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SEBASTIANO BREDA

Ricercatore t.d. art. 24 c. 3 lett. A
Dipartimento di Ingegneria "Enzo Ferrari"

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Pubblicazioni

2023 - A MATLAB/Simulink Model of a PEM Fuel Cell System Including Ageing Phenomenon [Relazione in Atti di Convegno]
Corda, G.; Breda, S.; D'Adamo, A.
abstract

This paper presents a numerical model of a Polymer Electrolyte Membrane Fuel Cell (PEMFC) system reproducing an automotive-type powertrain. The 0D model is developed in MATLAB/Simulink environment, and it incorporates all the main auxiliary components (air and hydrogen supply line, cooling circuit) as well as the PEMFC stack unit. The model includes an ageing model to estimate the PEMFC stack degradation over time, resulting in progressive efficiency loss as well as in increased auxiliary power and thermal dissipation demand. The presented model enables the estimation of both PEMFC duration and of the time-varying request of heat rejection, facilitating the selection of auxiliaries to optimize the lifelong performance. The model constitutes the backbone for the design and optimization of PEMFC systems for automotive applications, and the integration with a degradation model provides a comprehensive research tool to estimate the long-term performance and lifetime of PEMFC system.

2023 - Impact of fuel surrogate formulation on the prediction of knock statistics in a single cylinder GDI engine [Articolo su rivista]
Fontanesi, S.; Shamsudheen, F. A.; Gonzalez, E. G.; Sarathy, S. M.; Berni, F.; D'Adamo, A.; Borghi, M.; Breda, S.
abstract

The statistical tendency of an optically accessible single-cylinder direct-injection spark-ignition engine to undergo borderline/medium knocking combustion is investigated using 3D-CFD. Focus is made on the role of fuel surrogate formulation for the characterization of anti-knock quality and flame speed of the actual fuel. An in-house methodology is used to design surrogates able to emulate laminar flame speed and autoignition delay times of the injected fuel. Two different surrogates, characterized by increasing level of complexity, are compared. The most complex one (six components) improves the representation of the real fuel, highlighting the crucial role of accurate fuel kinetics to predict flame propagation and unburnt mixture reactivity. A devoted chemical mechanism including the oxidation pathways for all the species in the surrogate is also purposely developed for the current analysis. Knock is investigated using a proprietary statistical knock model (GruMo-UNIMORE Statistical Knock Model, GK-PDF), which can infer the probability of knocking events within a RANS formalism. Predicted statistical distributions are compared to measured counterparts. The proposed numerical/experimental comparison demonstrates the possibility to efficiently integrate complex-chemistry driven information in 3D-CFD combustion simulations without online solving chemical reactions: a combination of laminar flame speed correlations, ignition delay look-up tables, and a statistics-based knock model is adopted to estimate the percentage of knocking cycles in a GDI engine while limiting the computational cost of the simulations.

2021 - A Simple CFD Model for Knocking Cylinder Pressure Data Interpretation: Part 1 [Relazione in Atti di Convegno]
Corrigan, D. J.; Breda, S.; Fontanesi, S.
abstract

Knock is one of the main limitations on Spark-Ignited (SI) Internal Combustion Engine (ICE) performance and efficiency and so has been the object of study for over one hundred years. Great strides have been made in terms of understanding in that time, but certain rather elementary practical problems remain. One of these is how to interpret if a running engine is knocking and how likely this is to result in damage. Knocking in a development environment is typically quantified based on numerical descriptions of the high frequency content of a cylinder pressure signal. Certain key frequencies are observed, which Draper [1] explained with fundamental acoustic theory back in 1935. Since then, a number of approaches of varying complexity have been employed to correlate what is happening within the chamber with what is measured by a pressure transducer. Whilst such phenomena can be well described by 3D Computational Fluid Dynamics (CFD) with moving meshes, small time-steps and chemical kinetics, such an approach is computationally intensive. Analytical calculations or Finite Element Methods (FEM) on the other hand, can estimate modal frequencies but not their likelihood of occurrence. In the present work, a simple stationary 3D CFD model, taking inspiration from an experiment by Draper [1] in 1934, is implemented in STAR CCM+ software. One or more autoignition events are simulated, and the corresponding frequency spectra and modal pressure distributions are described. It is shown that the model can reproduce the expected knocking frequencies from numerical analysis and experimental data. Sensitivity to autoignition and pressure transducer location is commented upon. Time Frequency Analysis (TFA) is applied to moving mesh data and demonstrates that little accuracy is lost in considering the stationary case. The current model is considered to be an appropriate means for analysis of knocking cycles with trace and moderate intensity, and can be used to bridge the gap between what is measured by a pressure transducer and what is occurring in the combustion chamber.

2021 - Combustion modelling of turbulent jet ignition in a divided combustion chamber [Articolo su rivista]
Olcuire, M.; Iacovano, C.; D'Adamo, A.; Breda, S.; Lucchini, T.; Fontanesi, S.
abstract

Turbulent jet ignition is seen as one of the most promising strategies to achieve stable lean-burn operation in modern spark-ignition engines thanks to its ability to promote rapid combustion. A nearly stoichiometric mixture is ignited in a small-volume pre-chamber, following which multiple hot turbulent jets are discharged in the main chamber to initiate combustion. In the present work, a detailed computational investigation on the turbulent combustion regime of premixed rich propane/air mixture in a quiescent divided chamber vessel is carried out, to study the characteristics of the jet flame without the uncertainties in mixing and turbulent conditions typical of real-engine operations. In particular, the paper investigates the dependency of flame propagation on nozzle diameter (4, 6, 8, 12 and 14 mm) and pre-chamber/main-chamber volume ratio (10% and 20%); CFD results are compared to the experimental outcomes. Results show that the combustion regime in the quiescent pre-chamber follows a well-stirred reaction mode, rendering the limitation in using conventional flamelet combustion models. Furthermore, due to the very high turbulence levels generated by the outflowing reacting jets, also the main chamber combustion develops in a well-stirred reactor type, confirming the need for a kinetics-based approach to combustion modelling. However, the picture is complicated by thickened flamelet conditions possibly being verified for some geometrical variations (nozzle diameter and pre-chamber volume). The results show a general good alignment with the experimental data in terms of both jet phasing and combustion duration, offering a renewed guideline for combustion simulations under quiescent and low Damköhler number conditions.

