Effect of Methane Number in a Diesel Engine Converted to Natural Gas Spark Ignition 2019-24-0008
Natural gas (NG) is an alternative fuel for spark-ignition engines. In addition to its cleaner combustion, recent breakthroughs in drilling technologies increased its availability and lowered its cost. NG consists of mostly methane, but it also contains heavier hydrocarbons and inert diluents, the levels of which vary substantially with geographical source, time of year, and treatments applied during production or transportation. To investigate the effects of NG composition on engine performance and emissions, a 3D CFD model of a heavy-duty diesel engine retrofitted to spark ignition operations simulated engine operation under lean-combustion, low-speed, and medium load conditions. To eliminate the effect of different gas energy density, three NG blends of similar lower heating value but different H/C ratio have been investigated at fixed spark timing. The results indicated that a lower H/C ratio increased flame propagation and thus increased in-cylinder pressure and indicated mean effective pressure. In addition, the lower H/C ratio increased thermal efficiency despite the higher heat transfer, most probably due to the more advanced combustion phasing. Also, a higher H/C ratio reduced nitrogen oxides emissions but increased unburned hydrocarbons (UHC) emissions. Moreover, while the UHC emissions had a similar H/C ratio as the NG, there was no correlation between the carbon monoxide emissions and the fuel H/C ratio.