Various measurement techniques were employed to quantify sulfuric acid deposition levels and concentration of sulfuric acid in the condensate from the recirculated exhaust gas heat exchanger of a 1995 Cummins M11 heavy-duty diesel engine. Methods employed included a modified version of the sulfur species sampling system developed by Kreso et al. (1)*, rinsing the heat exchanger, and experiments employing a condensate collection device (CCD).
The modified sampling system was applied to the inlet and outlet of the heat exchanger in order to quantify the changes in various sulfur compounds. Doped sulfur fuel (3300 to 4000 ppm S) was used to increase the concentrations of the various oxides of sulfur (SOx). These tests were performed at mode 9 of the old EPA 13-mode test cycle (1800 RPM, 932N*m) with 17-20% exhaust gas recirculation (EGR) at two EGR outlet temperatures: 160°C and 103°C. The particle-associated sulfate levels decreased from the EGR heat exchanger inlet to outlet at both EGR outlet temperatures. The particle-associated sulfate levels increased when the temperature of the EGR outlet was decreased from 160 to 103°C. Temperature change had little effect on sulfate deposition levels in the EGR heat exchanger.
The level of deposition of EGR heat exchanger sulfate increased when the conventional sulfur fuel (370 ppm S) was replaced with doped sulfur fuel (3600-4000 ppm S). The increase in fuel sulfur also led to a larger number of ultrafine (< 50 nm diameter) particles in the diluted exhaust. This supports the conclusion that the sulfate levels in the exhaust gas are increased with increasing fuel sulfur level, and that they condense to form ultrafine particles.
Operating the engine with an EGR outlet temperature of 103°C led to the formation of 20 to 24-ml of liquid condensate per hour of engine operation, having a concentration of 1.1 to 1.3% sulfuric acid. The CCD experiments yielded higher sulfate deposition rates than EGR heat exchanger rinses while the exhaust gas temperature was maintained above the water dew point. These higher levels could either be due to lower exhaust gas temperatures or to geometric differences between the CCD and the EGR heat exchanger.