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The present work investigates the effect of fuel injection timing on combustion stratification and soot formation in an optically accessible, single cylinder light duty diesel engine. The engine operated under low load and low engine speed conditions, employing a single injection scheme. The conducted experiments considered three different injection timings, which promoted Partially Premixed Combustion (PPC) operation. The fuel quantity of the main injection was adjusted to maintain the same Indicated Mean Effective Pressure (IMEP) value among all cases considered. Findings were analysed via means of pressure trace and apparent heat transfer rate (AHTR) analyses, as well as a series of optical diagnostics techniques, namely flame natural luminosity, CH* and C2* chemiluminescence high-speed imaging, as well as planar Laser Induced Incandescence (pLII). For the chemiluminescence high-speed imaging a novel methodology was developed to acquire the net spatially resolved chemiluminescence signal, using combustion spectroscopy to remove the background chemiluminescence and thermal radiation signal. The analysis suggests that when the injection timing advanced, the stratification of the combustion became weaker, as indicated by the natural luminosity signal, due to the better pre-mixing between the injected fuel and the surrounding air. The enhanced premixing resulted in lower levels of in-cylinder soot late in the combustion cycle and, presumably the lowest engine-out soot emissions, because of the limited formation of locally fuel-rich areas inside the cylinder. This came at the price of increased fuel consumption.

The work investigates the effect of various post-injection strategies on the flame patterns in a Ricardo Hydra optical single-cylinder, light-duty diesel engine, operated in a partially premixed combustion mode, under low load (IMEP: ca. 2.3 bar), low speed (1200 rpm) conditions. The effect of post-injection fuel amount (12% and 24% of the total fuel quantity per cycle) and post-injection timing (0, 5, 10 deg aTDC) are investigated via pressure trace analysis and optical measurements. Flame propagation is captured by means of high-speed flame natural luminosity imaging and of CH*, C2*, and OH* line-of-sight chemiluminescence measurements. Results suggest that post-injections suppress mixture reactivity but enhance oxidation, and that a larger amount of fuel and/or later post-injection, leads to higher levels of natural luminosity, indicating possible higher soot-out emissions, while post-injection close to the main combustion event appears to have a beneficial effect on the soot oxidation processes.