Low temperature combustion modes, such as Homogeneous Charge Compression Ignition (HCCI) and Partially Premixed Combustion (PPC), have been researched over recent decades since the concepts show promise for high efficiency and low emissions compared to conventional diesel combustion. PPC is an intermediate combustion type ranging from HCCI-like to diesel-like combustion. The purpose of this paper is to optically study how the combustion and ignition is affected by different start of injection (SOI) timings. The study is carried out in an optically accessible heavy-duty single-cylinder engine with swirl. The intake pressure was kept constant while the intake temperature was varied to keep the combustion phasing (CA50) constant at ~3 CAD ATDC during an SOI sweep. The fuel used is a mix of primary reference fuels with octane number 81. In order to determine where the combustion starts, high speed combustion imaging is used to measure the natural luminosity. The liquid spray penetration is measured using Mie scattering, illuminating the combustion chamber by laser diodes. The results indicate that the change in ignition behavior is affected by the interaction between the fuel spray and the piston, as different fuel/air ratios are formed in the squish region and in the bowl.