Emission Characteristics of a Natural Gas Engine Operating in Lean-Burn and Stoichiometric Modes

To comply with the stringent Euro VI emission standards, two techniques have been proposed for natural gas engines, namely, lean-burn combustion with oxidation catalyst (OC) and stoichiometric combustion with three-way catalyst (TWC). In this paper, comparisons are drawn between the two techniques in terms of carbon monoxide (CO), nitrogen oxides (NOx), and methane (CH4) emissions and fuel economy. The results indicated that the postcatalyst CO emissions of lean-burn combustion with OC are lower, whereas the postcatalyst NOx and CH4 emissions are higher than those of stoichiometric combustion with TWC. This can be explained by the combined effects of lower engine-out CO emissions and higher engine-out NOx and CH4 emissions as well as lower overall conversion efficiency for all emissions with the adoption of lean-burn combustion with OC. Thus, it can be concluded that although the fuel economy of stoichiometric combustion with TWC is inferior to lean-burn combustion with OC, it is still considered a more preferred technique when aiming at achieving Euro VI emission standards for its distinctive advantages in emission performance. However, it is also pointed out that further optimization of the calibration of air-to-fuel ratio is necessary for the stoichiometric operation.