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Research

  • Automatic Combustion Control for Calibration Purposes in a GDI Turbocharged Engine

    Year: 2014

    Author: Enrico Corti, Claudio Forte, Nicolo Cavina, Giorgio Mancini, Vittorio Ravaglioli

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    Combustion phasing is crucial to achieve high performance and efficiency: for gasoline engines control variables such as Spark Advance (SA), Air-to-Fuel Ratio (AFR), Variable Valve Timing (VVT), Exhaust Gas Recirculation (EGR), Tumble Flaps (TF) can influence the way heat is released. The optimal control setting can be chosen taking into account performance indicators, such as Indicated Mean Effective Pressure (IMEP), Brake Specific Fuel Consumption (BSFC), pollutant emissions, or other indexes inherent to reliability issues, such as exhaust gas temperature, or knock intensity. Given the high number of actuations, the calibration of control parameters is becoming challenging.

    Many different approaches can be used to reach the best calibration settings: Design Of Experiment (DOE) is a common option when many parameters influence the results, but other methodologies are in use: some of them are based on the knowledge of the controlled system behavior, by means of models that are identified during the calibration process.

    The paper shows how the calibration can be managed using a different concept, based on the Extremum Seeking (ES) approach. The main idea consists in changing the values of each control parameter at the same time, identifying its effect on a cost or merit function (target function), allowing to shift automatically the control setting towards the optimum solution throughout the calibration procedure. The function is evaluated cycle by cycle, based on combustion analysis. Due to the control parameters continuous variations the target function values change: the ES objective is to drive the variations towards the setting minimizing the cost function.

    The methodology has been applied to data referring to a GDI turbocharged engine, trying to maximize IMEP or minimize BSFC, while limiting the knock intensity and exhaust gas temperature, using SA, AFR and VVT as control variables. Experimental data referring to the considered engine have been used to feed a combustion model, allowing to test the calibration approach: results show that the ES-based calibration is able to automatically change SA, lambda and VVT values, taking into account all the constraints, and finally reaching the optimal control setting, independently of the starting setting.

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  • Non-Intrusive Methodology for Estimation of Speed Fluctuations in Automotive Turbochargers under Unsteady Flow Conditions

    Year: 2014

    Author: Fabrizio Ponti, Vittorio Ravaglioli, Enrico Corti, Davide Moro, Matteo De Cesare

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    The optimization of turbocharging systems for automotive applications has become crucial in order to increase engine performance and meet the requirements for pollutant emissions and fuel consumption reduction. Unfortunately, performing an optimal turbocharging system control is very difficult, mainly due to the fact that the flow through compressor and turbine is highly unsteady, while only steady flow maps are usually provided by the manufacturer. For these reasons, one of the most important quantities to be used onboard for optimal turbocharger system control is the rotational speed fluctuation, since it provides information both on turbocharger operating point and on the energy of the unsteady flow in the intake and exhaust circuits.
    This work presents a methodology that allows determining the instantaneous turbocharger rotational speed through a proper frequency processing of the signal coming from one accelerometer mounted on the turbocharger compressor. Consequently, the developed algorithm can be used to determine both rotational speed mean value and the amplitude of speed fluctuations that are caused by unsteady flows. From this last evaluated quantity, it is also possible to obtain an estimation of power delivered by the turbine, that might be used for control and diagnostic purposes.
    The whole estimation algorithm has been developed and validated for a light duty turbocharged Diesel engine installed in a test cell at the University of Bologna. This paper reports the experimental layout used in this work and the accuracy obtained applying the speed fluctuations estimation procedure to the turbocharged Diesel engine under study.

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  • A RANS CFD 3D Methodology for the Evaluation of the Effects of Cycle By Cycle Variation on Knock Tendency of a High Performance Spark Ignition Engine

    Year: 2014

    Author: Claudio Forte, Enrico Corti, Gian Marco Bianchi, Stefania Falfari, Stefano Fantoni

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    Knocking combustions heavily limits the efficiency of Spark Ignition engines. The compression ratio is limited in the design stage of the engine development, letting to Spark Advance control the task of reducing the odds of abnormal combustions.
    A detailed analysis of knocking events can help improving engine performance and diagnosis strategies. An effective way is to use advanced 3D CFD (Computational Fluid Dynamics) simulation for the analysis and prediction of combustion performance. Standard 3D CFD approach is based on RANS (Reynolds Averaged Navier Stokes) equations and allows the analysis of the mean engine cycle. However knocking phenomenon is not deterministic and it is heavily affected by the cycle to cycle variation of engine combustions. A methodology for the evaluation of the effects of CCV (Cycle by Cycle Variability) on knocking combustions is here presented, based on both the use of Computation Fluid Dynamics (CFD) tools and experimental information. The focus of the numerical methodology is the statistical evaluation of the local air-to-fuel and turbulence distribution at the spark plugs and their correlation with the variability of the initial stages of combustion.
    CFD simulations have been used to reproduce knock effect on the in-cylinder pressure trace. The pressure signal holds information about waves propagation and heat losses: it is crucial to relate local pressure oscillations to knock severity. For this purpose, a CFD model has been implemented, able to predict the combustion evolution with respect to Spark Advance, from non-knocking up to heavy knocking conditions. The CFD model validation phase is essential for a correct representation of both regular and knocking combustions: the operation has been carried out by means of an accurate statistical analysis of experimental in-cylinder pressure data. The methodology is applied to a high performance engine, equipped with both mono-spark and twin-spark configurations, and proved to be an useful tool for the evaluation of knock tendency of the two different settings in Maximun Brake Torque condition.

