Modeling of Turbulence, Combustion and Knock for Performance Prediction, Calibration and Design of a Turbocharged Spark Ignition Engine

In this thesis work, a downsized VVA Spark Ignition engine is numerically and experimentally studied. In particular, the following topics are considered: •In-cylinder turbulence and combustion processes; •Knock and cycle by cycle variation (CCV) phenomena; •Techniques aiming to mitigate knock occurrence and improve fuel economy such as EGR and water injection methods; •Intake system redesign to reduce the emitted gas-dynamic noise; •Engine calibration. A deep experimental campaign is carried out to characterize the engine behaviour. Indeed, engine system is investigated both in terms of the overall performance (torque, power, fuel consumption, air flow rate, boost pressure etc.) and of the intake gas-dynamic noise at full load operation. In addition, proper experimental analyses are peformed on the engine to characterize the CCV phenomenon and the knock occurrence. Measured data are post-processed to derive experimental parameters which syntetize CCV and knock levels, according to the engine operating conditions. A 1D CFD model of the whole engine is realized in GT-PowerTM environment. Refined “in-house developed” sub-models capable to reproduce turbulence, combustion, CCVs and knock processes are introduced into 1D code through user routines. First of all, the whole engine model is validated against the experimental data both in terms of overall performance parameters and ensemble averaged pressure cycles and intake gas-dynamic noise at part and full load operation. Cycle by cycle variation is reproduced through a proper correlation and consequently a representative faster than average in-cylinder pressure cycle is obtained. Then, the knock model, with reference to the latter pressure cycle, allows to evaluate a proper knock index and to identify the knock limited spark advance (KLSA), basing on the same threshold level adopted in experimental knock analysis. In this way, the knock model taking into account the CCV is validated at full load operation. Once validated, the original engine architecture is modified by virtually installing a “Low pressure” EGR system. 1D simulations accounting for various EGR rates and mixture leaning are performed at full load points, showing improvements in the fuel economy with the same knock intensity of the base engine configuration. Water injection technique is also investigated by virtually mounting a water injector in the intake runners for each engine cylinder. In a similar way, 1D analyses are carried out for various water/fuel and air-to-fuel ratios, highlightinig BSFC improvements at full load operation. Since the engine under study is characterized by higher intake gas-dynamic noise levels, a partial redesign of the intake system is properly identified and subsequently tested with 1D and 3D CFD simulations to numerically quantify the gains in terms of reduction in the gas-dynamic noise emitted at the intake mouth. Finally, a numerical methodology aiming to calibrate the considered engine at high load knock-limited and at part load operations is developed. First, it shows the capability to identify with satisfactory accuracy the experimentally advised engine calibration. In addition, it allows the comparison of different intake valve strategies, underlining, in certain engine operating conditions, the fuel consumption benefits of an early intake valve closure (EIVC) strategy with respect to a Full Lift one, due to a better combustion phasing and a reduced mixture over-fuelling. The developed automatic procedure presents the capability to realize a “virtual” engine calibration on completely theoretical basis and proves to be very helpful in reducing time and costs related to experimental activities at the test bench

development of a virtual calibration methodology for a downsized si engine by using advanced valve strategies

Abstract The calibration phase of a new engine at test bench is an expensive and time-consuming process. To support the engine development process, in this paper a numerical methodology aiming to define the optimal control parameters is proposed for a downsized VVA SI engine. First, a 1D engine model is build-up in GT-Power and is enhanced with phenomenological sub-models. 1D model is then validated against the experimental findings, at high- and part-load operations. In a second stage, a numerical calibration strategy is defined, to automatically identify, for various engine loads/speeds, the control parameters, ensuring optimal performance and complying with proper system limitations. Complete engine maps are computed for different control strategies ( EIVC and Throttled ). An application example is also presented, where computed maps are embedded in a vehicle model to predict the CO 2 emission produced along a NEDC cycle

Numerical evaluation of heat transfer effects on the improvement of efficiency of a spark ignition engine characterized by cylinder variability

