Analysis of non-legislated engine-emission components, with
different exhaust-gas after-treatment techniques, is an important
air quality objective.
This paper reports the results for various nitrogen oxides,
ammonia and differentiated hydrocarbons emitted at part load from a
small 4-S SI engine. It was operated with gasoline, with CNG and
with two different three-way catalytic converters.
CNG produces less HC and less aromatics. But the HC conversion
rate is insufficient. This is due to the lower exhaust gas
temperatures, at part load with CNG, and due to the higher
stability of light HCs.
CNG affects the λ-regulation window, of the investigated system,
such that the NOx conversion rate is lowered.
In the rich domain of the λ-regulation window, the NO &
NOx emissions after catalyst were lowest, while the NH₃
formation was most intense, and vice versa.
The catalyst type: precious metal, and its content, influences
the conversion rates and emissions of the investigated components
during light-off.