The Effect of Inlet Valve Timing and Engine Speed on Dual Fuel NG-Diesel Combustion in a Large Bore Engine

High load (18 bar IMEP) dual fuel combustion of a premixed natural gas/air charge ignited by directly injected diesel fuel was studied in a large bore gas engine. A nozzle design with low flow rate was installed to inject a small diesel volume (10.4 mm³) equal an energetic amount of about two percent. The effect of compression end temperature on ignition and combustion was investigated using valve timings with early IVC (Miller) and maximum charging efficiency (MaxCC). Furthermore, the engine speed was reduced (1500 rpm to 1000 rpm) for the Miller valve timing to analyze the impact of the chemical time scale on the combustion process. During all experiments, the cylinder charge density was kept constant adjusting the intake pressure and the resulting air mass flow. Unlike a typical reactivity-controlled compression ignition (RCCI) combustion process, the combustion phasing of the investigated pilot-ignited natural gas combustion was also sensitive to the injection timing besides the air-fuel equivalence ratio (AFER). As a consequence of the lower compression end temperature with Miller valve timing, the operating range of the AFER was shifted to significantly lower values (λ = 1.53 to 1.63) compared to MaxCC (λ = 1.88 to 2.08) and knocking combustion was no longer observed. Furthermore, the bijective relation between the mass fraction burnt 50% (MFB50%) as a function of the start of energizing (SoE) at a constant air-fuel equivalence ratio was no longer valid. The experiments with MaxCC valve timing revealed that this behavior is not necessarily a characteristic of the pilot ignition. In fact, conditions inhibiting the ignition and engine knock result in loss of the one-to-one pairing between SoE and MFB50%. The reduction of the engine speed extended the lean misfire limit (λ = 1.53 to 1.75) and improved the engine start behavior.