Alfredo Gimelli

Study of a new mechanical variable valve actuation system: Part II—estimation of the actual fuel consumption improvement through one-dimensional fluid dynamic analysis and valve train friction estimation

It is commonly recognized that one of the most effective ways to improve Brake-Specific Fuel Consumption (BSFC) in a spark-ignition engine at partial load is the adoption of VVA strategies, which largely affect the pumping work. Many different solutions have been proposed, characterized by different levels of complexity, effectiveness and costs. VVA systems currently available on the market allow for variable valve timing and/or lift (VVA). The design of a new mechanical VVA system has been discussed in Part I of this article. That study led to the development of a four-element VVA mechanism. Now, to estimate the potential advantages of the studied system on engine performances, one-dimensional thermo-fluid dynamic analyses were conducted, considering both full load and partial load operating conditions. For this reason, this article addresses the definition of the one-dimensional model of a 638-cm3 single-cylinder engine under development, which will be equipped with the four-element VVA system. The findings from the one-dimensional study will be discussed in detail. In particular, the parametric analyses, which concern the engine power at wide open throttle and the SFC at partial load, will be presented. These results, however, are only theoretical results because the one-dimensional simulation is not able to take into account the increased friction losses due to the complexity of the VVA system. Therefore, to correctly quantify the actual fuel consumption allowed by the studied system (net of the generally increased power dissipated by friction when compared to a conventional valve train), a specific methodology, discussed in Part I, has been adopted.

Study of a new mechanical variable valve actuation system: Part I—valve train design and friction modeling:

This article addresses the design of a new mechanical Variable Valve Actuation (VVA) system. The basic scheme consists of three main elements, which enable valve lift variation. Although VVA systems could reduce the specific fuel consumption due to an important de-throttling of the intake system, the systems can lead to higher friction losses due to the increased number of components. For this reason, a specific numerical algorithm was implemented to determine either the cam profile or the kinematic and dynamic characteristics of the entire system. In this way, it was possible to estimate the instantaneous and average power dissipated by the frictions for the actuation of each valve. These evaluated frictions will be used in Part II for the estimation of the actual improvement in terms of specific fuel consumption at part load net of the increased mechanical power dissipated when compared to a conventional valve train. A preliminary thermo-fluid dynamic analysis revealed that the proposed variable valve actuation system is unable to significantly reduce the specific fuel consumption because of the inability to carry out valve actuation strategies that reduce the pumping work. A more flexible mechanical VVA system has been thus developed, which is able to allow intake valve deactivation, as well as variation in valve lift, timing and duration. Finally, in Appendix 1, an analytical procedure aimed at the determination of the geometry of the conjugate profiles of a generic mechanism has been described with the aim of obtaining a general methodology for the design of a mechanical VVA system.

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.

Reciprocating Compressor 1D Thermofluid Dynamic Simulation: Problems and Comparison with Experimental Data

The authors here extend a 0D-1D thermofluid dynamic simulation approach to describe the phenomena internal to the volumetric machines, reproducing pressure waves’ propagation in the ducts. This paper reports the first analysis of these phenomena in a reciprocating compressor. The first part presents a detailed experimental analysis of an open-type reciprocating compressor equipped with internal sensors. The second part describes a 0D-1D thermofluid dynamic simulation of the compressor. Comparison of computed and measured values of discharge mass flow rate shows a good agreement between results for compression ratio 𝛽<5. Then, to improve the model fitting at higher pressures, a new scheme has been developed to predict the blow-by through the ring pack volumes. This model is based on a series of volumes and links which simulate the rings’ motions inside the grooves, while the ring dynamics are imposed using data from the literature about blow-by in internal combustion engines. The validation is obtained comparing experimental and computing data of the two cylinder engine blowby. After the validation, a new comparison of mass flow rate on the compressor shows a better fitting of the curves at higher compression ratio.

Employing Micro-Turbine Components in Integrated Solar – MGT - ORC Power Plants

The authors propose in this paper the integration of a combined power plant, with a micro gas turbine (MGT) and an ORC system, and a solar field that allows a temperature increase of the air at the MGT recuperator inlet. Consequently, an increase is also obtained for the exhausts and this leads to an enhanced heat recovery in the ORC boiler and a greater availability of thermal energy.The purpose of the authors’ work is to analyze the effectiveness of the above proposal under several aspects, therefore, under a preliminary thermodynamic study of the plant set-up in CHP mode, the attention is paid to the choice of the organic fluid expander with a low-cost objective. Classical similarity criteria are helpful for a first estimate of the adaptability of a radial flow turbine from the down-sized turbocharger technology to an organic working fluid. A CFD validation of such choice is then carried out and this phase also determines the characteristic curves of the radial flow turbine.Basing on the availability of the characteristic maps of all the rotating components, a reliable off-design analysis of the solar-assisted CHP plant is performed under several load levels and environmental conditions, the latter strongly influencing the solar irradiance on the parabolic trough collectors.Finally, since the examples refer to both natural gas and biogas fuelling, a CFD analysis of the reacting flow through the MGT combustor checks the combustion effectiveness under challenging variations of the boundary conditions.Copyright