2020 - Application of the Sectional Method to Investigate Particle Number and Soot Mass in Ethanol and Gasoline Fueled Premixed Spark Ignition Engines [Relazione in Atti di Convegno]
Pessina, V.; Del Pecchia, M.; Breda, S.; Dalseno, L.; Borghi, M.
abstract

Emission modelling is still a timely topic in the engine research community. Soot emission reduction has gained its spotlight among the pollutants-related issues mainly due to the renewed interest in Gasoline Direct Injection. The conjunction of experimental measurements and numerical investigations provides an effective tool to cope with the constant evolution of the emission regulations. Thus, numerical models must be validated over a wide range of engine operating points and fuels. To this aim, the Sectional Method was applied to investigate Particulate Matter and Particle Number produced during combustion in a premixed spark ignition engine using 3D-CFD. Soot-related quantities were investigated for different values of equivalence ratio (from 1.0 up to 1.5) as well as for different fuels. Three different fuel types were examined: a commercial nonoxygenated American gasoline (TIER-2), a commercial Chinese gasoline (CHINA-6) with ethanol 10 %vol and pure Ethanol (E100). A detailed chemistry-based tabulated approach was exploited to compute a dedicated soot library, for each of the analyzed fuels, by means of 0D chemical kinetic simulations using a constant pressure reactor approach. Numerical results were compared to a database of experimental measurements collected from literature. The sooting tendency threshold dependency on equivalence ratio was also investigated and the results showed that the ethanol is the less sooting among the examined fuels, while the non-oxygenated gasoline exhibited the highest soot mass and Particle Number. This paper provides a CFD-based benchmark for soot mass and Particle Number for three fuel types with largely different chemical nature.

2020 - Development of a Sectional Soot Model Based Methodology for the Prediction of Soot Engine-Out Emissions in GDI Units [Relazione in Atti di Convegno]
Del Pecchia, M.; Sparacino, S.; Pessina, V.; Fontanesi, S.; Breda, S.; Irimescu, A.; Di Iorio, S.
abstract

With the aim of identifying technical solutions to lower the particulate matter emissions, the engine research community made a consistent effort to investigate the root causes leading to soot formation. Nowadays, the computational power increase allows the use of advanced soot emissions models in 3D-CFD turbulent reacting flows simulations. However, the adaptation of soot models originally developed for Diesel applications to gasoline direct injection engines is still an ongoing process. A limited number of studies in literature attempted to model soot produced by gasoline direct injection engines, obtaining a qualitative agreement with the experiments. To the authors' best knowledge, none of the previous studies provided a methodology to quantitatively match particulate matter, particulate number and particle size distribution function measured at the exhaust without a case-by-case soot model tuning. In the present study, a Sectional Method-based methodology to quantitatively predict gasoline direct injection soot formation is presented and validated against engine-out emissions measured on a single-cylinder optically accessible gasoline direct injection research engine. While adapting the model to the gasoline direct injection soot framework, attention is devoted to modelling the dependence of the processes involved in soot formation on soot precursors chemistry. A well-validated chemical kinetics mechanism is chosen to accurately predict soot precursors formation pathways retaining an accurate description of the main oxidation pathways for oxygenated fuel surrogates. To account for the prominent premixed combustion mode characterizing modern GDI units, a constant pressure reactor library is generated containing the rates for the chemistry-based processes involved in soot formation and evolution at engine-like conditions. The proposed methodology is successfully applied to a 3D computational fluid dynamics model of the engine to predict soot engine-out emissions at the exhaust.

2020 - Partial versus radical nephrectomy in very elderly patients: a propensity score analysis of surgical, functional and oncologic outcomes (RESURGE project) [Articolo su rivista]
Mir, M. C.; Pavan, N.; Capitanio, U.; Antonelli, A.; Derweesh, I.; Rodriguez-Faba, O.; Linares, E.; Takagi, T.; Rha, K. H.; Fiori, C.; Maurer, T.; Zang, C.; Mottrie, A.; Umari, P.; Long, J. -A.; Fiard, G.; De Nunzio, C.; Tubaro, A.; Tracey, A. T.; Ferro, M.; De Cobelli, O.; Micali, S.; Bevilacqua, L.; Torres, J.; Schips, L.; Castellucci, R.; Dobbs, R.; Quarto, G.; Bove, P.; Celia, A.; De Concilio, B.; Trombetta, C.; Silvestri, T.; Larcher, A.; Montorsi, F.; Palumbo, C.; Furlan, M.; Bindayi, A.; Hamilton, Z.; Breda, A.; Palou, J.; Aguilera, A.; Tanabe, K.; Raheem, A.; Amiel, T.; Yang, B.; Lima, E.; Crivellaro, S.; Perdona, S.; Gregorio, C.; Barbati, G.; Porpiglia, F.; Autorino, R.
abstract

Purpose: To compare the outcomes of PN to those of RN in very elderly patients treated for clinically localized renal tumor. Patients and methods: A purpose-built multi-institutional international database (RESURGE project) was used for this retrospective analysis. Patients over 75 years old and surgically treated for a suspicious of localized renal with either PN or RN were included in this database. Surgical, renal function and oncological outcomes were analyzed. Propensity scores for the predicted probability to receive PN in each patient were estimated by logistic regression models. Cox proportional hazard models were estimated to determine the relative change in hazard associated with PN vs RN on overall mortality (OM), cancer-specific mortality (CSM) and other-cause mortality (OCM). Results: A total of 613 patients who underwent RN were successfully matched with 613 controls who underwent PN. Higher overall complication rate was recorded in the PN group (33% vs 25%; p = 0.01). Median follow-up for the entire cohort was 35 months (interquartile range [IQR] 13–63 months). There was a significant difference between RN and PN in median decline of eGFR (39% vs 17%; p < 0.01). PN was not correlated with OM (HR = 0.71; p = 0.56), OCM (HR = 0.74; p = 0.5), and showed a protective trend for CSM (HR = 0.19; p = 0.05). PN was found to be a protective factor for surgical CKD (HR = 0.28; p < 0.01) and worsening of eGFR in patients with baseline CKD. Retrospective design represents a limitation of this analysis. Conclusions: Adoption of PN in very elderly patients with localized renal tumor does not compromise oncological outcomes, and it allows better functional preservation at mid-term (3-year) follow-up, relative to RN. Whether this functional benefit translates into a survival benefit remains to be determined.

