The increased availability of natural gas (NG) in the United States (US), and its relatively low cost compared to diesel fuel has heightened interest in the conversion of medium duty (MD) and heavy duty (HD) engines to NG fueled combustion systems. The aim is to realize fuel cost savings and reduce harmful emissions, while maintaining durability. This is a potential path to help the US reduce dependence on crude oil.Traditionally, port-fuel injection (PFI) or premixed NG spark-ignited (SI) combustion systems have been used for MD and HD engines with widespread use in the US and Europe; however, this technology exhibits poor cycle efficiency and is load limited due to knock phenomenon. Direct Injection of NG during the compression stroke promises to deliver improved thermal efficiency by avoiding excessive premixing and extending the lean limits which helps to extend the knock limit.In this work a single cylinder engine with 14:1 compression ratio (CR) was used to investigate spark ignited direct injection (SIDI) NG combustion using a novel spark injector-igniter. Two parameters were investigated, start of injection (SOI) timing and spark ignition (SPK) timing. The impact of these parameters on fuel consumption, combustion stability, phasing, knock, and emissions of NOx, HC, CO is reported. An optimum range of SOI timing targeted in a partially stratified combustion (PSC) range (−130 to −50 degATDC) was shown. Significant improvement in combustion stability, phasing, and HC emissions contributed to reduced fuel consumption by up to 40% over premixed SI mode using the same injector. SPK timing adjustment, in the PSC range, showed a trade-off of combustion knock and stability to within acceptable limits, thus demonstrating potential for increased load operation.