Engine experiments were carried out on a heavy-duty single-cylinder engine to investigate the effects of post injections on soot production. In this context, the term “post injection” refers to an injection shortly after the main, near firing Top Dead Center (TDC) injection. Both high load Gasoline Compression Ignition (GCI) post injection experiments and Conventional Diesel Combustion (CDC) post injection experiments were run. Load ranged from 16 – 21.5 bar gross Indicated Mean Effective Pressure (IMEPg) with the post injections extending the load up to 35%. Three post injection timings were used between 12 – 40 dATDC; at each injection timing, every load, and with both fuels, there was no soot benefit seen from using a post injection. This result was contradictory to what is seen in literature at low to mid load conditions. Thus, experiments were run at low to mid load, replicating conditions found in literature. The load in these experiments ranged from 3.8 – 8 bar IMEPg; preliminary experiments were conducted with diesel fuel with 0% and 15% Exhaust Gas Recirculation (EGR). At all loads and at both 0 and 15% EGR, the use of a post injection resulted in higher engine-out soot emissions. This led to investigating spray targeting and injector configuration. An injector tip with a larger included spray angle and smaller orifice diameter resulted in 66 – 98% lower engine-out soot emissions throughout the load range, but soot levels were at best the same as when no post injection was used. Fuel effects were then investigated by comparing CERTification (CERT) diesel and PRF 21 to Chevron diesel. Both CERT diesel and PRF 21 have a Cetane number of 45, lower than the Cetane number of Chevron diesel (53), and PRF 21 lacks aromatics. Both CERT diesel and PRF 21 showed moderate improvements in soot when using a post injection, but only at a few specific conditions. This led to the conclusion that overall, post injections are not an effective pathway to soot reduction in the present configuration.