Fabio Bozza

A Quasi-Dimensional Three-Zone Model for Performance and Combustion Noise Evaluation of a Twin-Spark High-EGR Engine

The paper reports the research activity related to the development of a twin-spark Sl engine equipped with a variable valve timing (VVT) device. Improvements on the fuel consumption at part load are expected when an high internal exhaust gas recirculation (internal EGR) level is realized with a proper phasing of the VVT device. The twin-spark solution is implemented to improve the burning speed at low load, and to increase the EGR tolerance levels. Both experimental and theoretical analyses are carried out to investigate the real advantages of the proposed engine architecture. In particular an original quasi-dimensional model for the simulation of the burning process in a twin-spark engine is presented. The model is mainly utilized to find the proper combination of VVT device position (and hence EGR level) and spark advance for different engine operating conditions. A comparison with the single-spark solution is also provided. In addition, a procedure for the estimation of the sound pressure levels originated from the combustion process is utilized, to estimate the increased radiated noise associated to the double ignition. The model is well suited to define the control strategy maps of the engine in its whole operating range.

Adapting the Micro-Gas Turbine Operation to Variable Thermal and Electrical Requirements

This paper examines the possibilities for a micro-gas turbine operation under a wide range of thermal and mechanical load requirements. The authors focus the attention on a partially recuperated thermal cycle based on a by-pass option towards the heat recovery boiler, in order to adapt the gas turbine operation to increasing needs of thermal output. In addition, a variable speed operation is considered as a more reliable method for decreasing the mechanical output without producing an excess in efficiency decay. The actual possibilities of the above-named regulation tools are examined by an integrated procedure which involves, besides an accurate thermodynamic preliminary analysis, the component matching study and the CFD based simulation of the combustion chamber.

Acoustic and fluid-dynamic optimization of an automotive muffler

In this work, the acoustic and fluid-dynamic performances of a commercial three-chamber perforated muffler were simulated with a three-dimensional boundary element method and also a one-dimensional approach. The inner insulating material (wool) was taken into account in the performed analyses, together with the presence of a mean flow across the muffler in order to predict both the transmission loss and the pressure drop Δp. Three-dimensional analyses were experimentally validated in a wide frequency range and in the absence of mean flow and were utilized to build a more precise one-dimensional representation of the device. In this way, better agreement between the one-dimensional results and the experimental data was realized, at least in the frequency range characterized by planar wave propagation (below 800 Hz). Once validated, the one-dimensional model was coupled to an external optimizer to perform acoustic and fluid-dynamic optimizations of the considered muffler. Initially, a genetic algorithm was employed to modify the internal muffler geometry and to improve the transmission loss, in the absence of mean flow, in the 100–800 Hz frequency range. A second optimization was also performed to identify the trade-off between the acoustic performance and the fluid-dynamic performance, in terms of the transmission loss and Δp, in the 100–400 Hz frequency range.

A numerical procedure for the calibration of a turbocharged spark-ignition variable valve actuation engine at part load

Referring to spark-ignition engines, the downsizing, coupled to turbocharging and variable valve actuation systems are very common solutions to reduce the brake-specific fuel consumption at low-medium brake mean effective pressure. However, the adoption of such solutions increases the complexity of engine control and management because of the additional degrees of freedom, and hence results in a longer calibration time and higher experimental efforts. In this work, a twin-cylinder turbocharged variable valve actuation spark-ignition engine is numerically investigated by a one-dimensional model (GT-Power™). The considered engine is equipped with a fully flexible variable valve actuation system, realizing both a common full-lift strategy and a more advanced early intake valve closure strategy. Refined sub-models are used to describe turbulence and combustion processes. In the first stage, one-dimensional engine model is validated against the experimental data at full and part load. The validated model is then integrated in a multipurpose commercial optimizer (modeFRONTIER™) with the aim to identify the engine calibration that minimizes brake-specific fuel consumption at part load. In particular, the decision parameters of the optimization process are the early intake valve closure angle, the throttle valve opening, the turbocharger setting and the spark timing. Proper constraints are posed for intake pressure in order to limit the gas-dynamic noise radiated at the intake mouth. The adopted optimization approach shows the capability to reproduce with good accuracy the experimentally identified calibration. The latter corresponds to the numerically derived Pareto frontier in brake mean effective pressure–brake specific fuel consumption plane. The optimization also underlines the advantages of an engine calibration based on a combination of early intake valve closure strategy and intake throttling rather than a purely throttle-based calibration. The developed automatic procedure allows for a ‘virtual’ calibration of the considered engine on completely theoretical basis and proves to be very helpful in reducing the experimental costs and the engine time-to-market.

Prediction and enhancement of the acoustic performance of a spark ignition engine intake air filter box

The present work has resulted in the development of numerical and experimental methodologies for prediction of the acoustic performance of an air filter box of an internal-combustion engine for automotive applications. With the aim of characterizing its fluid-dynamic behaviour and acoustic attenuation, the latter is first analysed as an isolated component. Typical performance parameters such as the discharge coefficient CD, the pressure losses, the noise reduction and the transmission loss are experimentally and numerically evaluated. Different numerical approaches are considered, including complete three-dimensional computational fluid-dynamics analyses and finite element–boundary element procedures. The latter also take into account the interaction with the vibrating structure. The preliminary air-box design is also modified through the introduction of a column resonator and a Helmholtz resonator. Both the base and the modified system are finally coupled to the whole engine to estimate the gas-dynamic noise emitted by the intake system. The new architectural solutions of the intake system are characterized by improved acoustic performances and also preserve a good power output of the entire engine system. In each case the comparisons with experimental findings showed good correlation with the numerical predictions.

