Optimization of a High-Speed Dual-Fuel (Natural Gas-Diesel) Compression Ignition Engine for Gen-sets

The goal of this study is to develop a clean and efficient thermal unit for a generator set (gen-set) rated at 80 kW, exploring the potential of Dual-Fuel (DF) combustion (Natural Gas-Diesel) on high-speed Compression Ignition (CI) engines. Typically, the most comparable commercial gen-sets are made up of Heavy-Duty (HD) Diesel engines, whose cost and complexity will probably increase to meet more stringent emissions standards. The conversion of a light-duty Diesel engine may permit to match the high efficiency of Diesels with the low emissions of DF combustion at an affordable cost. Moreover, the new thermal unit would be more compact and lighter. Running on Natural Gas (NG) is less expensive than using Diesel fuel, and it offers more opportunities to reduce the environmental impact (e.g., NG can be easily obtained from biomass, in the same site where the gen-set is installed). Last but not the least, in case of interruption of NG supply, the system can be easily switched to conventional Diesel operation, offering a higher fuel flexibility. Despite the large number of scientific publications concerning DF engines, very few of them consider high-speed units equipped with modern Common Rail injection systems. Even more limited are the investigations on the combustion process at medium-high loads (BMEP > 10 bar), carried out by measuring in-cylinder pressure and optimizing all the fundamental control parameters (injection strategy for both Diesel fuel and NG, boost pressure, EGR rates, etc.). It should be observed that the use of state-of-the-art injection systems and the accurate calibration of their parameters at each operating condition is the only way to maximize the benefits of NG in terms of reduction of soot emissions while addressing the well-known issues related to the increase of some pollutants (HC, CO, and NOx). This study reviews the results of a theoretical and experimental activity carried out on a four-cylinder, Common Rail, 2.8-liter turbocharged Diesel engine. A gas injection system is installed upstream of the intake plenum, and an open Electronic Control Unit (ECU) is used to calibrate all the most important engine parameters. Thanks to the deep insight into the combustion process provided by in-cylinder pressure analysis and measurement of pollutant emissions, the study presents some general guidelines for setting the control strategy in this type of DF engine. Considering the operating condition at maximum power (BMEP = 12 bar, 3000 rpm, brake power = 83 kW), the following advantages are observed with comparison to the standard Diesel engine: soot is more than halved, NOx emissions are reduced by 32% and CO2 by 31%, and Brake Thermal Efficiency (BTE) increases from 35.8% to 39%. The only drawback is the increase of one order of magnitude of both CO and HC, requiring a specific oxidation catalyst. Another outcome of the study is the limitation on the use of DF NG-Diesel combustion at low loads: the experimental activity demonstrates that it is very difficult to achieve complete combustion of an ultra-lean air-NG premixed charge so that BTE tends to drop. At these conditions, it appears to be more convenient to switch back to standard Diesel operations.