Lean combustion is a promising combustion technology that has the potential to improve engine efficiency while decreasing emissions. One reason why lean combustion has not been more widely implemented is that as the air-fuel ratio increases, the resulting flame propagation speed becomes slower and combustion becomes unstable. Turbulent jet ignition is a pre-chamber ignition enhancement concept that facilitates ultra-lean combustion by using a hot combusting jet as a distributed ignition source. The jet penetration allows for shorter flame travel distances, which decreases the overall burn duration and improves stability. By using a rich mixture in the pre-chamber, the pre-chamber mixture is easily ignitable and the transport of chemically active radical species and unburned fuel into the main-chamber charge improves ignition quality. In this paper a series of experiments are performed in an optically accessible rapid compression machine to demonstrate the extension of the lean limit due to the jet ignition process with auxiliary injection of liquid propane into the pre-chamber. Combustion is characterized by analyzing the pressure traces and optical data generated in each test. Using a pre-chamber pressure sensor, the flow interaction between the main-chamber and prechamber is also explored for different conditions. Combustion visualization using a high speed color camera gives further insight into the turbulent jet ignition process, and allows for comparison between different testing configurations.