This work explores the role of the ignition quality of a fumigated fuel on combustion phasing and brake thermal efficiency (BTE), which was investigated in a 2.5L turbocharged common rail light-duty diesel engine. Different combinations of dimethyl ether (DME) and propane were fumigated into the intake air and displaced some of the directly injected ultra-low sulfur diesel fuel (ULSD) needed to maintain the engine and a constant speed and load. Fumigation of DME and propane significantly increased BTE and reduced brake specific energy consumption (BSEC) compared to the baseline diesel condition with no fumigation. A mixture of 20% DME with 30% propane provided the maximum BTE, with 24% reduction in BSEC, however, at the expense of increasing peak cylinder pressure by 6 bar, which was even higher at greater DME substitutions. Fumigated DME auto-ignited early, ahead of top dead center (TDC), showing the typical low temperature and high temperature heat release events and propane addition suppressed the early low temperature heat release (LTHR), shifting more of the DME heat release closer to TDC. Total hydrocarbon emissions increased while NOx emissions reduced with increasing fumigation.