The fuel effect of diesel, biodiesel and biodiesel-ethanol on load limits of high-efficiency premixed low temperature combustion (LTC), defined by nitrogen oxides (NOx) <= 1 g/kg-fuel, smoke <= 0.5 filter smoke number (FSN) and combustion efficiency >= 96%, is investigated using a diesel engine in this study. Low load operation range is limited by a drop in combustion efficiency for all three test fuels. High load operation range is limited by a sharp increase in smoke with diesel fuel. Biodiesel produces lower smoke at high load but still exceed the limit of 0.5 FSN. Unlike diesel and biodiesel, smoke is no longer the limiting factor for high load operation with biodiesel-ethanol fuel. A blend of 20% ethanol in biodiesel results in ultra-low smoke emission (maximum FSN less than 0.25), which is thought to be caused by the joint effects of better fuel air mixing and higher fuel oxygen fraction. Since the high load operating limit of biodiesel-ethanol is caused by a drop in combustion efficiency and not smoke as for the other fuels, biodiesel-ethanol was used to further extend the LTC load limits at a higher boosting pressure. When intake pressure is increased from 120 to 150 kPa, biodiesel-ethanol still demonstrates ultra-low smoke, and its high-efficiency LTC operating range is extended from 0.40-0.65 to 0.35-0.82 MPa indicated mean effective pressure (IMEP). (c) 2012 Elsevier Ltd. All rights reserved.