Numerical analysis of the transient operation of a turbocharged diesel engine including the compressor surge

The paper deals with the simulation of a multi-cylinder turbocharged diesel engine for automotive applications, employing a one-dimensional approach with the aim of refining the turbocharger modelling during transient manoeuvres. The proposed methodology is able to handle stable compressor behaviour and also compressor surge. In addition, a waste-gate model is introduced to account for the instantaneous variation in the valve section as a result of the control signal, which is provided by the engine control unit, and the engine state. Preliminarily, the engine model is tuned against experimental data in terms of both the global performance parameters and the in-cylinder pressure cycles. The compressor performance is described through an ‘extended’ map obtained using a one-dimensional turbocharger model; in this way, a refined surge analysis can be performed, accounting for both direct flow compressor operations and reverse flow compressor operations. The one-dimensional model is applied to analyse different transient manoeuvres. First, the vehicles maximum speed is predicted and compared with the manufacturers data, during an acceleration manoeuvre. Then, a sudden part-to-full-load step is described with the aim of analysing in detail the turbo-lag. Finally, a full-to-part-transient manoeuvre is also analysed to verify the capability of the model to represent the compressor surge phenomenon. The numerical results provided in this work qualitatively reproduce the experimental observations available in the literature for transient operation of engines. Thus, the developed computational tool can be successfully used to support the design process and the transient analysis of turbocharged internal-combustion engines.

Optimal Design of a Two-Stroke Diesel Engine for Aeronautical Applications Concerning Both Thermofluidynamic and Acoustic Issues

Some aspects concerning the development of a prototype of a diesel engine suitable for aeronautical applications are discussed. The engine aimed at achieving a weight to power ratio equal to one kg/kW (220 kg for 220 kW) is conceived in a two stroke Uniflow configuration and constituted by six cylinders distributed on two parallel banks. Basing on a first choice of some geometrical and operational data, a preliminary fluid-dynamic and acoustic analysis is carried out at the sea level. This includes the engine-turbocharger matching, the estimation of the scavenging process efficiency, and the simulation of the spray and combustion process, arising from a Common Rail injection system. Both 1D and 3D CFD models are employed. In-cylinder pressure cycles are utilized to numerically predict the combustion noise. The acoustic study is based on the integration of FEM/BEM codes. In order to improve the engine performance and vibro-acoustic behaviour, the 1D model, tuned with information derived from the 3D code, is linked to an external optimiziation code (ModeFRONTIER™). A constrained multi-objective optimization is performed to contemporary minimize the fuel consumption and the maximum in-cylinder temperature and pressure gradient. In this way a better selection of a number of engine parameters is carried out (exhaust valve opening, closing and lift, intake ports heights, start of injection, etc). The best found solution is finally compared to the initial one and some substantial design improvements are discussed.Copyright

The employment of hydrogenerated fuels from natural gas reforming: Gas turbine and combustion analysis

An integrated method for power plant analysis, including rotating component matching and CFD simulation of the combustion process, is applied to the study of gas turbines supplied with hydrogenated fuels originating from the natural gas reforming. The method proposed by the authors allows estimation of the power plant performance and emission in the gas turbine operating range. A comparison is then carried out between the plant behavior with conventional fuelling and with decarbonised fuel supply. Attention is also paid to the study of the combustion regimes with either natural gas or fuels with increasing hydrogen contents, in order to achieve a realistic insight of both the temperature distributions and the growth of nitric oxides throughout the combustion chamber.

Performance Prediction and Combustion Modeling of Low-CO2 Emission Gas Turbines

The authors present a comprehensive analysis of innovative solutions for reducing the CO2 emission from gas turbine based power plants. Two different gas turbine types are considered, first in their baseline arrangement and then with modified configurations with regenerative, water-intercooled cycles.The carbon dioxide reduction is obtained through a partial oxidation of the natural gas upstream of the combustion chamber.The analyses are carried out with an increasing level of complexity, starting from a purely thermodynamic approach and then proceeding with a component matching based method.Finally, the comparison of the CFD simulation of the combustion processes of the conventionally fuelled combustor and of the one supplied with the decarbonized fuel is presented.Copyright

A Methodology for Establishing Optimal Part-Load Operation of Industrial Gas Turbines

The authors present the application of a method for gas turbine analysis, that they have recently introduced, based upon the selection of rotating components and on the study of off-design behaviour. The paper deals with an aero-derivative gas turbine, whose operating field is evaluated by an advanced cycle calculation which takes into account the fluid-dynamic and mechanical matching of the turbomachines.Different regulation systems are considered, like variable-geometry compressor and turbine vanes, whose effect is combined with the variations in rotational speed of the gas generator device.The simulation model allows definition of a performance map of the gas turbine, not only in terms of mechanical output and efficiency but also of pollutant emissions, as a result of the prediction of thermal NO formation inside the combustor.The method leads to the establishment of an optimal control and regulation strategy with respect to a prescribed objective, i.e., either for maximum in energy saving or for the best compromise between performance and emission levels. Cases are examined with reference to both mechanical energy production and combined heat and power generation.Copyright