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2022 - 2‐Stroke RCCI Engines for Passenger Cars [Articolo su rivista]
Mattarelli, E.; Rinaldini, C. A.; Marmorini, L.; Caprioli, S.; Legrottaglie, F.; Scrignoli, F.

Reactivity Controlled Compression Ignition (RCCI) is one of the most promising solutions among the low temperature combustion concepts, in terms of thermal efficiency and pollutant emissions. However, for values of brake mean effective pressure higher than 10 bar, in‐cylinder peak pressure rise rates tend to be too high, limiting the specific power of any 4‐Stroke (4S) engine. Such a limitation can be canceled by moving to the 2‐Stroke (2S) cycle. Among many alternatives, the “Uniflow” scavenging system with exhaust poppet valves on the cylinder head allows the designer to reproduce the same identical combustion patterns of a 4‐stroke RCCI engine, while increasing the indicated power output. The goal of the paper is to explore the potential of a 2‐stroke RCCI engine, on the basis of a comprehensive experimental campaign carried out on a modified automotive 2.0 L, 4‐stroke, four‐cylinder, four‐valve diesel engine. The developed prototype can run with one cylinder operating in 4‐stroke RCCI mode (gasoline–diesel), while the others work in the standard diesel mode. A One Dimensional‐Computational Fluid Dynamics (1D‐CFD) model has been built to predict the performance of the same prototype, when operating all four cylinders in RCCI mode. In parallel, an equivalent 2‐stroke RCCI virtual engine has been developed, by means of 1D‐CFD simulations and empirical assumptions. A numerical comparison between the 4S and the 2S engines is finally presented, in terms of performance and emissions at full load. The study demonstrates that a 2S RCCI engine can maintain all of the advantages of the RCCI combustion, strongly reducing the penalization in terms of performance, in comparison to a standard 4S diesel engine.

2021 - Application to micro-cogeneration of an innovative dual fuel compression ignition engine running on biogas [Articolo su rivista]
Legrottaglie, F.; Mattarelli, E.; Rinaldini, C. A.; Scrignoli, F.

Renewable sources and enhancement of energy conversion efficiency are the main paths chosen by the European Community to stop climate changes and environmental degradation, and to enable a sustainable growth. For this purpose, the construction of a new and more dynamic electricity distribution network is mandatory. This “smart grid” should also include small and medium-size companies, able to program the generation and use of energy from renewable sources (the so-called "prosumers"). In this frame, micro-cogeneration (rated electric power up to 50 kW) is one of the most promising techniques. In this work, the application to micro-cogeneration of an innovative Compression Ignition internal combustion engine, operated in Dual Fuel mode is proposed. Thanks to the specific combustion system (Reactivity Controlled Compression Ignition, RCCI: a lean homogenous mixture of air and biomethane or biogas is ignited by the injection of a small amount of Diesel fuel), brake thermal efficiency can be increased at all operating conditions, compared to a conventional Spark Ignition engine running on biomethane or biogas. The ensuing reduction of CO2 emissions is higher than 20%. Furthermore, the proposed engine can tolerate larger variations in the composition of the biogas, without a significant drop of thermal efficiency. Finally, in case of emergency, it is able to run on Diesel fuel only. The use of the engine is particularly suitable for a company operating in the agricultural field, such as a mid-size farm, that is able to produce biogas for its self-consumption. Therefore, a representative study case is selected, and the corresponding electrical and thermal energy needs are analysed throughout a typical year. The energetic analysis leads to the identification of the most suitable engine size and calibration settings, in order to reduce the purchase of electricity and natural gas, maximizing the use of the company's own renewable sources (biogas or biomethane). The final goal of the optimization process is to create a virtuous system, that can reduce the environmental impact and make the company almost independent from the energetic point of view.

2021 - Design of a Novel 2-Stroke SI Engine for Hybrid Light Aircraft [Relazione in Atti di Convegno]
Caprioli, S.; Rinaldini, C.; Mattarelli, E.; Savioli, T.; Scrignoli, F.

