This study evaluates a technology of integrated low heat rejection (LHR) and low temperature combustion (LTC) aiming to achieve improved energy conversion efficiency of internal combustion engines. Different from the use of thermal-barrier coatings in traditional LHR diesel engines, altering engine coolant temperature (ECT) is used to realize LHR in this study. In a separate study, the engine was overcooled to low ECTs and then increased to 100 degrees C in an effort to get trend-wise behavior without exceeding safe ECTs. This study uses an engine simulation of the same multi-cylinder, four-stroke, 1.9-L diesel engine operating at 1500 rpm and similar to 35% EGR to examine the engine performance and combustion phasing at different ECTs under the LTC mode. As compared to conventional LHR operation mode, also studied in a second separate study, increasing ECT yields more significant improvements in fuel conversion efficiency under LTC-LHR mode, 28.8% of indicated efficiency and a doubling of brake efficiency gains, pointing to a higher sensitivity to ECT variations under LTC conditions compared to conventional conditions.