Combustion Improvement of CNG Engines by Hydrogen Addition

This research aimed to identify how combustion characteristics are affected by the addition of hydrogen to methane, which is the main components of natural gas, and to study a combustion method that takes advantage of the properties of the blended fuel. It was found that adding hydrogen did not achieve a thermal efficiency improvement effect under stoichiometric conditions because cooling loss increased. The same result was obtained under EGR stoichiometric conditions. In contrast, under lean burn conditions, higher thermal efficiency and lower NOx than with methane combustion was achieved by utilizing the wide flammability range of hydrogen to expand the lean limit. Although NOx can be decreased easily by the addition of large quantities of hydrogen, the substantially lower energy density of the fuel causes a substantial reduction in cruising range. Consequently, this research improved the combustion of a CNG engine by increasing the tumble ratio to 1.8. In addition, by limiting the hydrogen volume ratio to 9 vol%, the NOx levels specified in the Euro6 regulations were achieved with a cruising range at the same level as a conventional CNG vehicle adopting stoichiometric combustion. In addition, thermal efficiency was improved by 9% compared to stoichiometric conditions, and tank-to-wheel (TtW) CO2 was decreased by 11%. It was found that an effective way of decreasing NOx is to synchronize fuel injection with the intake stroke.