Low temperature combustion through in-cylinder blending of fuels with different reactivity offers the potential to improve engine efficiency while yielding low engine-out NOx and soot emissions. A Navistar MaxxForce 13 heavy-duty compression ignition engine was modified to run with two separate fuel systems, aiming to utilize fuel reactivity to demonstrate a technical path towards high engine efficiency.The dual-fuel engine has a geometric compression ratio of 14 and uses sequential, multi-port-injection of a low reactivity fuel in combination with in-cylinder direct injection of diesel. Through control of in-cylinder charge reactivity and reactivity stratification, the engine combustion process can be tailored towards high efficiency and low engine-out emissions.Engine testing was conducted at 1200 rpm over a load sweep. In addition to conventional gasoline and diesel, a blend of ethanol in gasoline on a level of 85% by volume (E85) was also investigated to examine the impact of reduced charge reactivity and enhanced reactivity stratification on load extension and engine efficiency. At each test point, engine operation was optimized for best brake thermal efficiency (BTE) within the constraints of NOx < 0.2 g/bhp-hr and pressure rise rate < 15 bar/deg, by varying diesel injection strategy, percent of fuel delivered through port fuel injection (PFI%), and air system operating conditions. Using gasoline and diesel, dual-fuel low temperature combustion (LTC) operation reached 11.6 bar BMEP with NOx < 0.2 g/bhp-hr and a best brake thermal efficiency (BTE) of 43.6%. The use of E85 further extended LTC operation to 19 bar BMEP (21.7 bar gIMEP) with a best BTE of 45.1%. When using E85, throughout the test range, NOx was below 0.2 g/bhp-hr and smoke was below 0.2 FSN.