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Tests were performed on a single-cylinder direct-injection (DI) research diesel engine to investigate the influence of elevated combustion-chamber temperature on combustion performance. The test program examined the low-heat-rejection (LHR) approach by removing the coolant but without employing heat-insulated parts. Heat-release characteristics calculated from pressure-time histories were correlated with measured exhaust emissions. It was found that increasing temperature level decreases the ignition delay and consequently decreases the fraction of total fuel that burns in the premixed-combustion phase. Exhaust hydrocarbon, NOx and particulate emission were found generally to increase with increasing temperature. The premixed-combustion fraction is concluded not to be the main source of the increased emissions.