containing 230 wt。-ppm sulfur。 Every 50 hours the activity of the system was determined by running two times a simulated MVEG-A test on the dynamic engine bench with an inertia mass of 1900 kg。 The fuel used in this activity test was a low sulfur diesel fuel。 The NOx conversion in the MVEG-A test is quite sensitive to small changes in the activity of the complete system because of the relatively cold ECE part。 The results of this aging test are shown in figure 16。

This test shows that almost no activity loss can be observed during the 200 hours of intensive aging。 After aging no significant activity change is observed in the EUDC part of the cycle。 In the ECE part a decreasing activity can be observed as trend。 In parallel a sternly decreasing CO conversion is observed, so the activity loss is most probably due to a deactivation of the preoxidation catalyst, which also lowers the NO2 fraction in NOx in the exhaust gas at low temperatures。 After conditioning at a high speed and load operation point for 10 hours using the low sulfur diesel, this activity is restored completely。 The deactivation of the preoxidation catalyst is most probably due to sulfate deposition。 These results give confidence for the applicability of the system on a heavy duty engine, however, prolonged aging tests

have to be run to confirm the aging stability。

CONCLUSIONS

The results reported here show that the proposed SCR systems can give significant NOx reductions in the ESC test。 Already with a rude constant urea injection rate conversions of 77% can be achieved by injecting during all phases of the ESC test。 By optimizing the injection strategy, e。g。 by taking into account the actual level of NH3 stored in the system, it is expected to get to conversions higher than 75% based on the results reported here。 The engine aging results are promising with respect to the long term stability of the SCR catalyst system。 The catalyst system in itself is clearly technically feasible。

However introduction of the SCR system on heavy duty vehicles still depends on the solution of many other problems like e。g。 injection strategy, injection system, urea availability, OBD and the acceptability of the system to the end users。 All these problems can only be solved by a close cooperation of all the people involved。 This way the many hurdles for a successful introduction of the SCR system on heavy duty vehicles can be overcome。

                          The Diesel Engine

  In 1892 Rudolf Diesel developed and obtained the German patent for the diesel engine。 His goal was to create an engine that was highly efficient。 Much more efficient than the gasoline engine that was invented in 1876 and was not very efficient at all, especially at that point in time。 There are two main differences between a diesel engine and a gasoline engine:

  (1) A gasoline engine intakes a mixture of gas and air, compresses it and ignites the mixture with a spark。 A diesel engine takes in just air, compresses it and then injects fuel into the compressed air。 The heat of the compressed air lights the fuel spontaneously。(2) A gasoline engine compresses at a ratio of 8:1 to 12:1, while a diesel engine compresses at a ratio of 14:1 to as high as 25:1。

  The higher compression ratio of the diesel engine leads to better efficiency。 Gasoline engines use either a carburetor or a fuel injection system to deliver the fuel to the cylinder。 With a carburetor the fuel is mixed as it enters the intake manifold, long before it gets to the cylinders。 In a fuel injection system the fuel is injected just before the intake stroke at the intake valve。

  Diesel engines use direct fuel injection (DI), that is to say the diesel fuel is injected directly into the cylinder。 The diesel engine has no spark plugs。 The air it takes in is compressed and the fuel is injected directly into the cylinder where the heat caused by the air compression ignites the fuel。 In the old days this meant that it exploded and expanded very quickly, making a noisy engine。 This is why most diesel cars were IDI (indirect injection); the rough behavior was fixed by injecting the fuel into a small pre-combustion chamber that is connected to the cylinder by a narrow passage。