2019 - Comparison of library-based and detailed chemistry models for knock prediction in spark-ignition engines [Relazione in Atti di Convegno]
Cicci, Francesco; D’Adamo, Alessandro; Barbato, Alessio; Breda, Sebastiano
abstract

The present engine development pathway for increased specific power and efficiency is moving Spark-Ignition engines towards unprecedented levels of mean thermo-mechanical loading. This in turn promotes undesired abnormal combustion events in the unburnt mixture (also called “engine knock”), leading to solid parts failure and constituting a severe upper constraint to engine efficiency. In this context, CFD simulations are regularly used to investigate the fluid-dynamic reasons for engine knock and to address knock suppression strategies, using dedicated models to simulate the chemical reaction rate of the fuel/air/residual mixture at the same thermodynamics states as those encountered in engines. In this paper three different approaches are coherently compared to simulate knock occurrence on a turbocharged GDI engine, representing some of the most popular choices for modelers in the RANS framework. The first one considers the on-the-fly solution of chemical reactions, which represents the state-of-the-art knock modelling approach albeit its problematic computational cost for industrial turnaround times. The other two methods consider pre-calculated libraries of ignition delay times (calculated at constant pressure and volume, respectively) for the same fuel model, and knock timing is predicted using a classical Livengood-Wu approach coupled to the same main combustion model. All the analyzed models for the end-gas reaction rate are coupled with a dedicated combustion model for propagating flame (G-equation). A comprehensive analysis of computational cost and of knock prediction accuracy is carried out for library-based methods against the detailed chemistry model. Finally, results are critically discussed and explained using combined ignition delay time maps and traces for thermodynamic in-cylinder states, and guidelines for the a priori choice for constant pressure- or volume-generated libraries are provided. In this context, the use of a synthetic knock model combined with libraries of ignition delays calculated at constant volume emerges as an accurate and efficient modelling strategy. The study outlines a method for the well-supported use of simplified CPU-efficient models, with a promoted confidence in simulation results from the comparison with detailed chemistry.

2019 - Experimental and numerical study on the adoption of split injection strategies to improve air-butanol mixture formation in a DISI optical engine [Articolo su rivista]
Breda, S.; D'Orrico, F.; Berni, F.; d'Adamo, A.; Fontanesi, S.; Irimescu, A.; Merola, S. S.
abstract

Gasoline replacement with alternative non-fossil fuels compatible with existing units is widely promoted around the world to reduce the dependency on oil-based products by adopting domestic renewable sources. In this context, the possibility to obtain bio-alcohols from non-edible residues of food and plants is particularly attractive for gasoline replacement in SI (Spark Ignition) engines. Such bio-fuels are characterized by higher laminar flame speed (LFS) and octane rating, resulting in improved thermal efficiency and reduced regulated emissions. Low-carbon alcohols (e.g. ethanol) are disadvantageous as gasoline replacement due to poor energy density and high corrosive action on distribution pipelines, whereas high-carbon ones (e.g. n-butanol) are particularly promising candidates thanks to the physical properties and the energy density closer to those of gasoline. High latent heat of vaporization and low saturation pressure are the most relevant weaknesses of n-butanol related to gasoline replacement in DISI (Direct Injection SI) power units. On equal injection pressure and phasing, the slow evaporation rate of n-butanol leads to poor mixture preparation and larger fuel deposits. In particular, this is emphasized by low charge and wall temperatures during part load operation, reducing combustion efficiency and promoting the formation of pollutant particles. Split injection is a promising strategy to improve charge preparation contemporary reducing fuel deposits and improving mixture homogeneity, mostly for low-evaporating fuels. In the present work different split injection strategies are tested in an optically accessible SI engine fueled with n-butanol and simulated through CFD with the aim of identifying trends and understanding the root causes behind measured behaviors. CFD simulations help in understanding changes in charge stratification using different injection strategies, allowing to explain both combustion behavior and soot formation tendency from the analysis of fuel distribution. Mixture quality in the spark region and the presence of very rich mixture pockets in the combustion chamber are identified as the most critical aspects that should be optimized when changing the injection strategy; this in turn contributes to avoid slow burn rates or excessive soot production during operation with low evaporating fuels such as n-butanol. A strong correlation between diffusive flames and rich mixture pockets is found in terms of both location and intensity, proving the first order role of fuel deposits formation and mixture homogenization on both combustion development and soot formation.

2019 - Outcomes of Partial and Radical Nephrectomy in Octogenarians – A Multicenter International Study (Resurge) [Articolo su rivista]
Antonelli, A.; Veccia, A.; Pavan, N.; Mir, C.; Breda, A.; Takagi, T.; Rha, K. H.; Maurer, T.; Zhang, C.; Long, J. -A.; De Nunzio, C.; Lima, E.; Ferro, M.; Micali, S.; Quarto, G.; Linares, E.; Celia, A.; Schips, L.; Bove, P.; Larcher, A.; Fiori, C.; Mottrie, A.; Bindayi, A.; Trombetta, C.; Silvestri, T.; Palou, J.; Faba, O. R.; Tanabe, K.; Yang, B.; Fiard, G.; Tubaro, A.; Torres, J. N.; De Cobelli, O.; Bevilacqua, L.; Castellucci, R.; Tracey, A.; Hampton, L. J.; Montorsi, F.; Perdona, S.; Simeone, C.; Palumbo, C.; Capitanio, U.; Derweesh, I.; Porpiglia, F.; Autorino, R.
abstract

OBJECTIVE: To analyze the outcomes of partial nephrectomy (PN) and radical nephrectomy (RN) in octogenarian patients. METHODS: The RESURGE (REnal SUrgery in the Eldely) multi-institutional database was queried to identify patients ≥80 years old who had undergone a PN or RN for a renal tumor. Multivariable binary logistic regression estimated the association between type of surgery and occurrence of complications. Multivariable Cox regression model assessed the association between type of surgery and All-Causes Mortality. RESULTS: The study analyzed 585 patients (median age 83 years, IQR 81-84), 364 of whom (62.2%) underwent RN and 221 (37.8%) PN. Patients undergoing RN were older (P = .0084), had larger tumor size (P < .0001) and higher clinical stage (P < .001). At multivariable analysis for complications, the only significant difference was found for lower risk of major postoperative complications for laparoscopic RN compared to open RN (OR: 0.42; P = .04). The rate of significant (>25%) decrease of eGFR in PN and RN was 18% versus 59% at 1 month, and 23% versus 65% at 6 months (P < .0001). After a median follow-up time of 39 months, 161 patients (31%) died, of whom 105 (20%) due to renal cancer. CONCLUSION: In this patient population both RN and PN carry a non-negligible risk of complications. When surgical removal is indicated, PN should be preferred, whenever technically feasible, as it can offer better preservation of renal function, without increasing the risk of complications. Moreover, a minimally invasive approach should be pursued, as it can translate into lower surgical morbidity.

