The brake thermal efficiency and exhaust emission issues are still not fully-resolved to diesel/natural gas dual fuel engines. To better understand the effect of pilot diesel ignition mode on combustion and emissions characteristics of dual fuel engines, a detailed study concerned with diesel injection timing was conducted. The testing work was operated on a 6-cylinder turbocharged intercooler diesel/natural gas dual fuel heavy-duty engine at light load operations, and diesel injection timing was controlled over a very wide range. The investigated results show that the diesel injection timing (T-inj) has an obvious effect on pilot diesel ignition mode. A significant advancing Tinj leads to pilot diesel ignition mode differs from traditional diesel engine compression ignition mode in the sense that it does not occur at a specific place in the spray, which is a two-stage autoignition mode. With advancing Tinj, engine combustion and emissions characteristics, including cylinder pressure, cylinder temperature, heat release rate, start of combustion (SOC), ignition delay, combustion duration, crank angle of 50% heat release (CA50), nitrogen oxides (NOx) and total hydrocarbon (THC), show completely different variation trends in different ignition modes. Overall, higher thermal efficiency and lower emissions can be achieved simultaneously in two-stage autoignition mode. Satisfactory results can be obtained with higher brake thermal efficiency (35%), lower NOx (60 ppm) and THC (0.4%) emissions, when T-inj is 42.5 degrees CA BTDC. (C) 2015 Elsevier Ltd. All rights reserved.