Effects of Piston Bowl Diameter on Combustion Characteristics of a Natural gas/Diesel Dual Fuel Engine

Natural gas/diesel dual fuel engines have potential for a high thermal efficiency and low NOx emissions. However, they have the disadvantages of high unburned species emissions and lower thermal efficiencies at low loads (at low equivalence ratio). A way to solve this problem is to properly distribute the pilot fuel vapor in a natural-gas premixture. The combustion chamber geometry affects the combustion process since it influences the distribution of the pilot fuel vapor. This study investigates the influence of injection conditions and the piston bowl geometry on the performance and emissions of a dual fuel engine. Experiments were carried out using two pistons with different bowl diameters, 52 mm and 58 mm, at single-and two-stage diesel-fuel injection. The results show that the larger bowl provides lower hydrocarbon emissions at a lower equivalence ratio in the case of single-stage injection. For two-stage injection, the influence of the bowl diameter depends on the timing of the first injection. To elucidate the effects of pilot fuel distribution, computational fluid dynamics (CFD) calculations were conducted for non-reacting pilot fuel sprays under conditions equivalent to the experiments. As a result, the relative rich area of the pilot fuel in the case of the single injection in the large piston bowl do not impinge on the piston wall at the ignition timing of the experiment, which leads to higher heat release rate owing to the weak cooling effect.