• Energy Research

<div class="section abstract"><div class="htmlview paragraph">Polymer Electrolyte Membrane Fuel Cells (PEMFCs) are undergoing a rapid development, due to the ever-growing interest towards their use to decarbonize power generation applications. In the transportation sector, a key technological challenge is their thermal management, i.e. the ability to preserve the membrane at the optimal thermal state to maximize the generated power. This corresponds to a narrow temperature range of 75-80°C, possibly uniformly distributed over the entire active surface. The achievement of such a requirement is complicated by the generation of thermal power, the limited exchange area for radiators, and the poor heat transfer performance of conventional coolants (e.g., ethylene glycol). The interconnection of thermal/fluid/electrochemical processes in PEMFCs renders heat rejection as a potential performance limiter, suggesting its maximization for power density increase. To this aim, suspensions of coolants and nanoparticles (nanofluids) have been proposed for PEMFCs cooling, although their characterization has often been limited to the superior thermal conductivity, overlooking a comprehensive understanding, and leaving a relevant research gap.</div><div class="htmlview paragraph">In this paper, nanofluids cooling is simulated using 3D-CFD in a small laboratory scale (25 cm²) model of a hydrogen-air PEMFC with a liquid cooling circuit. The variation of the coolant fluid is studied considering flow uniformity, heat rejection, pressure losses, and power generation, ultimately leading to a high-level analysis on the trade-off between heat transfer/storage, relevant for coolant channels in PEMFCs. The study elucidates the membrane conditions and the compositional requirements for ethylene glycol and water based nanofluids to lead to a net gain in the generated power density, modelled in the range of +5/10% for high particle loading (10%) and envisaged to reach +15% for hypothesized ideal compositions. The study clarifies the role of nanofluids for PEMFC cooling and redefines their enabler contribution in the development of high power density PEMFCs, indicating guidelines for their application-designed formulation.</div></div>

A new generation of highly downsized SI engines with specific power output around or above 150 HP/liter is emerging in the sport car market sector. Technologies such as high-boosting, direct injection and downsizing are adopted to increase power density and reduce fuel consumption. To counterbalance the increased risks of pre-ignition, knock or mega-knock, currently made turbocharged SI engines usually operate with high fuel enrichments and delayed (sometimes negative) spark advances. The former is responsible for high fuel consumption levels, while the latter induce an even lower A/F ratio (below 11), to limit the turbine inlet temperature, with huge negative effects on BSFC. A possible solution to increase knock resistance is investigated in the paper by means of 3D-CFD analyses: water/methanol emulsion is port-fuel injected to replace mixture enrichment while preserving, if not improving, indicated mean effective pressure and knock safety margins. The peak power engine operation of a currently made turbocharged GDI engine is investigated comparing the adopted fuel-only rich mixture with stoichiometric-to-lean mixtures, for which water/methanol mixture is added in the intake port under constant charge cooling in the combustion chamber and same air consumption level. In order to find the optimum fuel/emulsion balance analytic considerations are carried out. Different strategies are evaluated in terms of percentage of methanol-water emulsion rate, to assess the effects of different charge dilutions and mixture compositions on knock tendency and combustion efficiency. Thanks to the lower chemical reactivity of the diluted end gases and the faster burn rate allowed by the methanol addition, the water/methanol-injected engine allows the spark advance (SA) to be increased; as a consequence, engine power target is met, or even crossed, with a simultaneous relevant reduction of fuel consumption.

Abstract Nowadays various technical solutions have been proposed in order to improve the performance of spark-ignition internal combustion engines both at part and full load operations, especially in terms of Brake Specific Fuel Consumption (BSFC). Among the most advanced technical solutions, a fully flexible valve control system (VVA – Variable Valve Actuation) appears a very robust and reliable approach to attain the above aim. In fact advanced valve strategies, such as Early Intake Valve Closure (EIVC) and Late Intake Valve Closure (LIVC), proved to be an effective way to decrease the fuel consumption: at part load through a reduction of the pumping work and, at high load, through a knock mitigation and an over-fueling reduction. In this paper, a comparative numerical study is realized to evaluate the influence of the intake valve strategy on the performance of a small-size turbocharged spark-ignition engine. The analyzed engine is equipped with a fully flexible VVA on the intake side, based on the “lost motion” principle and able to realize both EIVC and Full Lift strategies, while the virtual modification of the intake cam profile allows for the actuation of LIVC profiles. First, a 1D model of the tested engine is developed in GT-Power™ framework. It is integrated with in-house developed sub-models for the description of in-cylinder phenomena, including turbulence, combustion, knock and heat transfer. The adopted approach is validated against 3D turbulence results, measured global performance parameters and in-cylinder pressure cycles. The consistency of the proposed approach, without requiring any case-dependent tuning, is demonstrated at various speeds, loads and intake valve strategies. The validated engine model is used to perform a parametric analysis for different intake valve closure angles in two representative operating points at full and part load. The results point out that both EIVC and LIVC induce an improved fuel consumption with respect to a conventional Full Lift valve strategy. EIVC proves to be more effective at part load than LIVC, while similar BSFC advantages are obtained at high load. The proposed approach, based on refined sub-models for in-cylinder phenomena description, shows the capability to predict the effects of advanced valve strategies, making the implementation of a “virtual” calibration of a VVA engine possible.