2019 - The potential of statistical RANS to predict knock tendency: Comparison with LES and experiments on a spark-ignition engine [Articolo su rivista]
D'Adamo, A.; Breda, S.; Berni, F.; Fontanesi, S.
abstract

Pollutant regulations and fuel consumption concerns are the driving guidelines for increased thermal efficiency and specific power in current internal combustion engines. The achievement of such challenging tasks in Spark Ignition units is often limited by the onset of knock, which hinders the possibility to operate the engine with the optimal combustion phasing. The sporadic occurrence of individual knocking events is related to cycle-to-cycle variability of turbulent combustion. This is avoidable by only accepting a safety margin from its earliest onset. On one side, the stochastic nature of knock and turbulence-related combustion variability would indicate Large-Eddy Simulation (LES) as the most appropriate technique for CFD analyses. Nevertheless, Large-Eddy Simulation remains a very time- and CPU-demanding approach, hardly integrated in the industrial timeframe for the design exploration and development of new units. Therefore, Reynolds Averaged Navier Stokes (RANS) models representing the average flow are chosen to limit CPU and development times, though they suffer from the intrinsic inability to account for cycle-dependent phenomena (e.g. knock). This is particularly critical at knock-borderline conditions, where far-from-average knocking events may occur. A previously developed statistical RANS-PDF knock model partly overcomes this limitation using equations for mixture fraction and enthalpy variance, ultimately reconstructing log-normal distributions of knock intensity. This allows RANS simulations to be directly compared to the usual statistical knock analysis at the test-bench. In this paper all the mentioned modelling techniques (LES, RANS and RANS-PDF) are applied to simulate combustion and knock in a currently made turbocharged GDI engine under knock-safe, knock-limited and light-knocking conditions. The study relevance lays in the critical comparison of the results. The full potential of the statistical RANS-PDF model for engine development is highlighted on a coherent basis. The possibility to preserve the RANS formalism while enriching the results with knock statistical description is a relevant advancement in the virtual design of high-efficiency engines.

2019 - Validation of a sectional soot model based on a constant pressure tabulated chemistry approach for PM, PN and PSDF estimation in a GDI research engine [Relazione in Atti di Convegno]
Del Pecchia, M.; Sparacino, S.; Breda, S.; Cantore, G.
abstract

Findings from the International Agency for Research on Cancer (IARC) classified particulate matter (PM) as carcinogenic to humans. While being a promising solution to reduce greenhouse gases (GHG) emissions and increase engine fuel economy, Gasoline Direct Injected (GDI) engines produce a number of particles (PN) of fine size higher than Port Fuel Injected (PFI) ones. As a consequence, the EU commission significantly tightened the emission standards for passenger cars, following which all gasoline engines will have to meet the euro-6d regulation coming into force in 2020. Efforts are made by the research community to understand the root causes leading to soot formation and possibly identify technical solutions to lower it. An important piece of the puzzle is the investigation of soot formation via 3D-CFD. To this aim, relevant efforts have been and are still being paid to adapt soot emissions models, originally developed for Diesel combustion, for GDI units. Among the many available models, one of the most advanced is the so-called Sectional Method. So far, studies presented in literature were not able to formulate a methodology to quantitatively match experimental PM, PN and PSDF without a dedicated soot model tuning. In the present work, a Sectional Method-based methodology to quantitatively predict GDI soot is presented and validated against PM, PN and PSDF measurements on a optically accessible GDI research unit. While adapting the model to GDI soot, attention is devoted to the modelling of soot precursor chemistry: a customized version of a pre-existing chemical kinetics mechanism, used to predict the formation of the key PAH (Polycyclic Aromatic Hydrocarbons) species, is presented and validated via 1D numerical simulations on a premixed flat flame burner dataset available in literature. The present work demonstrates that a Sectional Method-based approach can be a powerful tool to quantitatively predict engine-out soot emissions.

2018 - Development of Chemistry-Based Laminar Flame Speed Correlation for Part-Load SI Conditions and Validation in a GDI Research Engine [Articolo su rivista]
Del Pecchia, Marco; Breda, Sebastiano; D'Adamo, Alessandro; Fontanesi, Stefano; Irimescu, Adrian; Merola, Simona
abstract

The detailed study of part-load conditions is essential to characterize engine-out emissions in key operating conditions. The relevance of part-load operation is further emphasized by the recent regulation such as the new WLTP standard. The combustion development at part-load operations depends on a complex interplay between moderate turbulence levels (low engine speed and tumble ratio), low in-cylinder pressure and temperature and stoichiometric-to-lean mixture quality (to maximize fuel efficiency at partial loads). From a modelling standpoint, the reduced turbulence intensity compared to full-load operations complicates the interaction between different sub-models (e.g. re-consideration of the flamelet hypothesis adopted by common combustion models). In this paper, the authors focus on chemistry-based simulations for laminar flame speed of gasoline surrogates at conditions typical of part-load operations. The analysis is an extension of a previous study focused on full-load operations of a methodology based on detailed chemistry 1D simulations of the flame structure. The comparison with the previous research reveals that flames at partial loads experience analogous temperature levels, despite the generally lower pressure. Therefore, particular attention will be devoted to the temperature scaling of flame speed, as well as to the extension to lean mixtures. The proposed correlation is applied to simulate the combustion development on a single-cylinder research engine operated at a 0.7 bar absolute pressure part-load condition provided with an optical access to the combustion chamber. The experimental data derived by the aforementioned kind of equipment allows a detailed description of the flame development since early flame kernel growth and, therefore, the role of an accurate laminar flame speed modelling is discussed in details. The correlation for laminar flame speed proposed by the authors constitutes a useful reference for similar studies and it can be used in conjunction with the most common CFD combustion models.

