High-pressure direct-injection (HPDI) in heavy duty engines allows a natural gas (NG) engine to maintain diesel-like performance while deriving most of its power from NG. A small diesel pilot injection (5-10% of the fuel energy) is used to ignite the direct injected gas jet. The NG burns in a predominantly non-premixed combustion mode which can produce particulate matter (PM). Here we study the effect of injection strategies on emissions from a HPDI engine in two parts. Part-I will investigates the effect of late post injection (LPI) and Part II will study the effect of slightly premixed combustion (SPC) on emission and engine performance. PM reductions and tradeoffs involved with gas late post-injections (LPI) was investigated in a single-cylinder version of a 6-cylinder,15 liter HPDI engine. The post injection contains 10-25% of total fuel mass, and occurs after the main combustion event. When timed appropriately, LPI results in significant PM reductions with only small effects on other emissions and engine performance. The morphology of particles produced by LPI is similar to that from conventional HPDI (and also from diesel), but the size and number concentration are reduced. Pulse Isolation experiments and reacting-flow computational fluid dynamics (CFD) modelling indicated that the main PM reduction from LPI comes from reducing the amount of fuel in the first injection, leading to lower PM formation in the main combustion event. The second injection makes an insignificant net contribution to the total PM.