Before every experiment the catalyst system was preconditioned to eliminate any adsorbed NH3 by running the engine 10 minutes at a high load and intermediate speed point giving high temperatures in the hydrolysis and SCR catalysts。 Low sulfur fuel was used in the experiments described here。

SYSTEM ACTIVITY MAPS – By varying the injected amount of urea solution (the NH3/NOx ratio, alpha) and the engine load at a constant speed of 1400 rpm (European Stationary Cycle, ESC, test cycle base speed), curves for the NOx conversion and NH3 slip versus alpha ratio for different temperatures could be obtained (see figure 11)。 The data show clearly the effects discussed extensively in the model gas experiments section。 The conversion increases linearly with the alpha ratio up to a critical alpha ratio。 Above this alpha ratio the NOx conversion does not increase significantly and NH3 slip occurs。 Above temperatures of 300° C NH3 slip free conversions above 80% can be achieved。

Using these curves, the NOx conversion and alpha ratio at starting NH3 slip were determined for different engine operation points。 Using different volumes of the SCR catalyst, a map of NOx conversion as function of temperature and space velocity was obtained for both the basic HSO system and the VHSO system (see figures 12 and 13)。 The basic HSO system results show a clear effect of temperature and space velocity on NOx conversion。 Below temperatures of ca。 300° C the conversion is limited to 55%。 This behavior changes dramatically when using a preoxidation catalyst (VHSO system)。 Now the conversion below 300° C varies between 50 and 85%。 Moreover the effect of the space velocity is far less pronounced than in the basic system。

ESC TEST RESULTS – To show the NOx reduction potential of a SCR system ESC tests were run with different catalyst systems: a basic HSO system with 9。2 L SCR catalyst volume, a VHSO system with 9。2 L SCR catalyst volume and a VHSO system with 4。6 L SCR catalyst volume。 The temperature at the inlet of the SCR catalyst varied between 300° and 450° C。 Only during the first phase (“idling”) the temperature was 200°C。 The urea solution was injected starting from the second phase (mode 2) of the ESC with a constant injection rate。 This injection rate is sufficient to convert in every phase of the ESC test all NOx stoichiometrically。 The results are shown in figure 14 and table 2。

The data shows that significant NOx conversions can be reached: 45 % for the basic HSO system with 9。2 L SCR catalyst volume。 Conversions for the inpidual phases vary between 25% and 95%。 Adding a preoxidation catalyst (VHSO system) the SCR system converts 75% with conversions in the inpidual phases varying between 70% and 100%。 Even when the SCR catalyst volume is reduced to 4。6 L the total ESC conversion is significantly higher than in the basic HSO system: 65%。

The conversions obtained here could be increased by injection of urea during the first phase as this part contributes for almost 5% to the total ESC test result。 Assuming 40% for the VHSO system in the first phase, the total ESC conversion could amount to 77%。

ENGINE AGING – To determine the aging behavior of the SCR catalyst system, a VHSO system with respectively a 1。6 pre-oxidation catalyst, a 1。25 hydrolysis catalyst, a 4。9 SCR catalyst and a 1。25 L guard oxidation catalyst was mounted on a 2。5 L DI TCI passenger vehicle engine on a dynamic engine bench。 There the catalyst system was aged for 200 hours using the aging cycle depicted in figure 15。 This cycle consist of different phases which all expose the system to conditions which are considered to be extreme for aging。 The maximum temperature observed during this aging cycle was 500° C。 During all the phases urea solution was injected in amounts determined by the NOx raw emissions and the temperature for that phase。 At temperatures below 250° C an alpha ratio of 1 was used。 At temperatures above 250° C the alpha was kept at 0。7。 The fuel used for aging was a standard diesel fuel