2018 - Understanding the origin of cycle-to-cycle variation using large-eddy simulation: Similarities and differences between a homogeneous low-revving speed research engine and a production DI turbocharged engine [Articolo su rivista]
D'Adamo, Alessandro; Breda, Sebastiano; Berni, Fabio; Fontanesi, Stefano
abstract

A numerical study using large-eddy simulations (LES) to reproduce and understand sources of cycle-to-cycle variation (CCV) in spark-initiated internal combustion engines (ICEs) is presented. Two relevantly different spark-ignition (SI) units, that is, a homogeneous-charge slow-speed singlecylinder research unit (the transparent combustion chamber (TCC)-III, Engine 1) and a stratifiedcharge high-revving speed gasoline direct injection (GDI) (Engine 2) one, are analyzed in fired operations. Multiple-cycle simulations are carried out for both engines and LES results well reproduce the experimentally measured combustion CCV. A correlation study is carried out, emphasizing the decisive influence of the early flame period variability (1% of mass fraction burnt (MFB1)) on the entire combustion event in both ICEs. The focus is moved onto the early flame characteristics, and the crucial task to determine the dominant causes of its variability (if any) is undertaken. A two-level analysis is carried out: the influence of global parameters is assessed at first; second, local details in the ignition region are analyzed. A comparison of conditions at combustion onset is carried out and case-specific leading factors for combustion CCV are identified and ranked. Finally, comparative simulations are presented using a simpler flame deposition ignition model: the simulation flaws are evident due to modeling assumptions in the flame/flow interaction at ignition. The relevance of this study is the knowledge extension of turbulence-driven phenomena in ICEs allowed by advanced CFD (Computational Fluid Dynamics) simulations. The application to different engine types proves the soundness of the used models and it confirms that CCV is based on enginespecific factors. Simulations show how CCV originates from the interplay of small- and large-scale factors in Engine 1, due to the lack of coherent flows, whereas in Engine 2 the dominant CCV promoters are local air-to-fuel ratio (AFR) and flow velocity at ignition. This confirms the absence of a generally valid ranking, and it demonstrates the use of LES as a development and designorienting tool for next-generation engines.

2017 - A RANS knock model to predict the statistical occurrence of engine knock [Articolo su rivista]
D'Adamo, Alessandro; Breda, Sebastiano; Fontanesi, Stefano; Irimescu, Adrian; Merola, Simona Silvia; Tornatore, Cinzia
abstract

In the recent past engine knock emerged as one of the main limiting aspects for the achievement of higher efficiency targets in modern spark-ignition (SI) engines. To attain these requirements, engine operating points must be moved as close as possible to the onset of abnormal combustions, although the turbulent nature of flow field and SI combustion leads to possibly ample fluctuations between consecutive engine cycles. This forces engine designers to distance the target condition from its theoretical optimum in order to prevent abnormal combustion, which can potentially damage engine components because of few individual heavy-knocking cycles. A statistically based RANS knock model is presented in this study, whose aim is the prediction not only of the ensemble average knock occurrence, poorly meaningful in such a stochastic event, but also of a knock probability. The model is based on look-up tables of autoignition times from detailed chemistry, coupled with transport equations for the variance of mixture fraction and enthalpy. The transported perturbations around the ensemble average value are based on variable gradients and on a local turbulent time scale. A multi-variate cell-based Gaussian-PDF model is proposed for the unburnt mixture, resulting in a statistical distribution for the in-cell reaction rate. An average knock precursor and its variance are independently calculated and transported; this results in the prediction of an earliest knock probability preceding the ensemble average knock onset, as confirmed by the experimental evidence. The proposed model estimates not only the regions where the average knock is promoted, but also where and when the first knock is more likely to be encountered. The application of the model to a RANS simulation of a modern turbocharged direct injection (DI) SI engine with optical access is presented and the analysis of the knock statistical occurrence obtained by the proposed model adds an innovative contribution to overcome the limitation of consolidated “average knock” analyses typical of a RANS approach.

2017 - CFD Optimization of n-Butanol Mixture Preparation and Combustion in an Research GDI Engine [Relazione in Atti di Convegno]
Breda, Sebastiano; D'Adamo, Alessandro; Fontanesi, Stefano; Del Pecchia, Marco; Merola, Simona; Irimescu, Adrian
abstract

The recent interest in alternative non-fossil fuels has led researchers to evaluate several alcohol-based formulations. However, one of the main requirements for innovative fuels is to be compatible with existing units' hardware, so that full replacement or smart flexible-fuel strategies can be smoothly adopted. n-Butanol is considered as a promising candidate to replace commercial gasoline, given its ease of production from bio-mass and its main physical and chemical properties similar to those of Gasoline. The compared behavior of n-butanol and gasoline was analyzed in an optically-accessible DISI engine in a previous paper [1]. CFD simulations explained the main outcomes of the experimental campaign in terms of combustion behavior for two operating conditions. In particular, the first-order role of the slower evaporation rate of n-butanol compared to gasoline was highlighted when the two fuels were operated under the same injection phasing. The poor n-butanol/air mixture homogeneity was found to be a major limiting factor on the potential benefit of the use of n-butanol. This outcome is further deepened in this paper by numerically exploring different mixture preparation strategies for n-butanol. To this aim, variations of the injection phasing and profile are analyzed, including the use of multiple injection strategies. An optimized fuel injection strategy is then numerically identified considering mixture homogeneity, engine torque output and tailpipe emissions of soot and NOx. In order to confirm the validity of the CFD approach, this strategy is experimentally tested to finally draw conclusions on the potential of n-butanol in modern GDI units.

2017 - Chemistry-Based Laminar Flame Speed Correlations for a Wide Range of Engine Conditions for Iso-Octane, n-Heptane, Toluene and Gasoline Surrogate Fuels [Relazione in Atti di Convegno]
D'Adamo, Alessandro; Del Pecchia, Marco; Breda, Sebastiano; Berni, Fabio; Fontanesi, Stefano; Prager, Jens
abstract

CFD simulations of reacting flows are fundamental investigation tools used to predict combustion behaviour and pollutants formation in modern internal combustion engines. Focusing on spark-ignited units, most of the flamelet-based combustion models adopted in current simulations use the fuel/air/residual laminar flame propagation speed as a background to predict the turbulent flame speed. This, in turn, is a fundamental requirement to model the effective burn rate. A consolidated approach in engine combustion simulations relies on the adoption of empirical correlations for laminar flame speed, which are derived from fitting of combustion experiments. However, these last are conducted at pressure and temperature ranges largely different from those encountered in engines: for this reason, correlation extrapolation at engine conditions is inevitably accepted. As a consequence, relevant differences between proposed correlations emerge even for the same fuel and conditions. The lack of predictive chemistry-grounded correlations leads to a wide modelling uncertainty, often requiring an extensive model tuning when validating combustion simulations against engine experiments. In this paper a fitting form based on fifth order logarithmic polynomials is applied to reconstruct correlations for a set of Toluene Reference Fuels (TRFs), namely iso-octane, n-heptane, toluene and for a commercial gasoline fuel surrogate. Experimental data from literature are collected as well as existing computations for laminar flame speed. These last are extended up to full-load engine-relevant conditions, where experiments are not available; they constitute a model-based prediction of flame behaviour at such states. The mentioned literature and calculated data, which are shown to be representative of a wide range of engine-typical operating points, constitute the target values for the fitting polynomials. The model-based correlations of this study constitute a reference to increase the accuracy of flamelet combustion simulations, and to reduce the modelling approximations when dealing with full-load engine operations.