In this work, the effects of in-cylinder heat transfer on indicated thermal efficiency of a spark ignition engine showing a cylinder-to-cylinder variation are numerically analyzed. A 1D CFD model of engine is developed and integrated with a turbulent combustion sub-model and with a refined thermal sub-model for cylinders and exhaust pipes. The model is validated against the engine measurements. Thermal sub-model includes a Finite Element (FE) approach to predict the temperatures of cylinders and of exhaust pipes. The model correctly reproduces the thermodynamic behavior of cylinders at varying the operating condition. Simulations at low load and speed indicate that in-cylinder heat transfer represents a relevant percentage on total fuel energy entering the cylinder. Therefore, heat transfer exerts an important influence on the improvement of engine indicated thermal efficiency when considering the sole combustion phasing optimization of cylinders and the suppression of cylinder-to-cylinder variation

A Tabulated-Chemistry Approach applied to a Quasi-Dimensional Combustion Model for a Fast and Accurate Knock Prediction in Spark-Ignition Engines

The description of knock phenomenon is a critical issue in a combustion model for Spark-Ignition (SI) engines. The most known theory to explain this phenomenon is based on the Auto-Ignition (AI) of the end-gas, ahead the flame front. The accurate description of this process requires the handling of various aspects, such as the impact of the fuel composition, the presence of residual gas or water in the burning mixture, the influence of cool flame heat release, etc. This concern can be faced by the solution of proper chemistry schemes for gasoline blends. Whichever is the modeling environment, either 3D or 0D, the on-line solution of a chemical kinetic scheme drastically affects the computational time. In this paper, a procedure for an accurate and fast prediction of the hydrocarbons auto-ignition, applied to phenomenological SI engine combustion models, is proposed. It is based on a tabulated approach, operated on both ignition delay times and reaction rates. This technique, widely used in 3D calculations, is extended to 0D models to overcome the inaccuracies typical of the most common ignition delay approaches, based on the Livengood-Wu integral solution. The aim is to combine the predictability of a detailed chemistry with an acceptable computational effort. First, the tabulated technique is verified through comparisons with a chemical solver for a semi-detailed kinetic scheme in constant-pressure and constant-volume configurations. Then a phenomenological model, based on the end-gas AI computation, is utilized to predict the knock occurrence in different SI engines, including both naturally-aspirated and turbocharged architectures. 0D/1D simulations are performed both with an online solution of the chemistry and employing the tabulated approach. Assessment with reference KLSA values shows that the knock model, based on the tabulated chemistry, is able to well reproduce the essential features of the auto-ignition process in the analyzed engines, with a limited impact on the computational time

EGR Systems Employment to Reduce the Fuel Consumption of a Downsized Turbocharged Engine at High-load Operations☆

Abstract In this work, a promising technique, consisting in an introduction of the external low pressure cooled EGR system, is analyzed by means of a 1D numerical approach with reference to a downsized spark-ignition turbocharged engine. The effects of various EGR amounts are investigated in terms of fuel consumption at full load operations. The proposed results highlight that EGR allows for increasing the knock safety margin. Fuel economy improvements however depend on the overall engine recalibration, consisting in proper settings of the A/F ratio and spark advance, compatible with knock occurrence. The numerical recalibration also accounts for additional limitations on the turbocharger speed, boost level, and turbine inlet temperature. The maximum BSFC improvement by the proposed solution is 5.9%

Refinement of a 0D Turbulence Model to Predict Tumble and Turbulent Intensity in SI Engines. Part II: Model Concept, Validation and Discussion

