Dual Fuel (DF) combustion can help to reduce the environmental impact of internal combustion engines, since it may provide excellent Brake Thermal Efficiency (BTE) combined with ultra-low emissions. This technique is particularly attractive when using biofuels, or fuels with a low Carbon content, such as Natural Gas (NG). Unfortunately, as engine load decreases and the homogeneous NG-air mixture tends to become very lean, the high chemical stability of NG can be a serious obstacle to the completion of combustion. As a result, BTE drops and UHC and CO emissions become very high. A possible way to address this problem could be the addition of hydrogen (H2) to the NG-air mixture. In this paper, a numerical study has been carried out on an automotive Diesel engine, modified by the authors in order to operate in both conventional Diesel combustion and DF NG-diesel mode. A previous experimental characterization of the engine is the basis for the CFD-3D modeling and calibration of the DF combustion process, using a commercial software. The effects on combustion stability and emissions of different NG-H2 mixtures (six blends with 5%, 10%, 15%, 20%, 25%, and 30% by volume of hydrogen) are numerically investigated at a low load (BMEP = 2 bar, engine speed 3000 rpm). The results of the CFD-3D simulations demonstrate that NG-H2 blends are able to decrease strongly CO, UHC, and CO2 emissions at low loads. Advantages are also found in terms of thermal efficiency and NOx emissions.

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Two-Stroke (2S) Compression Ignition (CI) engines have been used in aviation since World War II, for their excellent fuel efficiency and lightweight construction. In a modern light aircraft, these advantages still remain, along with the capability to run on jet fuel instead of gasoline. However, the design of these engines must be deeply revised, in order to incorporate the recent technologies, and it must be optimized with the support of CAE tools. The paper presents a CFD optimization of a turbocharged 2S CI 5.6 L flat-six aircraft engine developed by CMD. The scavenging system is of the Uniflow type, with exhaust poppet valves and a set of piston-controlled ports along the cylinder liner. Two mechanical superchargers are serially connected to the turbochargers. The crankshaft can be directly coupled to the propeller, thanks to its excellent balance and the relatively low maximum engine speed (2600 rpm). Differently from previous papers published on the same engine, the current study is focused on two crucial design topics: the optimization of the supercharging system and of the injection strategy. A customized version of KIVA-3V is used for the 3D-CFD cylinder analyses, while a commercial 1D-CFD code is employed to model the whole engine. The study is supported by a comprehensive experimental campaign, that permitted the accurate calibration of the numerical models. In comparison to any Four-Stroke delivering the same maximum power (400 HP at 2600 rpm, sea level), the analyzed engine is very light (about 220 kg), and efficient (211 g/kWh at typical cruise conditions). Despite the high performance, peak cylinder pressure and turbine inlet temperature at sea level are relatively low: 125 bar, 850 K. At rated power (360 HP@2400 rpm, sea level) combustion is complete and smoke-less, thanks to the optimization of the injection strategy, supported by the previous CFD analysis. The engine can operate at altitudes as high as 5500 m (18,000 feet), still delivering 270 HP at 2400 rpm, without relevant reduction of fuel efficiency. The key for the performance at high altitudes is the choice of the turbocharger considering the compressor choke limit easly reachable at high altitudes.

Micro-cogeneration with locally produced biogas from waste is a proven technique for supporting the decarbonization process. However, the strongly variable composition of biogas can make its use in internal combustion engines quite challenging. Dual-fuel engines offer advantages over conventional SI and diesel engines, but there are still issues to be addressed, such as the low-load thermodynamic efficiency and nitrogen oxide emissions. In particular, it is highly desirable to reduce NOx directly in the combustion chamber in order to avoid expensive after-treatment systems. This study analyzed the influence of the combustion system, especially the piston bowl geometry and the injector nozzle, on the performance and emissions of a dual-fuel diesel–biogas engine designed for micro-cogeneration (maximum electric power: 50 kW). In detail, four different cylindrical piston bowls characterized by radii of 23, 28, 33 and 38 mm were compared with a conventional omega-shaped diesel bowl. Moreover, the influence of the injector tip position and the jet tilt angle was analyzed over ranges of 2–10 mm and 30–120°, respectively. The goal of the optimization was to find a configuration that was able to reduce the amount of NOx while maintaining high values of brake thermal efficiency at all the engine operating conditions. For this purpose, a 3D-CFD investigation was carried out by means of a customized version of the KIVA-3V code at both full load (BMEP = 8 bar, 3000 rpm, maximum brake power) and partial load (BMEP = 4 bar, 3000 rpm). The novelty of the study consisted of the parametric approach to the problem and the high number of investigated parameters. The results indicated that the standard design of the piston bowl yielded a near-optimal trade-off at full load between the thermodynamic efficiency and pollutant emissions; however, at a lower load, significant advantages could be found by designing a deeper cylindrical bowl with a smaller radius. In particular, a new bowl characterized by a radius of 23 mm was equivalent to the standard one at BMEP = 8 bar, but it yielded a NOx-specific reduction of 38% at BMEP = 4 bar with the same value of BTE.