Heat Transfer plays a fundamental role in internal combustion engines, as able to affect several aspects, such as efficiency, emissions and reliability. As for this last, a proper heat transfer prediction is mandatory for the estimation of the engine temperatures at peak power condition, it being the most critical one from a thermal point of view. At part-load/low revving speed operations, heat transfer is detrimental for the engine efficiency, deeply reducing indicated work of the burnt gases on the piston.Focusing on the in-cylinder domain, 3D-CFD simulations represent an irreplaceable tool for the estimation of gas-to-wall heat fluxes. Several models have been developed in the past, aiming at providing a reliable estimation of the heat transfer at any condition in terms of load and revving speed. To save computational cost and time, the most diffused wall approach for the numerical simulation of confined reacting flows is the high-Reynolds one, which means that heat transfer model is based on a thermal wall function. Unfortunately, wall functions (logarithmic profiles of the inertial layer) can be claimed only at restricted conditions, such as isothermal steady-state flow, velocity parallel to the wall and negligible pressure gradient. In practice, none of these assumptions is valid for industrial applications such as an in-cylinder simulation. Therefore in these cases, as demonstrated by different works in the past, wall functions do not exist and their adoption leads to a non-negligible error in the estimation of the heat transfer.The main goal of this work is to build up a methodology able to investigate the presence of wall functions in actual industrial applications, in particular in 3D-CFD in-cylinder analyses. Compared to previous works available in literature, where DNS or LES are carried out on simplified geometries and/or at low revving speed conditions because of the computational cost, in the present paper a RANS approach to turbulence and a low-Reynolds wall treatment are adopted. Moreover, a new strategy to obtain dimensionless profiles of velocity and temperature from computed fields is introduced. At first, the proposed methodology is validated on a 2D plane channel. Then, a preliminary application on a research engine, namely the GM Pancake engine, is proposed, showing that dimensionless profiles of velocity and temperature calculated on the combustion chamber walls are remarkably different from standard analytical wall functions.

The paper proposes a methodology to perform sloshing analyses through multidimensional Computational Fluid Dynamics (CFD), with particular focus on a lubricant tank of a high-performance sports car. Lubricant tanks are usually fed by a mixture of oil and air, which makes Volume of Fluid (VoF) models unsuitable for this kind of simulation. Hence, a different approach based on a Eulerian MultiPhase (EMP) model is investigated and adopted. In contrast to the VoF approach, which is the most consolidated technique to handle the numerical analysis of sloshing problems, the EMP accounts for interactions between liquid and gaseous phases, such as mixing and separation. It also reduces numerical constraints on time-step and mesh size. EMP is therefore applied to the analysis of a sports car lubricant tank where mist and foam formation and subsequent phase separation are of primary importance. Comparison between the EMP and VoF approach is performed on cases of increasing complexity. Firstly, a rectangular tank with internal baffles and under pitch oscillations, for which experimental measurements are available, is analyzed. The EMP approach shows improved responsiveness in representing both phase mixing and separation. Secondly, a current production lubricant oil tank, for which experimental test-rig measurements of foam percentage shortly upstream the feeding pump are available, confirms the ability of the EMP approach to quantitatively estimate foam formation. Thirdly, the analysis of a current production lubricant oil tank subject to typical racetrack maneuvers is performed. Such final step confirms the ability of the EMP approach to simulate complex interactions between the phases, which largely affect tank and lubricating circuit performance in high-performance sports car applications. Moreover, the EMP approach allows a massive reduction of computational time compared to VoF.