2017 - Development of a RANS-Based Knock Model to Infer the Knock Probability in a Research Spark-Ignition Engine [Articolo su rivista]
D'Adamo, Alessandro; Breda, Sebastiano; Iaccarino, Salvatore; Berni, Fabio; Fontanesi, Stefano; Zardin, Barbara; Borghi, Massimo; Irimescu, Adrian; Merola, Simona
abstract

Engine knock is one of the most limiting factors for modern Spark-Ignition (SI) engines to achieve high efficiency targets. The stochastic nature of knock in SI units hinders the predictive capability of RANS knock models, which are based on ensemble averaged quantities. To this aim, a knock model grounded in statistics was recently developed in the RANS formalism. The model is able to infer a presumed log-normal distribution of knocking cycles from a single RANS simulation by means of transport equations for variances and turbulence-derived probability density functions (PDFs) for physical quantities. As a main advantage, the model is able to estimate the earliest knock severity experienced when moving the operating condition into the knocking regime. In this paper, improvements are introduced in the model, which is then applied to simulate the knock signature of a single-cylinder 400cm3 direct-injection SI unit with optical access; the engine is operated with two spark timings, under knock-safe and knocking conditions respectively. The statistical prediction of knock resulting from the presented knock model is compared to the experimental evidence for both investigated conditions. The agreement between the predicted and the measured knock distributions validates the proposed knock model. Finally, limitations and some unprecedented possibilities given by the model are critically discussed, with particular emphasis on the meaning of RANS knock prediction.

2017 - Numerical Simulation and Flame Analysis of Combustion and Knock in a DISI Optically Accessible Research Engine [Articolo su rivista]
Iaccarino, Salvatore; Breda, Sebastiano; D'Adamo, Alessandro; Fontanesi, Stefano; Irimescu, Adrian; Merola, Simona
abstract

The increasing limitations in engine emissions and fuel consumption have led researchers to the need to accurately predict combustion and related events in gasoline engines. In particular, knock is one of the most limiting factors for modern SI units, severely hindering thermal efficiency improvements. Modern CFD simulations are becoming an affordable instrument to support experimental practice from the early design to the detailed calibration stage. To this aim, combustion and knock models in RANS formalism provide good time-to-solution trade-off allowing to simulate mean flame front propagation and flame brush geometry, as well as “ensemble average” knock tendency in end-gases. Still, the level of confidence in the use of CFD tools strongly relies on the possibility to validate models and methodologies against experimental measurements.In the paper, two sets of cycle-resolved flame visualizations are available from a single-cylinder 400 cm3 direct-injection spark-ignition (DISI) unit with optical access. The engine is operated at two spark timings, ranging from knock-safe to light-knock conditions.On this basis, a numerical analysis is carried out to reproduce flame kernel growth and propagation using the well-known ECFM-3Z combustion model for all the operating conditions. CFD results are compared in terms of enflamed volume and flame morphology against cycle averaged experimental data. In addition, average knock is simulated by means of the in-house built UniMORE Knock Model [1] in terms of knock onset location and phasing.The agreement between predicted and measured position of the flame front and knock inception location for the two different operating conditions confirms the validity of the adopted models and proves their predictive capability for engine design and optimization

2017 - Numerical simulation of gasoline and n-butanol combustion in an optically accessible research engine [Articolo su rivista]
Breda, Sebastiano; D'Adamo, Alessandro; Fontanesi, Stefano; D’Orrico, Fabrizio; Irimescu, Adrian; Merola, Simona; Giovannoni, Nicola
abstract

Conventional fossil fuels are more and more regulated in terms of both engine-out emissions and fuel consumption. Moreover, oil price and political instabilities in oil-producer countries are pushing towards the use of alternative fuels compatible with the existing units. N-Butanol is an attractive candidate as conventional gasoline replacement, given its ease of production from bio-mass and key physico-chemical properties similar to their gasoline counterpart. A comparison in terms of combustion behavior of gasoline and n-Butanol is here presented by means of experiments and 3D-CFD simulations. The fuels are tested on a single-cylinder direct-injection spark-ignition (DISI) unit with an optically accessible flat piston. The analysis is carried out at stoichiometric undiluted condition and lean-diluted mixture for both pure fuels. Numerical simulations are carried out on the same operating points and a dedicated set of detailed chemistry simulations are used to accurately predict laminar flame speed for both gasoline and n-Butanol at selected engine-relevant conditions. Moreover, a method to accurately fit target results is presented and it is applied to obtain a polynomial form of laminar flame speed for both fuels. Mixture preparation and combustion development are carefully analyzed to explain the experimental evidence and to argument the differences between the two fuels, as well as the fuel-specific tolerance to mixture leaning. Finally, conclusions are drawn to summarize the obtained results and to outline the foreseen advantages of using n-Butanol as gasoline substitute in modern SI engines.

2016 - A RANS-Based CFD Model to Predict the Statistical Occurrence of Knock in Spark-Ignition Engines [Articolo su rivista]
D'Adamo, Alessandro; Breda, Sebastiano; Fontanesi, Stefano; Cantore, Giuseppe
abstract

Engine knock is emerging as the main limiting factor for modern spark-ignition (SI) engines, facing increasing thermal loads and seeking demanding efficiency targets. To fulfill these requirements, the engine operating point must be moved as close as possible to the onset of abnormal combustion events. The turbulent regime characterizing in-cylinder flows and SI combustion leads to serious fluctuations between consecutive engine cycles. This forces the engine designer to further distance the target condition from its theoretical optimum, in order to prevent abnormal combustion to severely damage the engine components just because of few individual heavy-knocking cycles. A RANS-based model is presented in this study, which is able to predict not only the ensemble average knock occurrence but also a knock probability. This improves the knock tendency characterization, since the mean knock onset alone is a poorly meaningful indication in a stochastic event such as engine knock. The model is based on a look-up table approach from detailed chemistry, coupled with the transport of the variance of both mixture fraction and enthalpy. These perturbations around the ensemble average value are originated by the turbulent time scale. A multivariate cell-based Gaussian-PDF model is proposed for the unburnt mixture, resulting in a statistical distribution for the in-cell reaction rate. An average knock precursor and its variance are independently calculated and transported, and the earliest knock probability is always preceding the ensemble average knock onset, as confirmed by the experimental evidence. This allows to identify not only the regions where the average knock first occurs, but also where the first knock probability is more likely to be encountered. The application of the model to a RANS simulation of a modern turbocharged direct injection (DI) SI engine is presented and a small percentage of knocking cycles is predicted by the model although the average behavior is knock-free, in agreement with the experiments. The estimate of the knocking probability improves the consolidated “average knock” RANS analysis and gives an indication of the statistical knock tendency of the engine