As known, reliable information about underlying turbulence intensity is a mandatory pre-requisite to predict the burning rate in quasi-dimensional combustion models. Based on 3D results reported in the companion part I paper, a quasi-dimensional turbulence model, embedded under the form of "user routine" in the GT-Power\u2122 software, is here presented in detail. A deep discussion on the model concept is reported, compared to the alternative approaches available in the current literature. The model has the potential to estimate the impact of some geometrical parameters, such as the intake runner orientation, the compression ratio, or the bore-to-stroke ratio, thus opening the possibility to relate the burning rate to the engine architecture. Preliminarily, a well-assessed approach, embedded in GT-Power commercial software v.2016, is utilized to reproduce turbulence characteristics of a VVA engine. This test showed that the model fails to predict tumble intensity for particular valve strategies, such LIVC, thus justifying the need for additional refinements. The model proposed in this work is conceived to solve 3 balance equations, for mean flow kinetic energy, tumble vortex momentum, and turbulent kinetic energy (3-eq. concept). An extended formulation is also proposed, which includes a fourth equation for the dissipation rate, allowing to forecast the integral length scale (4-eq. concept). The impact of the model constants is parametrically analyzed in a first step, and a tuning procedure is advised. Then, a comparison between the 3- and the 4-eq. concepts is performed, highlighting the advantages of the 3-eq. version, in terms of prediction accuracy of turbulence speed-up at the end of the compression stroke. An extensive 3-eq. model validation is then realized according to different valve strategies and engine speeds. The user-model is then utilized to foresee the effects of main geometrical parameters analyzed in part I, namely the intake runner orientation, the compression ratio, and the bore-to-stroke ratio. A two-valve per cylinder engine is also considered. Temporal evolutions of 0D- and 3D-derived mean flow velocity, turbulent intensity, and tumble velocity present very good agreements for each investigated engine geometry and operating condition. The model, particularly, exhibits the capability to accurately predict the tumble trends by varying some geometrical parameter of the engine, which is helpful to estimate the related impact on the burning rate. Summarizing, the developed 0D model well estimates the in-cylinder turbulence characteristics, without requiring any tuning constants adjustment with engine speed and valve strategy. In addition, it demonstrates the capability to properly take into account the intake duct orientation and the compression ratio without tuning adjustments. Some minor tuning variation allows predicting the effects of bore-to-stroke ratio, as well. Finally, the model is verified to furnish good agreements also for a two-valve per cylinder engine, and with reference to two different high-performance engines

Performance and Emissions of an Advanced Multi-Cylinder SI Engine Operating in Ultra-Lean Conditions

In this work the performance and noxious emissions of a prototype Spark Ignition (SI) engine, working in ultra-lean conditions, are investigated. It is a four-cylinder engine, having a very high compression ratio, and an active pre-chamber. The required amount of air is provided by a low-pressure variable geometry turbocharger, coupled to a high-pressure E-compressor. The engine is equipped with a variable valve timing device on the intake camshaft. The goal of this activity is to support the development and the calibration of the described engine, and to exploit the full potential of the ultra-lean concept. To this aim, a combustion model for a pre-chamber engine, set up and validated in a previous paper for a similar single-cylinder unit, is utilized. It is coupled to additional in-house developed sub-models, employed for the prediction of the in-cylinder turbulence, heat transfer, knock and pollutant emissions. Such a complex architecture, schematized in a commercial 1D modeling framework, presents several control parameters which have to be properly selected to maximize the engine efficiency and minimize the noxious emissions over its whole operating domain. A Rule-Based (RB) calibration strategy is hence implemented in the 1D model to identify the optimal values of each control variable. The reliability of the RB calibration is also demonstrated through the comparison with the outcomes of a general-purpose optimizer, over a load sweep at a constant speed. The 1D model and the RB methodology are then applied for the performance prediction over the whole engine operating domain. The predicted performances show the possibility to achieve a wide zone of very high efficiency, with limited penalizations only at very low loads. Main advantages of the lean-combustion concept are highlighted, concerning a higher specific heat ratio, reduced heat losses, improved knock mitigation, and abatement of pollutant emissions, especially regarding CO and NOx. The presented methodology demonstrates to be a valuable tool to support the development and calibration of the considered high-efficiency engine architecture

Performance Optimization of a Turbocharged Spark-Ignition VVA Engine at Part Load

Nowadays, modern internal combustion engines show more and more complex architectures in order to improve their performance. Referring to the spark-ignition (SI) engines, downsizing philosophy and Variable Valve Actuation (VVA) systems allow to reduce the Brake Specific Fuel Consumption (BSFC) at low and medium load, while maintaining the required performance at high load. On the other hand, the above solutions introduce additional degrees of freedom for the engine control, requiring longer calibration time and experimental effort. In the present work, a twin-cylinder turbocharged VVA SI engine is numerically investigated by a one-dimensional (1D) model (GT-PowerTM). The considered engine is equipped with a fully flexible VVA actuation system, realizing an Early Intake Valve Closure (EIVC) strategy. Proper "user routines" are implemented in the code to simulate turbulence and combustion processes. In a first stage, 1D engine model is validated against the experimental data under part load condition, both in terms of overall performance and combustion evolution. The validated 1D engine model is then integrated in a multipurpose commercial optimizer (mode FRONTIERTM) with the aim to identify the engine calibrations that simultaneously minimize BSFC and Brake Mean Effective Pressure (BMEP) under part load operation at a specified engine speed of 3000rpm. In particular, the decision parameters of the optimization process are the EIVC angle, the throttle valve opening and the waste-gate valve opening and combustion phasing. Proper constraints are assigned for the pressure in the intake plenum in order to limit the gas-dynamic noise radiated by the intake mouth. The adopted optimization approach shows the capability to reproduce with a very good accuracy the experimentally advised optimal calibration, corresponding to the numerically derived Pareto frontier in the Brake Mean Effective Pressure (BMEP)-BSFC tradeoff. The optimization also underlines the advantages of an engine calibration based on a combination of EIVC 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

