A clean premixed Diesel combustion strategy, called low temperature combustion (LTC), was able to achieve very low nitrogen oxide emissions (<35 ppm) through use of exhaust gas recirculation (12.1% inlet oxygen), and reduced particulate matter (PM) emissions (<0.05 FSN) through advanced fuel injection timing (-24 degrees aTDC). When varying the injection timing by relatively small increments, large changes in PM mass and number emissions were measured within the premixed LTC regime. A discrepancy is investigated between expected reductions in PM emissions by simple fuel-air premixing and combustion temperature metrics, and actual PM emissions measurements when advancing the fuel injection timing earlier than -24 degrees aTDC. For these earlier injection timings, particle numbers were seen to increase in two distinct particle size modes, whereas only one particle size mode existed at the minimum PM emissions -24 degrees aTDC injection timing. Additional parameters from a 1D free fuel spray model were used to suggest new information that could explain the cause of these unexpected increases in PM. Using 0D and 1D calculations, the engine-out particle size and number emissions are analyzed to better understand their sensitivity to changes in the fuel injection timing within the early injection timing LTC regime. (C) 2011 Elsevier Ltd. All rights reserved.