OPTIMIZATION OF THE COMBUSTION CHAMBER DESIGN OF A NATURAL GAS-DIESEL DUAL FUEL ENGINE RUNNING AT LOW LOAD

A 3D-CFD investigation has been performed to optimize the design of the combustion system of a Dual Fuel (DF) natural gas-Diesel 4-Stroke engine, employed as a gen-set. The novelty of the study is the focus on low load operations (3000 rpm - 2 bar BMEP; 25 % load), where the control of CO and UHC emissions is quite critical, due to the large air excess of the premixed charge. Differently from high load conditions, here the combination of the combustion chamber shape and of the diesel spray patterns plays a fundamental role. Three piston bowl geometries, with same compression ratio ( 17.5), have been numerically compared: a conventional Omega-shaped bowl (baseline), an open shallow bowl and a hemispherical bowl. For each bowl, five Diesel sprays included angles have been considered: 170 degrees, 150 degrees (baseline), 120 degrees, 90 degrees and 60 degrees. For all the configurations, it was found that performance improves as the spray angle increases. Moreover, the hemispherical combustion chamber behaves significantly better than the baseline in a range of spray included angle between 170 degrees and 120 degrees. The effect of the Diesel injection strategy has been also investigated, starting from the best combustion chamber configuration (hemispherical bowl with Diesel spray included angle = 170 degrees): Results show that the best SOI for single-Diesel injection is -10 degrees CA AFTDC. In comparison to the baseline configuration, the optimized combustion system permits to improve the gross indicated thermal efficiency by 49.3 %, while UHC and CO emissions decrease by about 72 % and 61 %, respectively. The only drawback is the higher value of NOx emissions (+ 41 %), which should be not critical at these low loads.