containing more biodiesel. Retarding the injection to post-TDC (top dead center) lowers the peak heat release and flattens the heat release curve.

For cases 2 (SOI -10) and 3 (SOI +3), NOx emissions increase with biodiesel, which is consistent with previously published results. For case 3, NOx increases 3.61%, 8.43%, and 15.7% for B20, B50, and B100, respectively. However, for early pre-TDC injection timings (case 1, SOI -25) , NOx emissions decrease with higher biodiesel content. Then, after the biodiesel content passes a critical value, NOx emissions increase again if more biodiesel is used.

This implies a tradeoff between different parameters. For case 1, the reductions of NOx are 6.62% for B20, 17.1% for B50, and 13.1% for B100, compared with B0. Low NOx is observed for a retarded post-TDC injection strategy due to HCCI-like low-temperature combustion. Averaged reductions of NOx with late injection are 69.3% for B0, 69.7% for B20, 72.7% for B50, and 74.2% for B100 compared with conventional diesel combustion within the engine. These results showed that late injection (low-temperature combustion) could effectively reduce NOx emissions.

In other words, retarding injection timing benefited NOx emissions because of less thermal NOx formation during low-temperature combustion. Although NOx emission increases with higher percentage of biodiesel for both conventional and late injection strategies, on average, NOx reduces 68.1% for B20, 66.7% for B50, and 64.4% for B100 by use of late injection strategy. As for PM emission during low-temperature combustion, it will also be reduced due to lower soot formation rate as reported by many papers. The emission of CO, however, will generally increase. But CO is relatively easier to remove than PM and NOx in the exhaust.

...Retarding fuel injection until after TDC is an effective method for simultaneous reduction of soot and NOx emissions. Meanwhile, problems associated with biodiesel NOx emissions can be solved by low-temperature combustion. Therefore, low-temperature HCCI or PCCI combustion modes can be promising solutions for low-emission biodiesel engines.

—Fang et al. (2008)

The research was supported by the Department of Energy and by the Ford Motor Company.

Resources

  • Tiegang Fang, Yuan-Chung Lin, Tien Mun Foong, and Chia-fon Lee (2008) Reducing NOx Emissions from a Biodiesel-Fueled Engine by Use of Low-Temperature Combustion. ASAP Environ. Sci. Technol., doi: 10.1021/es8001635

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