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Reduction of engine-out NOx emissions to ultra-low levels is facilitated by enabling low temperature combustion (LTC) strategies. However, there is a significant energy penalty in terms of combustion efficiency as evidenced by the high levels of hydrocarbon (HC), carbon monoxide (CO), and hydrogen emissions. In this work, the net fuel energy lost as a result of incomplete combustion in two different LTC regimes is studied—partially premixed compression ignition (PPCI) using in-cylinder injection of diesel fuel and reactivity controlled compression ignition (RCCI) of port injected gasoline and direct injected diesel. A detailed analysis of the incomplete combustion products was conducted. Test results indicated that carbon monoxide (CO), hydrogen, and light hydrocarbon (HC) made up for most of the combustion in-efficiency in the PPCI mode, while heavier HC and aromatics were significantly higher in the RCCI mode.