The trend of powertrain electrification is quickly spreading from the automotive field into many other sectors. For ultra-light aircraft, needing a total installed propulsion power up to 150 kW, the combination of a specifically developed internal combustion engine (ICE) integrated with a state-of-the-art electric system (electric motor, inverter and battery) appears particularly promising. The dimensions and weight of ICE can be strongly reduced (downsizing), so that it can operate at higher efficiency at typical cruise conditions; a large power reserve is available for emergency maneuvers; in comparison to a full electric airplane, the hybrid powertrain makes possible to fly at zero emissions for a much longer time, or with a much heavier payload. On the other hand, the packaging of a hybrid powertrain into existing aircraft requires a specific design of the thermal engine, that must be light, compact, highly reliable and fuel efficient. The last aspect has a direct impact on the performance of the aircraft, since the mission range depends on the capacity of the fuel tanks, which, in turn, is limited by the aircraft total weight. The two-stroke cycle engine is far from a novelty for ultra-light aircraft; unfortunately, the specific fuel consumption and pollutant emissions of the conventional engines is quite high, in comparison to their 4-Stroke (4S) counterparts. The aim of the project presented in this paper is to develop a new type of 2-Stroke SI engine, able to match lightness, fuel efficiency and low pollutant emissions at a reasonable cost. The proposed ICE weights less than 60 kg, it delivers 110 kW@6000 rpm, along with a brake specific fuel consumption lower than 260 g/kWh in all the most relevant operating conditions. The paper describes the design of the new engine, with particular attention to the optimization of the scavenging system (without poppet valves) and the design of a low pressure direct injection system. The process is supported by CFD 1D and 3D simulations. As far as the design of the injection system is concerned, the main goal was to obtain a fuel trapping ratio higher than 95%, along with a properly stratified charge at combustion onset, when considering the most critical operating condition (maximum engine speed and load). The main optimized parameters include the number of injectors, their locations, the injection timing and duration.

2021 - Influence of H2 enrichment for improving low load combustion stability of a Dual Fuel lightduty Diesel engine [Articolo su rivista]
Mattarelli, E.; Rinaldini, C.; Caprioli, S.; Scrignoli, F.

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.

2021 - Optimization of a High-Speed Dual-Fuel (Natural Gas-Diesel) Compression Ignition Engine for Gen-sets [Articolo su rivista]
Mattarelli, E.; Rinaldini, C. A.; Savioli, T.; Scrignoli, F.

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.

2020 - Parametric Study on Electric Turbocharging for Passenger Cars [Relazione in Atti di Convegno]
Mattarelli, E.; Scrignoli, F.; Rinaldini, C. A.

The motor generator unit installed on the turbocharger shaft (MGU-H) provides a fundamental contribution to the amazing performances and efficiency of the last Formula 1 power units. The excess of exhaust gas energy - normally dumped through the waste-gate - can be converted into electric energy and used to push the car, by means of a second motor generator unit installed on the engine crankshaft (MGU-K). The goal of this paper is to assess pros and cons of the MGU-H technology when applied to a family of engines of different displacement, installed on a typical passenger car. The influence of engine size and cylinders layout is investigated, under the same set of hypotheses, considering both transient and steady engine operations. The baseline engine is a commercial 2.0 L, SI, 4-cylinder in-line, rated at 200 HP at 4500-5000 rpm. The study considers the following other SI configurations: a) 1.5L, 3-cylinder in-line, 150 HP; b) 3.0L, V6, 300 HP; c) 4.0L, V8, 400 HP; d) 6.0L, V12, 600 HP. It is assumed that all the 5 engines have the same unit displacement and the same maximum load, expressed in terms of brake mean effective pressure as a function of rotational speed. The study is carried out using an experimentally calibrated GT-Power model of the baseline engine, and considering the same class C vehicle. A Matlab/Simulink model is also developed for the analysis of the WLTP driving cycle. The study demonstrates that the MGU-H technology can be conveniently applied to all the considered engines. The maximum advantage in terms of fuel saving on a driving cycle is obtained on the smallest. However, in the V6, V8 and V12 configurations, the installation of one electric turbocharger instead of two, strongly simplifies the engine layout, and it allows the designer to find some space for additional powertrain components, such as electric motors, battery packs, etc. Moreover, the elimination of the turbo-lag problem, gives the designer much more freedom, enabling the adoption of more fuel efficient engine settings.

2019 - Development of a Hybrid Power Unit for Formula SAE Application: ICE CFD-1D Optimization and Vehicle Lap Simulation [Relazione in Atti di Convegno]
Mattarelli, E.; Rinaldini, C. A.; Scrignoli, F.; Mangeruga, V.

The paper reviews the CFD optimization of a motorcycle engine, modified for the development of a hybrid powertrain of a Formula SAE car. In a parallel paper, the choice of the donor engine (Ducati 959 Panigale: 2-cylinder, V90, 955 cc, peak power 150 HP at 10500 rpm, peak torque 102 Nm at 9000 rpm) is thoroughly discussed, along with all the hardware modifications oriented to minimize the new powertrain dimensions, weight and cost, and guarantee full reliability in racing conditions. In the current paper, the attention is focused on two main topics: 1) CFD-1D tuning of the modified Internal Combustion Engine (ICE), in order to comply with the Formula SAE regulations, as well as to maximize the power output; 2) simulation of the vehicle in racing conditions, comparison with a conventional combustion car and a full electric vehicle. The stock engine has been strongly modified, since the head of the vertical cylinder has been replaced by the electric motor, and the intake system of the other cylinder now includes a 20 mm restrictor. Despite these constraints, the tuned ICE is able to deliver more than 70 HP. Finally, the study shows that the hybrid car is not only more efficient (-26% of specific CO2), but also 1.48 s faster on each lap than the corresponding Combustion single seater.

