• Energy Research

Abstract This work describes the development and the application of a quasi-3D method for the simulation of turbochargers for automotive applications under unsteady flow conditions. The quasi-3D approach is based on the solution of conservation equations for mass, momentum and energy for unsteady flows and applied to 0D and 1D elements arbitrarily oriented in the space. The compressor is divided into different regions, each one treated numerically in a different way. For the impeller region a relative reference system has been used and the presence of a centrifugal force field has been introduced both in the momentum and energy conservation equation. The direction of the ports at the inlet and outlet of the impeller are used to determine the design flow angles and therefore the deviation during off-design conditions. Conversely in the vaneless diffuser the conservation of the angular momentum of the flow stream has been imposed in the tangential direction and then combined with the solution of the momentum equation in the radial direction. The model has been validated against measurements carried out on the test bench of the University of Genoa both in diabatic and adiabatic conditions.

In the last few years, the effect of diabatic test conditions on compressor performance maps has been widely investigated leading some authors to propose different correction models. The accuracy of performance maps constitutes the basis of the turbocharger matching with the engine, for which 1D procedures are more and more adopted. The classical quasi-steady approach generally used is based on the employment of compressor and turbine characteristic maps assuming adiabatic turbocharger conditions. The aim of the paper is to investigate the effect of heat transfer phenomena on the experimental definition of turbocharger maps, focusing on compressor performance. This work was developed within a collaboration between the Polytechnic School of the University of Genoa and CRITT M2A. The compressor steady flow behavior was analyzed through tests performed on different test rigs operating at the University of Genoa and at CRITT M2A, under various heat transfer conditions. The experimental campaign was developed on a small waste-gated turbocharger for gasoline engine application considering the effect of different heat transfer states by varying turbine inlet temperature, oil and coolant temperature and compressor inlet pressure. Non-destructive measurements were performed in order to get a proper database, including quasi-adiabatic maps, as a reference for the comparison of different correction models. A first comparison between raw results highlighted differences up to 30 percentage points of compressor efficiency at the lowest tested turbocharger rotational speed (30% of maximum speed). Three different correction models, selected in the available open literature for their simplicity and applicability especially in terms of input data required (geometrical and thermo-physical turbocharger characteristics) were then applied after slight modifications. The relevant results are presented and discussed in the paper.

Turbocharging is playing today a fundamental role not only to improve automotive engine performance, but also to reduce fuel consumption and exhaust emissions for both Spark Ignition and diesel engines. Dedicated experimental investigations on turbochargers are therefore necessary in order to get a better understanding of its performance. The availability of experimental information on realistic turbine steady flow performance is an essential requirement to optimize engine-turbocharger matching calculations developed in simulation models. This aspect is more noticeable as regards turbine efficiency, since its swallowing capacity can be accurately evaluated through the measurement of mass flow rate, inlet temperature and pressure ratio across the machine. Actually, in the case of a turbocharger turbine, isentropic efficiency directly evaluated starting from measurement of thermodynamic parameters at the inlet and outlet sections can give significant errors. This inaccuracy is mainly related to the difficulty of a correct evaluation of the turbine outlet temperature due to the flow field and temperature distribution at the exit of the machine. In the paper a preliminary experimental analysis on the direct evaluation of turbine isentropic efficiency is reported. In particular, a specific “hand-made” three holes probe was adopted to measure flow field distribution and thermodynamic quantities downstream the turbine.

