The objective of this study is to understand the effects of intake pressure and intake oxygen concentration on combustion and emissions of ultra-low sulfur diesel ("Diesel'') and 100% soybean methylene ether biodiesel ("B100'') and investigate on the low-temperature combustion (LTC) regime that has a potential to simultaneously reduce nitric oxides (NOx) and soot emissions. The present study is an experimental investigation with a 1-liter single-cylinder direct-injection diesel engine. Engine experiment was conducted under intake pressures and oxygen concentrations of 100-250 kPa absolute and 5-19% by volume (vol%), respectively. Thermal efficiency and carbon monoxide (CO) contour maps revealed two distinctive regions divided at an intake oxygen level of 8 vol% (or an equivalence ratio of 0.85). In the oxygen-sufficient region intake pressure exhibited dominant impacts on combustion duration, stability, and thermal efficiency. The NOx-soot contour map clarifies the location and size of the "soot barrier'' in Diesel, which was not observed in B100. B100 achieved simultaneous NOx and soot reduction to 0.2 g/kW h and 0.1 filter smoke number (FSN), respectively, at 150 kPa intake pressure with 11 vol% intake oxygen concentration. Corresponding thermal efficiency was approximately 43%. Increased intake pressure over 150 kPa lowered thermal efficiency at the maximum brake torque (MBT) timing. The apparent net heat release rate traces indicated that most heat release event occurred before top dead center (TDC) and burn duration was elongated when intake pressure was above 150 kPa. (c) 2014 Elsevier Ltd. All rights reserved.