2016 - CFD Analysis of Combustion and Knock in an Optically Accessible GDI Engine [Articolo su rivista]
Breda, Sebastiano; D'Adamo, Alessandro; Fontanesi, Stefano; Giovannoni, Nicola; Testa, Francesco; Irimescu, Adrian; Merola, Simona; Tornatore, Cinzia; Valentino, Gerardo
abstract

The occurrence of knock is the most limiting hindrance for modern Spark-Ignition (SI) engines. In order to understand its origin and move the operating condition as close as possible to onset of this potentially harmful phenomenon, a joint experimental and numerical investigation is the most recommended approach. A preliminary experimental activity was carried out at IM-CNR on a 0.4 liter GDI unit, equipped with a flat transparent piston. The analysis of flame front morphology allowed to correlate high levels of flame front wrinkling and negative curvature to knock prone operating conditions, such as increased spark timings or high levels of exhaust back-pressure. In this study a detailed CFD analysis is carried out for the same engine and operating point as the experiments. The aim of this activity is to deeper investigate the reasons behind the main outcomes of the experimental campaign. A tabulated knock model is presented, based on detailed chemical mechanism for the surrogate gasoline. Combustion and knock simulations are carried out in a RANS framework through the use of validated models and the results are compared with cycle-resolved acquisition from the test-bed. The results of the CFD analysis explain the experimentally observed flame behavior and allow to proficiently understand the reasons of the sensitivity to knock of the analyzed unit

2015 - A Numerical Investigation on the Potentials of Water Injection as a Fuel Efficiency Enhancer in Highly Downsized GDI Engines [Relazione in Atti di Convegno]
D'Adamo, Alessandro; Berni, Fabio; Breda, Sebastiano; Lugli, Mattia; Fontanesi, Stefano; Cantore, Giuseppe
abstract

Engine downsizing is gaining popularity in the high performance engine market sector, where a new generation of highly downsized engines with specific power outputs around or above 150 HP/litre is emerging. High-boost and downsizing, adopted to increase power density and reduce fuel consumption, have to face the increased risks of pre-ignition, knock or mega-knock. To counterbalance autoignition of fuel/air mixture, such 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 reduces performance and induces an even lower A/F ratio (below 11), to limit the turbine inlet temperature, with huge negative effects on BSFC. A relatively simple yet effective solution to increase knock resistance is investigated by means of 3-D CFD analyses in the paper: water is port injected to replace mixture enrichment while preserving, if not improving, indicated mean effective pressure and knock safety margins. Full-load engine operations of a currently made turbocharged GDI engine are investigated comparing the adopted fuel-only rich mixture with stoichiometric mixtures, for which water is added in the intake port under constant charge cooling in the combustion chamber. In order to find the optimum fuel/water balance, preliminary analyses are carried out using a chemical reactor to evaluate the effects of charge dilution and mixture modification on both autoignition delays and laminar flame speeds. Thanks to the lower chemical reactivity of the diluted end gases, the water-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.

2015 - A numerical investigation on the potentials of water injection to increase knock resistance and reduce fuel consumption in highly downsized GDI engines [Relazione in Atti di Convegno]
Berni, Fabio; Breda, Sebastiano; Lugli, Mattia; Cantore, Giuseppe
abstract

3D CFD analyses are used to analyse the effects of port-injection of water in a high performance turbocharged GDI engine. Particularly, water injection is adopted to replace mixture enrichment while preserving, if not improving, indicated mean effective pressure and knock resistance. A full-load / maximum power engine operation of a currently made turbocharged GDI engine is investigated comparing the actual adopted fuel-only rich mixture to stoichiometric-to-lean mixtures, for which water is added in the intake port under constant charge cooling in the combustion chamber. In order to find the optimum fuel/water balance, preliminary analyses are carried out using a chemical reactor to evaluate the effects of charge dilution and mixture modification on both autoignition delays and laminar flame speeds. Thanks to the lower chemical reactivity of the diluted end gases, the water-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.

2015 - CFD Analysis of the Effects of Fuel Composition and Injection Strategy on Mixture Preparation and Fuel Deposit Formation in a GDI Engine [Relazione in Atti di Convegno]
Giovannoni, Nicola; Breda, Sebastiano; Paltrinieri, Stefano; D'Adamo, Alessandro; Fontanesi, Stefano; Pulvirenti, Francesco
abstract

In spark-ignited direct-injected engines, the formation of fuel pools on the piston is one of the major promoters of unburnt hydrocarbons and soot: in order to comply with the increasingly stringent emission regulations (EU6 and forthcoming), it is therefore necessary to limit fuel deposit formation. The combined use of advanced experimental techniques and detailed 3D-CFD simulations can help to understand the mechanisms driving fuel pool formation. In the paper, a combined experimental and numerical characterization of pool formation in a GDI engine is carried out to investigate and understand the complex interplay of all the mentioned factors. In particular, a low-load low-rpm engine operation is investigated for different ignition phasing, and the impact of both fuel formulation and instantaneous piston temperature variations in the CFD analyses are evaluated. The investigated engine operation shows some interesting features which are suited to deeply investigate the interplay between fuel film formation, mixing and soot. In particular, the relatively low wall temperature and low injection pressure allow the fuel to form deposits and then slowly evaporate, with possible presence of liquid fuel at the time of ignition. The simultaneous presence of slow fuel evaporation, reduced turbulence and presence of liquid fuel leads to the formation of extremely rich mixture pockets (with equivalence ratios well above 5) which are the major promoters for soot inception. Four different start of injection (hereafter SOI) values are analyzed, for which tailpipe Soot concentration measurements are available. For one SOI value, two different injection profiles are also evaluated. In particular, the analyses focus on the formation of fuel pads on the combustion chamber walls and on the mixture stratification, and a correlation between these two factors and the tailpipe soot level is found. The proposed methodology proves to be able to capture the Soot trend for the different SOI values without simulating the combustion process; it is therefore promising since it avoids the need for a dedicated calibration of the combustion model parameters and provides reasonable results (at least in terms of trends) with limited computational resources.

