Experimental and numerical investigation on H2/CO formation and their effects on combustion characteristics in a natural gas SI engine

Abstract

Dedicated exhaust gas recirculation (D-EGR) is to generate H2 via fuel-rich combustion and viewed as a potential technique to meet future emission regulations without further after-treatment. In this study, firstly, the H2/CO formation through fuel-rich combustion in a single-cylinder natural gas spark ignition engine was quantitatively characterized by gas chromatography. Then, the effect of H2/CO addition on stoichiometric natural gas combustion performance and emission characteristics at 15% and 20% EGR levels was investigated. Finally, reaction path analysis and the brute-force sensitivity of ignition delay were conducted to evaluate the effect of H2 addition on reaction process. The yields of H2 and CO approximately linearly increased from ∼2% to ∼10% as the equivalence ratio varied from 1.1 to 1.5. The H2/CO addition accelerated the flame speed of mixture and significantly shortened the combustion duration, significantly improving the indicated thermal efficiency and the total unburned hydrocarbon with the acceptable penalty of increased NOx and CO emissions. Numerical results revealed that OH + H2 = H + H2O and H + O2 = O + OH were the most sensitive reactions with the presence of H2. This study delivered a quantitative basis for the optimization of D-EGR fueling strategies in natural gas engines.