Recent diesel engine technologies, developed for enhanced regulation of exhaust emissions, are characterized by high exhaust gas recirculation (EGR) rates and high-pressure fuel injection. The use of high EGR rates, by which high temperatures can be avoided in a cylinder, is an effective method for reduction of nitrogen oxide (NOx) emissions. High-pressure fuel injection leads to smaller soot particles, which decreases mass-based soot emission. These technologies, however, also have effects on particulate matter (PM) characteristics that are closely related to EGR cooler fouling. High-pressure fuel injection, which makes smaller soot particles, increases the concentration of soot particles, and a high EGR rate, which lowers the temperature in the cylinder, increases the soluble organic fraction (SOF). In this study, we evaluated the effects of these changes in PM characteristics on EGR cooler fouling. Instead of an engine-based experiment, in which a parametric study is nearly impossible, a laboratory experiment was performed to separate variables. A soot generator was used to make model exhaust gas because the variables could be controlled separately (e.g., mean particle size and concentration of soot particles) and this improved the reproducibility of the experiments. Additionally, n-dodecane, a model compound representing diesel fuel, was vaporized and injected into the exhaust gas to test the effects of SOF on cooler fouling. For particle sizes ranging from 41 to 190 nm in diameter, the deposition fraction was inversely proportional to particle size. Thus, smaller soot particles in the exhaust gas were more likely to cause formation of thermophoretic deposits on the wall of the EGR cooler. At an EGR gas temperature of 350 °C, the deposition fraction was greatest (84%) for the smallest particle size of 41 nm, whereas the deposition fraction was least (7%) for the largest particle size, 190 nm. The performance degradation of the EGR cooler showed a similar trend to the measured deposit mass. As coolant temperature decreased, the effect of n-dodecane injection on the growth of PM deposits increased, which significantly reduced the overall thermal conductivity of the EGR cooler. This result showed that 'wet soot' caused greater contamination of the EGR cooler.
Bibliographical noteFunding Information:
The authors gratefully acknowledge financial support from GM Daewoo Auto & Technology Company .
All Science Journal Classification (ASJC) codes
- Environmental Science(all)
- Atmospheric Science