2015 - Effects on knock intensity and specific fuel consumption of port water/methanol injection in a turbocharged GDI engine: Comparative analysis [Relazione in Atti di Convegno]
Breda, Sebastiano; Berni, Fabio; D'Adamo, Alessandro; Testa, Francesco; Severi, Elena; Cantore, Giuseppe
abstract

The recent rise in fuel prices, the need both to reduce ground transport-generated emissions (increasingly constrained by legislation) and to improve urban air quality have brought fuel-efficient, low-emissions powertrain technologies at the top of vehicle manufacturers' and policy makers' agenda. To these aims, engine design is now oriented towards the adoption of the so-called downsizing and down-speeding techniques, while preserving the performance target. Therefore, brake mean effective pressure is markedly increasing, leading to increased risks of knock onset and abnormal combustions in last-generation SI engines. 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. Possible solutions to increase knock resistance are investigated in the paper by means of 3D-CFD analyses: water, water/methanol emulsion and methanol are port-fuel injected to replace mixture enrichment while preserving, if not improving, indicated mean effective pressure and knock safety margins. The aim of the work is therefore the replacement of the gasoline-only rich mixture with a global stoichiometric one while avoiding power loss and improving fuel consumption. In order to maintain the same knock tendency, water, methanol or a mixture of the two is then added in the intake port to keep the same charge cooling of the original rich mixture. Different strategies in terms of methanol/water ratios of the port injected mixture are compared in order to find the best trade-off between fuel consumption, performance and knock tendency.

2015 - LES Modelling of Spark-Ignition Cycle-to-Cycle Variability on a Highly Downsized DISI Engine [Articolo su rivista]
D'Adamo, Alessandro; Breda, Sebastiano; Fontanesi, Stefano; Cantore, Giuseppe
abstract

The paper reports an activity aiming at characterizing cycle-to-cycle variability (CCV) of the spark-ignition (SI) process in a high performance engine. The numerical simulation of spark-ignition and of early flame kernel evolution are major challenges, mainly due to the time scales of the spark discharge process and to the reduced spatial scales of flame kernel. Typical mesh resolutions are insufficient to resolve the process and a dedicated treatment has to be provided at a subgrid level if the ignition process is to be properly modelled. The focus of this work is on the recent ISSIM-LES (Imposed Stretch Spark-Ignition Model) ignition model, which is based on an extension of the flame surface density (FSD) transport equation for a dedicated flame kernel treatment at subgrid scales. The FSD equation is solved immediately after spark discharge. The interaction of the flame kernel with the flow field is fully accounted for since spark formation and a transition is provided from ignition to propagation phase. The comparison is carried out with the AKTIM-Euler ignition model in terms of flame interaction with the flow field (e.g. arc convection, flame blow-off, flame holder effect). A multiple cycle LES activity provided a set of cycle-resolved conditions for spark-ignition comparisons, and the flame kernel development is carefully analyzed for the two ignition models on a wide range of thermo-physical conditions. Spark-ignition cyclic variability and combustion traces are compared with experiments. Results confirm that the simulated cycle-to-cycle variability increases through the adoption of the ISSIM-LES ignition model.

2015 - Large-eddy simulation of cycle-resolved knock in a turbocharged SI engine [Relazione in Atti di Convegno]
D'Adamo, Alessandro; Breda, Sebastiano; Cantore, Giuseppe
abstract

The paper presents a numerical study of cycle-to-cycle variability in a turbocharged GDI engine. The Large-Eddy Simulation technique is adopted in this study in conjunction with the recent ISSIM-LES model for spark-ignition, allowing a dedicated treatment of both the flame kernel formation and flame development phases. Numerical results are compared with an extended dataset of experimental test-bed acquisitions, where the engine is operated at knock-limited spark advance. The agreement of both ensemble averaged combustion pressure history and of its standard deviation confirm the validity of the adopted numerical framework able to correctly quantify the degree of CCV measured by the experiments. Knock tendency is evaluated by means of an in-house developed knock model, based on a tabulation technique for AI delays of the same RON98 gasoline as the one used in experiments. The results confirm the knock-free condition of the experimental KLSA, for which the cycle-resolved knock signature is extremely weak just for the cycles in the highest band of the CCV-affected combustion. The visualization of the pressure wave allows to identify the exhaust side as the most knock-prone region. Finally, spark-advance is increased by 3 CA with respect to the experimental edge-of knock limit, in order to simulate an experimentally prevented operating condition. Local pressure measurements mimicking flush-mounted transducers confirm the severe knock damage related to this condition. The predictive capability of the combustion CCV and of the adopted knock model confirm the heavy and recurrent cycle-resolved knock damage.

2015 - Numerical Investigation on the Effects of Water/Methanol Injection as Knock Suppressor to Increase the Fuel Efficiency of a Highly Downsized GDI Engine [Relazione in Atti di Convegno]
Berni, Fabio; Breda, Sebastiano; D'Adamo, Alessandro; Fontanesi, Stefano; Cantore, Giuseppe
abstract

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.

2015 - Numerical investigation on the effects of bore reduction in a high performance turbocharged GDI engine. 3D investigation of knock tendency [Relazione in Atti di Convegno]
Severi, Elena; D'Adamo, Alessandro; Berni, Fabio; Breda, Sebastiano; Lugli, Mattia; Mattarelli, Enrico
abstract

Downsizing 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.

2015 - Two-stage turbocharging for the downsizing of SI V-engines [Relazione in Atti di Convegno]
Rinaldini, Carlo Alberto; Breda, Sebastiano; Fontanesi, Stefano; Savioli, Tommaso
abstract

One of the most critical challenges for the specific power increase of turbocharged SI engines is the low end torque, limited by two aspects. First, the big size of the compressor necessary to deliver the maximum airflow does not allow high boost pressures at low speed, due to the surge line proximity. Second, the flame front velocity may become slower than the end gas auto-ignition rate, thus increasing the risk of knocking. This study is based on a current SI GDI V8 turbocharged engine, modeled by means of CFD tools, both 1d and 3d. The goal of the activity is to lower by 20% the displacement, without reducing brake torque, all over the engine speed range. It was decided to adopt a smaller bore, keeping stroke constant. Obviously, the combustion chamber, the valves and the intakeexhaust ports have been re-designed, as well as the whole intake and exhaust system. Instead of the two turbochargers, one for each bank of cylinders, a triple-turbocharger layout has been considered. The development of the engine has been carried out by means of 1D engine cycle simulations, using predictive knock models, calibrated with the support of both experiments and CFD-3d simulations. A few operating conditions for the final configuration have been also analyzed by means of a 3-d CFD tool. The paper presents the results of this activity, and describes in details the guidelines followed for the development of the engine.