Abstract
Ammonia (NH3), which has been considered as a threaten to human health, becomes increasingly concerned recently. In order to comply with the increasingly stringent emission standards, it is widely accepted that the technique combining stoichiometric operation with Three Way Catalyst (TWC) is a more efficient and cost-effective solution for emission control in natural gas engines. However, when equipped with TWC, part of the NOx emissions generated during the combustion process will be converted to NH3, making natural gas engines a significant contributor to NH3 emissions. With the promulgation of the Euro VI, the addition provision for the limitation of NH3 emissions further motivates the investigations on the NH3 emissions of natural gas engines equipped with TWC. In this paper, the effects of air/fuel ratio, engine speed and engine load on the NH3 emission characteristics of a natural gas engine operating at stoichiometric and equipped with TWC are first studied under steady-state conditions. Then the NH3 emissions are further evaluated and analyzed during both the WHSC steady-state test cycle and the WHTC transient test cycles. The results indicated that NH3 emissions have the highest values at air/fuel ratio of 0.96 and are lower when operating at stoichiometric conditions. With the increase of engine speed, NH3 emissions show a trend of first decrease and then increase while witness an increasing trend with the increase of engine load. During WHSC test cycle, NH3 emissions are depended on the trend of the raw NOx and raw CO emissions while during the WHTC test cycles, the effects of the oxygen induced during the motoring process should be taken into account. In summary, with the application of the combination of stoichiometric operation and TWC, NH3 emissions of the investigated natural gas engine could not meet the demand of the Euro VI emission standard, which means that the addition of an Ammonia Slip Catalyst(ASC) device is essential.
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