Investigation of the effect of additives to natural gas on heavy-duty SI engine combustion characteristics

This work investigates the implications of natural-gas composition on the combustion in a heavy-duty natural-gas engine and on the associated pollutant emissions. Natural gas is injected in ports and mixes with air before entering the cylinder. For the ignition source, both a spark plug and diesel pilot, which is injected before the top-dead center in the cylinder, are used. The effect of additives such as hydrogen, ethane and nitrogen on the output power and efficiency of the engine and emission levels are examined. The results indicate that these additives had no significant effect on the engine’s power or fuel consumption. Emissions of unburned fuel are reduced for all additives through either enhanced ignition or combustion processes. Adding ethane and H2 to the fuel increases the in-cylinder pressure and NOx emission, while fuel dilution with N2 has a critical amount. Black carbon particulate matter emissions are increased by ethane, but are virtually eliminated by including nitrogen or hydrogen in the fuel. The results show the higher flame speed of ethane compared to hydrogen, and hydrogen compared to methane. Thus, to reach the MBT condition, the spark time of ethane is the most retarded one and for methane it is the most advanced