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Natural gas offers the potential to reduce greenhouse gas emissions from heavy-duty on-road transportation. One of the challenges facing natural gas as a fuel is that its composition can vary significantly between different fuel suppliers and geographical regions. In this work, the impact of fuel composition variations on a heavy-duty, direct injection of natural gas engine with diesel pilot ignition is evaluated. This combustion process results in a predominantly non-premixed gaseous fuel combustion event; as a result, end-gas autoignition (knock) is not a concern. Changes in the fuel composition do still impact the combustion, both through the changes in the chemical kinetics of the reactions and due to changes in the density of the fuel. Increasing concentrations of heavier hydrocarbons, such as ethane or propane, in the fuel lead to higher fuel densities and hence greater fuel mass being injected for a given injection duration.