Direct injection of methane or hydrogen on port-fueled methane combustion in a heavy-duty optical spark ignition engine

The growing interest in utilizing natural gas for heavy-duty internal combustion engines is driven by its potential to mitigate CO₂, particulate matter, and NO_x emissions. However, natural gas engines operated with port-fuel injection (PFI) often experience slow burn rates and high cyclic variability, particularly when burning lean. Direct injection (DI) of a portion of the fuel, increases the system complexity but provides a route to introduce charge stratification, leading to more stable combustion. This study evaluates the effect of methane or hydrogen direct injection on port-fueled methane combustion in a heavy-duty optical diesel engine converted to spark-ignition operation. The fuel volume fractions through DI and PFI were varied keeping a constant overall air-excess ratio. The results obtained from methane combustion using PFI indicate unstable combustion at λ=1.4, with COV of IMEPg of 16.21 %. Incorporating direct injection of 40 % methane by volume (40 % by energy) significantly improved stability in combustion, bringing the COV of IMEPg down to 2.85 %. To extend the methane's lean limit while maintaining a similar COV of IMEPg of 3.17 %, a hydrogen direct injection of 60 % by volume (31.4 % by energy) was required. Direct injections of both methane and hydrogen improved the stability in combustion and extended the lean limit in methane combustion, with hydrogen achieving this at a lower energy ratio due to its higher reactivity. The increased DI volume fraction of methane or hydrogen advances the combustion phasing towards the top dead center (TDC) with a reduction in combustion duration, indicating a faster combustion process. Further, at λ = 1.6 and λ = 1.8, hydrogen direct injection of 60 % by volume resulted in stable combustion and improved gross indicated efficiency. However, increasing DI volume flow rate raised the soot formation concerns, suggesting further studies are needed in thermodynamic engines for quantifying emissions.