EGR Cooler Performance and Degradation: Effects of Biodiesel Blends 2008-01-2473
Exhaust gas recirculation (EGR) coolers experience degradation of performance as a result of the buildup of material in the gas-side flow paths of the cooler. This material forms a deposit layer that is less thermally conductive than the stainless steel of the tube enclosing the gas, resulting in lower heat exchanger effectiveness. Biodiesel fuel has a fuel chemistry that is much more susceptible to polymerization than that of typical diesel fuels and may exacerbate deposit formation in EGR coolers. A study was undertaken to examine the fundamentals of EGR cooler deposit formation by using surrogate tubes to represent the EGR cooler. These tubes were exposed to engine exhaust in a controlled manner to assess their effectiveness, deposit mass, and deposit hydrocarbon content. The tubes were exposed to exhaust for varying lengths of time and for varying coolant temperatures. The results show that measurable differences in the response variables occur within a few hours. Deposits containing more than 10-15% hydrocarbons exhibit a lower thermal resistance for the same deposit mass. No significant differences in effectiveness due to use of 5% and 20% biodiesel blends were observed at this “best-case” condition, but the data suggest that study of a “worst-case” condition is also needed to rule out fuel-specific effects.