Simultaneous improvements of thermal efficiencies and smoke emissions in semi-premixed diesel combustion with dual-injectors and a spatially divided combustion chamber

To simultaneously establish the high thermal efficiencies and the low smoke emissions in the semi-premixed diesel combustion with a twin peaked heat release consisting of the first-stage premixed combustion and the second-stage spray diffusive combustion, a combination of dual-injectors and a newly designed divided combustion chamber was proposed and the combustion characteristics were investigated. The divided chamber with a lip at the middle of the side wall can prevent the second fuel spray from entering the burned region of the premixture from the first-stage injection, realizing a suitable spatial distribution of spray flames and suppressing the soot formation. The dual-injectors can independently provide the fuel sprays into the upper and the lower layers of the divided chamber with the optional quantities and timings for the first and second injections, establishing the optimum combustion phasing. The distributions of the equivalence ratios and the soot formation characteristics in the divided chamber as well as in an ordinary re-entrant chamber as a reference were analysed with CFD simulations, showing that the soot formation is smaller in the divided chamber. The second-stage spray diffusive combustion is improved as well as the late afterburning and the combustion duration are reduced with the suppression of interference between the spray flames in the divided chamber. The cooling loss in the divided chamber is smaller due to the weaker gas motion and the lower heat transfer coefficient. The experiments also showed that the thermal efficiencies are better and the smoke emissions are lower in the divided chamber at medium loads. However, at higher loads, the smoke emissions increase due to the offset injector arrangement, and the CFD analysis showed that lower smoke emissions can be expected with optimization of the injector arrangement to locate the injector for the second fuel injection at the centre of cylinder.