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  • Automatic Calibration of Control Parameters based on Merit Function Spectral Analysis

    Year: 2014

    Author: E Corti, A Cerofolini, N Cavina, C Forte, G Mancini, D Moro, F Ponti, V Ravaglioli

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    The number of actuations influencing the combustion is increasing, and, as a consequence, the calibration of control parameters is becoming challenging. One of the most effective factors influencing performance and efficiency is the combustion phasing: for gasoline engines control variables such as Spark Advance (SA), Air-to-Fuel Ratio (AFR), Variable Valve Timing (VVT), Exhaust Gas Recirculation (EGR) are mostly used to set the combustion phasing.
    The optimal control setting can be chosen according to a cost function, taking into account performance indicators, such as Indicated Mean Effective Pressure (IMEP), Brake Specific Fuel Consumption (BSFC), pollutant emissions, or other indexes inherent to reliability issues, such as exhaust gas temperature, or knock intensity.
    The paper proposes the use of the extremum seeking approach during the calibration process. The main idea consists in changing the values of each control parameter at the same time, identifying its effect on the monitored cost function, allowing to shift automatically the control setting towards the optimum solution throughout the calibration procedure. Obviously, the nodal point is to establish how the various control parameters affect the monitored cost function and to determine the direction of the required variation, in order to approach the optimum. This task is carried out by means of a spectral analysis of the cost function: each control variable is varied according to a sine wave, thus its effect on the cost function can be determined by evaluating the amplitude of the Fast Fourier Transform (FFT) of the cost function, for the given excitation frequency. The FFT amplitude is representative of the cost function sensitivity to the control variable variations, while the phase can be used to assess the direction of the variation that must be applied to the control settings in order to approach the optimum configuration. Each control parameter is excited with a different frequency, thus it is possible to recognize the effect of a single parameter by analyzing the spectrum of the cost function for the given excitation frequency.
    The methodology has been applied to data referring to a PFI engine, trying to maximize IMEP, while limiting the knock intensity and exhaust gas temperature, using SA and AFR as control variables. The approach proved to be efficient in reaching the optimum control setting, showing that the optimal setting can be achieved rapidly and consistently.

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  • Transient Spark Advance Calibration Approach

    Year: 2014

    Author: Enrico Corti, Nicolò Cavina, Alberto Cerofolini, Claudio Forte, Giorgio Mancini, Davide Moro, Fabrizio Ponti, Vittorio Ravaglioli

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    Combustion control is assuming a crucial role in reducing engine tailpipe emissions while maximizing performance. The effort in the calibration of control parameters affecting the combustion development can be very demanding. One of the most effective factors influencing performance and efficiency is the combustion phasing: in Spark Ignition (SI) engines it is affected by factors such as Spark Advance (SA), Air-Fuel Ratio (AFR), Exhaust Gas Recirculation (EGR), Variable Valve Timing (VVT).
    SA optimal values are usually determined by means of calibration procedures carried out in steady state conditions on the test bench by changing SA values while monitoring performance indicators, such as Brake and Indicated Mean Effective Pressure (BMEP, IMEP), Brake Specific Fuel Consumption (BSFC) and pollutant emissions. The effect of SA on combustion is stochastic, due to the cycle-to-cycle variation: the analysis of mean values requires many engine cycles to be significant of the performance obtained with the given control setting. Moreover, often the effect of SA on engine performance must be investigated for different settings of other control parameters (EGR, VVT, AFR). The calibration process is time consuming involving exhaustive tests followed by off-line data analysis.
    This paper presents the application of a dynamic calibration methodology, with the objective of reducing the calibration duration. The proposed approach is based on transient tests, coupled with a statistical investigation, allowing reliable performance analysis even with a low number of engine cycles. The methodology has been developed and tested off-line, then it has been implemented in Real-Time. The combustion analysis system has been integrated with the ECU management software and the test bench controller, in order to perform a fully automatic calibration.