Performance Optimization of a Turbocharged Spark-Ignition VVA Engine at Part Load

Nowadays, modern internal combustion engines show more and more complex architectures in order to improve their performance. Referring to the spark-ignition (SI) engines, downsizing philosophy and Variable Valve Actuation (VVA) systems allow to reduce the Brake Specific Fuel Consumption (BSFC) at low and medium load, while maintaining the required performance at high load. On the other hand, the above solutions introduce additional degrees of freedom for the engine control, requiring longer calibration time and experimental effort. In the present work, a twin-cylinder turbocharged VVA SI engine is numerically investigated by a one-dimensional (1D) model (GT-PowerTM). The considered engine is equipped with a fully flexible VVA actuation system, realizing an Early Intake Valve Closure (EIVC) strategy. Proper "user routines" are implemented in the code to simulate turbulence and combustion processes. In a first stage, 1D engine model is validated against the experimental data under part load condition, both in terms of overall performance and combustion evolution. The validated 1D engine model is then integrated in a multipurpose commercial optimizer (mode FRONTIERTM) with the aim to identify the engine calibrations that simultaneously minimize BSFC and Brake Mean Effective Pressure (BMEP) under part load operation at a specified engine speed of 3000rpm. In particular, the decision parameters of the optimization process are the EIVC angle, the throttle valve opening and the waste-gate valve opening and combustion phasing. Proper constraints are assigned for the pressure in the intake plenum in order to limit the gas-dynamic noise radiated by the intake mouth. The adopted optimization approach shows the capability to reproduce with a very good accuracy the experimentally advised optimal calibration, corresponding to the numerically derived Pareto frontier in the Brake Mean Effective Pressure (BMEP)-BSFC tradeoff. The optimization also underlines the advantages of an engine calibration based on a combination of EIVC 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

A 1D/3D Methodology for the Prediction and Calibration of a High Performance Motorcycle SI Engine

AbstractNowadays internal combustion engines development is widely supported by 1D and 3D codes. On the other hand, an extensive experimental activity at test bench involves increased development cost and time-to-market of a new engine. For this reason, a numerical methodology capable to provide a reliable estimation of engine performance starting from a reduced set of measured data represents a very promising approach.In this paper, a hierarchical 1D/3D numerical procedure is proposed with reference to a motorcycle naturally aspirated spark ignition engine to predict its performance even in absence of experimental data.To this aim, the real engine is geometrically characterized through a reverse engineering process. Different measurements including 3D cylinder geometry, main lengths and diameters of intake /exhaust systems and valve lift profiles, are carried out. Then, a 1D model of the whole engine is realized within the GT-Power™ code, while a 3D model of the sole cylinder is developed within ANSYS Fluent™ environment.The exchange of data between 1D and 3D models starts with preliminary 3D CFD analyses, performed to evaluate the discharge coefficients of intake and exhaust valves. The latter are passed to 1D model to compute the time-varying boundary conditions at the intake and exhaust head ducts, under motored operation. Multi-cycle 3D CFD analyses are hence carried out to describe the in-cylinder mean and turbulent flow fields in motored conditions. The mass-averaged 3D results are then used to tune the turbulence sub-model included in the 1D engine model.The last step of the procedure is the computation of the engine performance under fired conditions at full load by means of the 1D simulation. The numerical/experimental comparison of performance parameters demonstrates that the proposed methodology is capable to satisfactory describe the overall engine behavior even in absence of detailed experimental data