2019 - Experimental investigation on a diesel engine operated in RCCI combustion mode [Relazione in Atti di Convegno]
Legrottaglie, F.; Mattarelli, E.; Rinaldini, C. A.; Savioli, T.; Scrignoli, F.

Low Temperature Combustion (LTC) concepts have been investigated in many recent studies, aiming to improve engine efficiency and minimize pollutant emissions. One of the most promising techniques is represented by the Reactivity Controlled Compression Ignition (RCCI), that can be obtained combining a low reactivity fuel (such as gasoline, natural gas, ethanol, etc) and a high reactivity fuel (such as Diesel oil). The former is injected in the intake manifold, and it generates a homogeneous mixture before the start of combustion; the latter is injected directly into the combustion chamber. This technology can be easily applied to existent Diesel engines, implementing a low pressure injection system for the low-reactivity fuel. This work presents the most important results of a preliminary experimental study, conducted on a light duty Diesel engine, modified in order to operate in RCCI combustion mode. In particular, four gasoline injectors have been installed between the intercooler and the intake plenum, while the injection strategy of both fuels has been optimized, along with boost pressure. Experiments show that at low loads it is possible to substitute most of Diesel fuel with gasoline, maintaining or even improving brake thermal efficiency. This result was obtained by optimizing the Diesel fuel injection strategy, without the support of EGR. However, at medium loads, it was not possible to achieve relevant reductions of Diesel fuel, due to the high risk of knocking.

2019 - Numerical optimization of the injection strategy on a light duty diesel engine operating in dual fuel (CNG/diesel) mode [Articolo su rivista]
Cantore, G.; Mattarelli, E.; Rinaldini, C. A.; Savioli, T.; Scrignoli, Francesco

The next generation of light duty Diesel engines will face increasingly stringent emissions regulations, as well as the restrictions enforced by some local administrations. As a result, many manufacturers are starting to abandon this technology, because of the high costs and the reduced appeal on customers. On the other hand, Spark Ignition (SI) engines are not able to match the thermal efficiency of diesels, as well as their low emission of carbon dioxide (CO2): therefore, it would be highly desirable to identify cost effective solutions that permit to overcome the limits of Diesel engines, in particular soot emissions, while maintaining all the above-mentioned advantages. Dual fuel combustion, combining Natural Gas and Diesel fuel, is a well-proven technique for reducing soot emissions, while maintaining, or even increasing fuel efficiency. Moreover, this technology can be directly applied to existent Diesel engines with a few hardware modifications. However, to achieve the best results, a brand new calibration of the engine control parameters is needed. CFD-3D combustion simulation is the most cost effective tool to drive the experimental calibration process. Obviously, the numerical models must be previously calibrated against a first set of experimental data.The first part of this study, based on a previous work [9], reviews the building and experimental validation of a CFD 3D model, able to analyze this type of Dual Fuel concept applied to a current production light duty turbocharged Diesel engine, suitable for many different applications. A good agreement between simulation and experiments is found. In the second part of the paper, the calibrated model is used to investigate Dual Fuel combustion, analyzing the effects of Diesel oil injection strategies.

2019 - Potential of Electrification Applied to Non-Road Diesel Engines [Relazione in Atti di Convegno]
Mattarelli, E.; Rinaldini, C. A.; Scrignoli, F.; Fregni, P.; Gaioli, S.; Franceschini, G.; Barater, D.

The new Stage 5 European regulation for Non Road Mobile Machinery has lowered the limits on pollutant emissions for all the categories of internal combustion engines. An interesting alternative to the implementation of sophisticated after-treatment systems is to downsize the engine, and provide the extra power for peak demands with an electric motor, installed in place of the flywheel. The paper explores the potential of this concept, applied to an industrial engine, manufactured by Kohler, and delivering a maximum power of 56 kW@2600 rpm. The study is supported by a comprehensive experimental characterization of the internal combustion engine and of the electric components. A representative duty cycle is also defined, on the basis of a set of measures, taken in real operating conditions. The analysis of this reference cycle is performed by using a GT-Suite model, comparing different power split strategies. It is found that the ICE total displacement can be reduced from 2.5 to 1.9 L (from 4 to 3 cylinders), without any penalization on powertrain performance and weight. A relevant reduction of soot (22%) and NOx (16%) emissions is observed, along with a slight reduction of fuel consumption.