Abstract The use of centrifugal compressors has been increasing tremendously in the last decade as they are a key component in the present energy scenario both in the modern internal combustion engine design and in advanced cycles and innovative plant layouts as fuel cell systems. Instability phenomena limit the operating range of the whole compressor system, especially during fast transients. The target is therefore to extend the minimum flow limit in order to improve the operability of each unit while avoiding compressor surge operation and guaranteeing safe operation. For this reason, it is necessary to develop a monitoring system capable of preventing surge and extending the operating range of these machines, their performance, and reliability to allow the integration with the other plant components. The experimental investigation carried out at the University of Genoa turbocharger test facility and presented in this work, consists of steady-state and transient measurements used to characterize and identify compressor behavior in correspondence of surge inception conditions to determine different techniques which could represent surge precursors. The data analysis concentrates on pressure and vibro-acoustic signals by applying different signal processing techniques in the time and frequency domain to classify compressor operation as stable or unstable. The cross correlation function and wavelet analysis have been identified as techniques to define a precursor able to detect incipient surge conditions. Through cross correlation function analysis, it has been possible to identify the presence of propagation phenomena in the system and to evaluate how these events become more significant near an unstable low-mass flow rate condition. At low mass flow rate condition, spikes of significant amplitude are well detectable in the cross correlation function indicating the rise of significant random content in the system responses associated with the rise of incipient surge condition. Additionally, the continuous wavelet transform has been applied to operational signals to show how their time-dependent spectral structure responses can highlight the rise of unstable phenomena, not easily identifiable with traditional signal processing techniques. Exploiting its features in terms of good frequency and time resolution allowed us to identify different contents in system responses regarding phenomena that take place close to surge line and were able to detect their nature in conditions very close to deep surge ones (e.g., rotating stall with its intermitting characteristic nature). Moreover, system response was studied in high frequency range and through a demodulation technique, it was found how blade passes frequency energy content change interacting with rotating stall inception, moving close to surge line. The obtained results provide an interesting diagnostic and predictive solution to detect compressor instabilities at low mass flow rate operating conditions and to prevent compressor fails.

<div class="section abstract"><div class="htmlview paragraph">Turbochargers are still one of the most common solutions to improve internal combustion engines performance. The correct evaluation of turbochargers characteristic maps is one of the main issues to achieve a good matching with internal combustion engines. In a 1D procedure the accuracy of performance maps constitutes the basis of the turbocharger matching with the engine. The classical quasi-steady approach assumes that compressor and turbine characteristic maps are evaluated under the hypothesis of adiabatic turbocharger behavior. The aim of the paper is the investigation of the effect of heat transfer phenomena on the measured turbocharger maps. A model to correct compressor efficiency evaluated starting from measured data, thus removing the heat transfer effects, is proposed. The compressor steady flow behavior has been analyzed through specific tests performed at the test rig for components of propulsion systems of the University of Genoa, under various heat transfer conditions. The experimental campaign was conducted on a water-cooled turbocharger for spark ignition engines and on an uncooled turbocharger for diesel engines considering the effect of different heat transfer conditions. Then, measurements were carried out under quasi-adiabatic conditions, maintaining a constant temperature between the compressor - intermediate casing - turbine to validate the proposed model. Thanks to this specific campaign it was possible to validate the proposed model highlighting a high degree of accuracy. The main advantage of the method presented here compared to others developed by the authors or found in the open literature is its ease of use, thus requiring a small amount of geometric and physical information that can be obtained in a standard turbocharger test bench.</div></div>

ICE2007-SAE International Conference on Internal Combustion Engines, Capri (Napoli, Italy), September 17-20, 2007

Abstract An experimental and theoretical investigation is being performed to evaluate exhaust emissions and fuel consumption of Heavy Duty Vehicles (HDVs) circulating in urban areas and involved in commercial shipping activities. The study is focused on the city of Genoa, whose urban road network is influenced by highway connections and shipping activities, as seven motorway exits and more than twenty accesses to port area are located within the urban area. In a first step, the HDV flows crossing highway exits, urban zones and port areas were evaluated, as well as the relevant vehicle classes. The typical urban trips linking highway exits to port gates and the HDV mission profiles within the port area were then defined, whose validation was performed through an experimental campaign for HDV instantaneous speed measurements on urban trips and in port zones. The availability of speed patterns enabled the application of Passenger Car and Heavy Duty Emission Model (PHEM) for the estimation of fuel consumption and emission factors for selected HDV classes. The main results of the different investigation steps are presented and discussed in the paper, outlining the specific activities of HDVs in port area and the relevant emissive behaviour.