An experimental investigation of low temperature combustion (LTC) cycles is conducted with diesel and ethanol fuels on a high compression ratio (18.2:1), common-rail diesel engine. Two LTC modes are studied; near-TDC injection of diesel with up to 60% exhaust gas recirculation (EGR), and port injected ethanol ignited by direct injection of diesel with moderate EGR (30-45%). Indicated mean effective pressures up to 10 bar in the diesel LTC mode and 17.6 bar in the dual-fuel LTC mode have been realized. While the NOx and smoke emissions are significantly reduced, a thermal efficiency penalty is observed from the test results. In this work, the efficiency penalty is attributed to increased HC and CO emissions and a non-conventional heat release pattern. The influence of heat release phasing, duration, and shape, on the indicated performance is explained with the help of parametric engine cycle simulations.Empirical and parametric analyses suggest that while HC and CO emissions are consistently high for the LTC test cases studied, their contribution to efficiency penalty largely depends on the engine load. The heat release profile has a significant effect on the maximum attainable thermal efficiency within the mechanical operating limits. Specifically, the thermal efficiency is the highest within a narrow combustion phasing window, identified as 7-12 °CA after top dead center for the test engine. A shorter burning duration has a positive effect on thermal efficiency while the peak cylinder pressure and pressure rise rate are negatively impacted. The heat release shaping and splitting have minimum effects on the thermal efficiency but influence the peak cylinder pressure rise rate. The results and discussion presented in this paper highlight the trade-offs experienced in enabling LTC with diesel and diesel-ethanol combination in comparison to the conventional diesel combustion.