This paper presents a detailed experimental analysis of diesel - ignited propane dual fuel combustion on a 12.9 - l, six - cylinder, production heavy - duty diesel engine. Gaseous propane was fumigated upstream of the turbocharger air inlet and ignited using direct injection of diesel sprays. Results are presented for brake mean effective pressures (BMEP) from 5 to 20 bar and different percent energy substituted (PES) by propane at a constant engine speed of 1500 rpm. The effect of propane PES on apparent heat release rates, combustion phasing and duration, fuel conversion and combustion efficiencies, and engine - out emissions of oxides of nitrogen (NOx ), smoke, carbon monoxide (CO), and total unburned hydrocarbons (HC) were investigated. Exhaust particle number concentrations and size distributions were also quantified for diesel - ignited propane combustion. With stock engine parameters, the maximum propane PES was limited to 86%, 60%, 33%, and 25% at 5, 10, 15, and 20 bar BMEPs, respectively, either by high maximum pressure rise rates (MPRR) or by excessive HC and CO emissions. With increasing PES, while fuel conversion efficiencies increased slightly at high BMEPs or decreased at low BMEPs, combustion efficiencies uniformly decreased. Also, with increasing PES, NOx and smoke emissions were generally decreased but these reductions were accompanied by higher HC and CO emissions. Exhaust particle number concentrations decreased with increasing PES at low loads but showed the opposite trends at higher loads. At 10 bar BMEP, by adopting a different fueling strategy, the maximum possible propane PES was extended to 80%. Finally, a limited diesel injection timing study was performed to identify the optimal operating conditions for the best efficiency-emissions-MPRR tradeoffs. (c) 2014 Elsevier Ltd. All rights reserved.