Abstract Last generation spark ignition (SI) engines are characterized by a simultaneous reduction of the engine displacement and an increase of the brake power; such conflicting targets are achieved through the adoption of several techniques such as turbocharging, direct fuel injection, variable valve timing and variable port lengths. This design approach, referred to as “engine downsizing”, leads to a remarkable increase in the thermal loads acting on the engine components facing the combustion chamber. Hence, an accurate evaluation of the thermal field is of primary importance in order to avoid thermo-mechanical failures. Moreover, the correct evaluation of the temperature distribution improves the prediction of point-wise abnormal combustion onset. Due to the complexity of the experimental measurement of instantaneous gas-to-wall heat fluxes, 3D-CFD simulations of the in-cylinder processes are a fundamental tool to evaluate not only the global amount of heat transferred to the combustion chamber walls, but also its point-wise distribution. Several heat transfer models and thermal laws of the wall are available in literature, most of which were developed in the past decades and calibrated against experiments carried out in research laboratories at relatively low-load/low-speed engine operations. In the present paper two widely adopted heat transfer models are proved to be effective at such conditions to predict gas-to-wall heat flux, as demonstrated by their application to the well-known GM pancake engine test case. However, despite such comforting results, they manifest evident shortages when used for highly-charged/highly-downsized current production SI engines, since operated at specific thermal loads and engine speeds very different from the above experiments. In particular, overestimations of the wall heat transfer predicted by such thermal laws of the wall are pointed out thanks to experimental engine thermal surveys and temperature measurements on four current production engines. Therefore an alternative heat transfer model is proposed by the authors and tested on such currently made turbocharged SI engines, operated at different conditions. Compared to the existing models differences are pointed out, especially in terms of law of the wall expression. Experimental engine thermal survey and point-wise temperature measurements are used to validate the numerical heat flux. In particular the increased predictive capabilities of the 3D-CFD gas-to-wall heat transfer simulations are revealed both in terms of global thermal balance and temperature distribution of the metal for all the investigated engines. In fact model adoption in a combined in-cylinder/CHT (Conjugate Heat Transfer) simulation loop leads to a correct characterization of the thermal status of all the analyzed engines. Finally, alternative model adoption for the investigated current production high specific power DISI turbocharged engines operated at full load and high revving speed is critically motivated adopting the “isothermicity parameter” ζ which represents an indication of the thermal state of the boundary layer, being a characteristic scale of the ratio between gas and wall temperatures.

Abstract A novel methodology for the estimation of the mass flow rate delivered by each hole of a multi-hole GDI nozzle is presented and discussed in this paper. The proposed method is based on the measurement of the impact force of each jet and it is able to evaluate the individual hole injection rate and injected mass using the relationship between the spray momentum flux and the mass flow rate. Three different nozzles are tested, one featuring equal hole diameters and two with differentiated hole-to-hole dimensions. Firstly, results are validated in terms of total injection rate, comparing the sum of the individual hole flow rates with the signal from a Zeuch-based flow meter. Secondly, outcomes are compared with the direct measurement of the injected quantity by means of a special device able to collect and weight the fuel delivered from each hole. Results evidence an excellent agreement in terms of mass flow rate dynamic profile as the proposed method is able to detect both opening and closure transients and the static flow rate. The proposed method proved to be able to capture the dynamic mass flow rate in transient conditions, i.e. very short injections, overcoming a limitation of the methodology proposed in a previous study. Moreover, in terms of injected mass, the results show a percentage error lower than 5% for medium and long energizing times and a maximum error of 9.5% for short injections in ballistic operating conditions.

AbstractDownsizing is a must for current high performance turbocharged SI engines. This is often achieved through the reduction of cylinder number, while keeping constant unit displacement and increasing boost pressure. However, the ensuing higher loads strongly increases the risk of abnormal combustion and thermo-mechanical failures. An alternative path to downsizing is the reduction of cylinder bore: this approach is more expensive, requiring a brand new design of the combustion system, but it also provides some advantages.The goal of the present paper is to explore the potential of bore reduction for achieving a challenging downsizing target, while preserving the engine knock safety margins.A current V8 GDI turbocharged sporting engine is taken as a reference, and a preliminary CFD-3D analysis is carried out in order to define the most suitable bore-to-stroke ratio. On this basis, bore is reduced by 11% at constant stroke, thus obtaining a reduction of about 20% on the engine displacement.In order to achieve the same peak power target, both engine boost and spark advance are adjusted until the knock safety margin of the original engine is met. 3D CFD tools, accurately calibrated on the reference engine, are used to address engine design and the calibration of the operating parameters.