The increased availability of natural gas (NG) in the United States (US) and its relatively low cost versus diesel fuel has increased interest in the conversion of medium duty (MD) and heavy duty (HD) engines to NG fueled combustion systems. The aim for development for these NG engines is to realize fuel cost savings and increase operating range while reduce harmful emissions and maintaining durability.Traditionally, port-fuel injection (PFI) or premixed NG spark-ignited (SI) combustion systems have been used for light duty LD, and MD engines with widespread use in the US and Europe . However, this technology exhibits poor thermal efficiency and is load limited due to knock phenomenon that has prohibited its use for HD engines. Spark Ignited Direct Injection (SIDI) can be used to create a partially stratified combustion (PSC) mixture of NG and air during the compression stroke. PSC promises to deliver improved thermal efficiency by avoiding excessive premixing and extending the lean limits which helps to extend the knock and load limit.In this work, a CAT 3401 SCOTE single cylinder engine with 14:1 compression ratio (CR) was used to investigate SIDI NG combustion using an integrated spark injector-igniter. The injector nozzle included angle (NIA) was varied from 80 degrees to 150 degrees to determine the impact on fuel consumption, combustion stability, phasing, and emissions. Start of injection (SOI) and spark ignition (SPK) timings were determined from previous work for a near stoichiometric operation (AFR 16 to 20:1). The nozzle included angle of 100 degrees resulted in up to 43% indicated thermal efficiency across a part-load operating range 3 to 11 bar and provided the best trade-off between fuel economy, emissions and combustion stability. This work demonstrated the optimization potential of a 100 degree NIA for high thermal efficiency of a heavy duty engine over the low load operating conditions.