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  • Managing Wind Variability with Pumped Hydro Storage and Gas Turbines

    Year: 2014

    Author: Michele Bianchi, Lisa Branchini, Nicolò Cavina, Alberto Cerofolini, Enrico Corti, Andrea De Pascale, Valentina Orlandini, Francesco Melino, Davide Moro, Antonio Peretto, Fabrizio Ponti

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    Load and wind energy profiles are totally uncorrelated, therein lies the problem of variable energy sources. Managing load with increasing wind penetration may call for operational ranges that conventional systems cannot readily access. Storage technologies could allow tolerating the unsteadiness of renewable sources with smaller fossil fuel plants capacity. In this paper, we analyze the operational requirements of about 1 GW nameplate wind farm to provide firm power to the grid when Pumped Hydro Storage (PHS) is compound with Gas Turbines (GTs) to smooth out wind variability. GTs and PHS operational constrains are included in the system model in order to correctly demonstrate how wind variability influences GTs and PHS performance. Taking into account different days of wind generation, integrated system behavior has been analyzed comparing different strategies: i) an in-house control strategy and ii) an optimal control strategy based on Dynamic Programming. The latter requires a careful approach to system modeling, aimed at reducing its complexity and the consequent computational load. The challenge is to implement only the smallest set of state and control variables needed to define the optimization problem. For the selected days of analysis, GTs and PHS output profile, fuel consumption and storage reservoir operation are presented.

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  • CFD Methodology for the Evaluation of Knock of a PFI Twin Spark Engine

    Year: 2014

    Author: Claudio Forte, Gian Marco Bianchi, Enrico Corti, Stefano Fantoni, Marco Costa

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    A methodology for the evaluation of the effects of twin spark ignition system on CCV (Cycle by Cycle Variability) and knocking combustions is here presented, based on both the use of Computation Fluid Dynamics (CFD) tools and experimental information. The focus of the numerical methodology is the statistical evaluation of the local air-to-fuel distribution at the spark plugs and its correlation with the variability of the initial stages of combustion.
    A detailed analysis of knocking events can help improving engine performance and diagnosis strategies. The use of twin spark ignition system can enhance the probability that the initial kernel could come across a zone with the correct air-to-fuel ratio, thus lowering the initial combustion instabilities. Moreover the lower distance swept from the flame fronts can considerably reduce the time for the unburnt mixture to auto-ignite, thus reducing the risk of knocking combustion.
    CFD simulations have been used to reproduce knock effect on the in-cylinder pressure trace. The pressure signal holds informa- tion about waves propagation and heat losses: it is crucial to relate local pressure oscillations to knock severity. For this purpose, a CFD model has been implemented, able to predict the combustion evolution with respect to Spark Advance, from non-knocking up to heavy knocking conditions. The CFD model validation phase is essential for a correct representation of both regular and knocking combustions: the operation has been carried out by means of an accurate statistical analysis of experimental in-cylinder pressure data.

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  • Evaluation of the Mixture Formation Process of High Performance Engine with a Combined Experimental and Numerical Methodology

    Year: 2014

    Author: Claudio Forte, Gian Marco Bianchi, Enrico Corti, Buono Michele, Fantoni Stefano

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    The combustion process of a Port Fuel Injection (PFI) engine is deeply influenced by the mixture formation process. The needs of reducing engine emission and fuel consumption push the engine manufactures to implement new advanced experimental and numerical techniques to better control the mixture quality. The local mixture air-index at the spark plug is closely related to combustion instabilities and the Indicated Mean Effective Pressure (IMEP) Coefficient of Variation (CoV) well correlates with the variability of the flame kernel development. The control of air to fuel ratio is especially critical for high performance engines: due to the low stroke-to-bore ratio the maximum power is reached at very high regimes, letting little time to the fuel to evaporate and mixing with air. The injector located upstream the throttle causes a lot of fuel to impinge the throttle and intake duct walls, slowing down the dynamics of mixture formation under part load conditions.
    The aim of the paper is to present a multi-cycle methodology for the simulation of the injection and the mixture formation processes of high performance PFI engine, in order to evaluate both the quality of combustion and the fuel dynamics in the intake duct.
    The phenomena involved in the process are highly heterogeneous, and particular care must be taken to the choice of CFD models and their validation. In the present work all the main models involved in the simulations are validated against experimental tests available in the literature, selected based on the similarity of physical conditions of those of the engine configuration under analysis. The lagrangian spray is initialized with a semi-empirical methodology, based on available experimental data, and its interaction with the wall is simulated by means of the Kuhnke model, so to take into account the high wall temperature of the intake valves during impingement. The dynamics of the wall film are accurately represented by the activation of momentum equation of wall film and a validation of its dynamics is accomplished against proper test cases.
    The methodology is applied in two different engine configurations with separate objectives: a part load condition, where the dynamics of fuel in the intake duct is crucial to ensure drivability, a full load configuration, for the evaluation of mixture quality and combustion stability.

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