Engine calibration issues

Once the engine under study has been deeply specified in terms of sub-systems and all the operating parameters (i.e. degrees of freedom of the engine system) have been properly defined as well as the performance requirements, the engine calibration phase is realized with aim to identify the optimum set of engine variables for various operating conditions by taking into account the system performance targets (minimum fuel consumption and pollutant emissions at part load, maximum power and torque at full load, combustion stability at idle, etc.). As general consideration, modern spark ignition engines are characterized by complex architectures, showing a significant number of sub-systems and operating variables, making the engine calibration a challenging phase during the system development. Indeed, from an experimental point of view, the calibration requires an high number of tests to explore the whole operating domain of the control parameters and the entire engine operating map. This involves a negative impact on the engine development time and costs.

In order to avoid the latter problems, the engine calibration phase is often supported by numerical

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tools and methodologies. As an example, Design of Experiment (DoE) methodologies are successfully employed to reduce the number of experimental tests. A further reduction in the experimental activity can be obtained using proper numerical models that, once tuned, are able to achieve a pre-calibration of the engine, following a fully-numerical approach. Consequently, activity at test-bench can, hence, be limited to verify and refine the desired performance targets. For numerical calibration purposes, among several modeling approaches, 1D engine model, properly validated, coupled to an optimizer tool has been successfully employed to perform multi-objective analyses allowing to numerically identify the optimal set of engine parameters. First, the considered optimization approach shows the capability to reproduce with good accuracy the experimentally advised optimal calibration. Second, for a predefined engine operating point, it is possible to select the new optimal set of engine variables capable to achieve the desired trade-off among different performance requirements. For the downsized engine under study, as said before, the performance at high and full load operation are mainly limited by the knock onset, which influences the engine calibration, causing the adoption of delayed spark timing and over-fuelling mixture. For this reason, the use of proper sub-model for the description of knock phenomenon is mandatory. Only in this way, the reliability of an integrated 1D model/optimizer approach can be ensured and an overall

“virtual” calibration of the engine, including all the control variables (i.e. valve strategy, combustion phasing, mixture quality, throttle opening, WG opening) can be accomplished. In particular, the virtual calibration at high loads, for a selected engine speed and intake valve lift strategy, is oriented to the identification of engine parameters allowing to maximize the brake torque while simultaneously minimize the fuel consumption under knock limited condition. In this way, the adopted numerical approach can also underline the potentiality of an engine calibration at high load, based on the actuation of innovative intake valve lift strategies by the VVA sub-system rather than a purely throttle based calibration. A similar virtual engine calibration can be also performed at part load for various VVA strategies. In conclusion, once the 1D engine model is validated against the experimental data, the discussed numerical approach allows for a virtual engine calibration on theoretical basis and it proves to be an useful tool to support and reduce the experimental costs and the engine time-to-market.

65 1.7 Structure of this work

This work is mainly focused on the study of turbulence, combustion, knock and cycle-by-cycle variation of a downsized VVA Spark Ignition engine. A 0D/1D modeling approach is here adopted to reproduce the engine behavior in different operating conditions. Gas-dynamic noise at intake mouth for the considered engine is also taken into account by testing redesign solutions of the intake system through a 3D CFD model. Engine calibration is also analyzed from a numerical point of view and a proper methodology is presented aiming to perform fully numerical calibrations.

Thesis work is organized into eight chapters. A brief description of the content of each of them is reported below:

The present introductory chapter provides a description of characteristics and operation of Spark ignition engines and of the main technologies for SI engines capable to obtain efficiency improvements as well as pollutant emission reductions. Then, a description of SI turbulent combustion process, knock and cycle by cycle variation phenomena is also presented. Finally, the engine under study is analyzed, mainly focusing on the experimenal activities and calibration issues. Numerical methodologies to support the calibration phase are discussed, too.

The second chapter furnishes on overview of the modeling approaches for internal combustion engines, with particular reference to the in-cylinder processes. The benefits of a proper integration of modeling approaches are also discussed. The 0D modeling approach is mostly analyzed: factal combustion model and “in-house developed” turbulence model are described in detail as well as the 0D/3D hierarchical approach for turbulence modeling.

The third chapter describes the 0D/1D model of the whole engine, developed in GT-Power^TM environment. The single models of the main engine sub-systems and the numerical routines are considered. Once turbulence and combustion sub-models are tuned, the engine model is validated against the experimental data at full and part load operations.

In the fourth chapter various experimental knock detection techniques are analyzed. In particular, two different knock detection methods are utilized to perform a knock analysis for the examined engine. Knock and cyclic dispersion models are discussed and then the knock model taking into account the cycle by cycle variation is validated at full load operation.

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The fifth chapter deals with the adoption of an external EGR system for the engine under study.

Starting from the validated 1D engine model, a Low pressure EGR circuit is virtually mounted and the new engine configuration is tested by 1D simulations. Numerical results show that the adoption of the EGR circuit allows for fuel consumption reduction at full load points.

In the sixth chapter water injection technique at the engine intake ports is investigated by a 1D approach. The effects of the above technique are studied in various full load knock limited operating points and for various A/F levels and water content. The presented results highlight that the considered solution involves significant BSFC improvements, especially in the operating conditions at medium engine speeds.

In the seventh chapter the gas-dynamic noise emitted at the engine intake mouth is firstly reproduced by 1D and 3D CFD models. Once the models are validated at full load, some redesign solutions for the air-box device are proposed and numerically tested to evaluate the reduction in the gas-dynamic noise.

The eighth chapter deals with virtual engine calibration by employing 1D model coupled to external optimizer. Starting from the model validation for two different intake valve strategies, multi-objective optimizations at part and full load are performed, showing the capability to reproduce the experimentally identified calibrations. The effect of intake valve strategy on the fuel consumption is also discussed for various load levels.

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CHAPTER 2