contd...
About 2% of the main injection quantity is injected 90 to 200º CA after the main injection in the cylinder. The common rail injection system is well suited for providing post injection.
The best NOx storage and conversion efficiency of NSR catalysts are obtained in a narrow temperature range of 200-350º C. Peak conversion efficiency may reach around 55 to 60% but overall conversion efficiency under driving cycle conditions is only around 35%. A number of catalyst modules to reduce space velocity and improve over all conversion have been employed in prototypes.
Sulphur Poisoning of NSR Catalysts
Sulphur on combustion forms sulphur dioxide, which gets oxidized to SO3over the catalyst and reacts with the rare earth oxides to form their sulphates such as barium oxide present in washcoat gets converted to barium sulphate. The mechanism of sulphur poisoning is similar to the mechanism of NOx trapping by the catalyst. Hence, presence of sulphur in fuel reduces NOx trapping efficiency. Even with 5 ppm sulphur in fuel the conversion efficiency has been seen to drop by half after about 25000 kms of operation.
To improve the catalyst resistance to sulphur poisoning new formulation of the adsorber material are being developed. The NSR catalysts so far are not being applied in diesel engines.
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