D. Chalet

Analysis of Unsteady Flow Through a Throttle Valve Using CFD

Abstract In automotive application, modelling of the various singularities in pipe systems still remain challenging. The aim of this study is to develop a new throttle valve model which can be included in a one dimensional engine simulation code. First, experimental study was conducted to evaluate an existing literature model, and results show drawbacks in this model. Then after a new model was developed based on the experimental study and CFD analysis, with the throttle valve modeled as a function of flow characteristics and geometrical parameters. After including the new model in the engine simulation code, the engine performance at partial load was determined and results show good agreement with the experimental study.

Describing uncertainties encountered during laboratory turbocharger compressor tests

To improve the existing turbocharger compressor technologies (surge margin increase, improving compression abilities, etc.), compressor tests require accurate determination of efficiency, pressure, and exact surge line emplacement. Moreover, due to nonideal measurement conditions typically encountered during turbocharger compressor performance testing, it is imperative to define measurement uncertainties to the experiments. This article deals with how reliable the results of the turbocharger compressor tests for a given test facility are. This article begins with a general introduction to measurement uncertainties, and a description of the turbocharger test facility together with sources of error of the different transducers is then presented. Uncertainty equations are derived from compressor performance relations; transducer errors were introduced to uncertainty equations so that the efficiency, pressure, and mass flow uncertainties of compressor at different functioning points can be predicted before compressor testing. Uncertainty analysis of nine different compressor functioning points was carried out. Finally, parameters that mostly affect the compressor performance results were identified.

Transfer matrix measurements for studying intake wave dynamics applied to charge air coolers with experimental engine validation in the frequency domain and the time domain

The flows at the intake and the exhaust of an internal-combustion engine are most of the time simplified to a single space dimension, and the hyperbolic partial differential equations that govern the compressible and unsteady air flow are discretized and solved numerically. This method is the basis of today’s engine simulation codes. Models for complex parts such as the charge air coolers often need calibration with experimental engine data essentially for the pressure drop coefficients and the corrected lengths. Another technique for understanding wave action inside the pipes of an internal-combustion engine is to use the reciprocating nature of the engine itself and to gain access to the frequency spectrum of the pressure and the mass flow signals. This was achieved in this paper using a dedicated dynamic bench that identifies a transfer matrix which is defined in terms of the pressure and the mass flow rate. This new transfer matrix technique permits the dynamic pressure and the mass flow to be identified under similar conditions to those encountered in an engine. The transfer matrix is measured for two charge air cooler geometries and validated using experimental engine measurements. The results and methodology are explained in the frequency domain and the time domain, and the future objectives and perspectives discussed.

Boundary conditions modelling of one-dimensional gas flows in an internal combustion engine

Abstract The aim of this paper is to improve the boundary conditions modelling for one-dimensional simulation codes. First, a literature survey is made in order to present the main equations and literature models, then a detailed description of the methodology is presented. Cross-sectional area variations and bends are studied using a new methodology. New models are then established that depend on the geometrical configuration and the flow characteristics. Finally, numerical results are compared with experimental results from a shock test bench. The new models are found to give better results for the instantaneous pressure signals and to improve flow calculations significantly.

Simulation of near surge instabilities onset in a turbocharger compressor

This study focuses on numerical simulations of a small automotive turbocharger compressor stage. Two medium speed characteristics were reproduced from nominal operating points to surge and were compared with experimental measurements. The aim of this study is to analyze the flow unsteadiness occurring near the surge line. The complete geometry of the impeller is meshed; hence, no spatio-temporal hypothesis is done during simulations. The main flow patterns are investigated to identify structures that might be responsible of surge inception. Results show that both impeller and diffuser are affected by stall. Blade channels are affected by a complete shroud recirculation extending from upstream of the impeller inlet to the diffuser inlet. Three radial recirculation zones are detected on the vaneless diffuser walls, strongly influenced by the two-pike asymmetric pressure field induced by the volute tongue. Its influence is observed at the inlet of the compressor, increasing the inter-blade flow unsteadiness.

Wave dynamics measurement and characterization of a charge air cooler at the intake of an internal combustion engine with integration into a nonlinear code

There exist two fundamental tools for modeling and simulating wave action of internal combustion engines. The first is a nonlinear time domain solution of the Euler equations using a space–time meshing. The second is a frequency domain solution of the linear wave equations. These two methods exist with a wide range of complexity and sophistication. Hybrid coupled methods also exist; they attempt to bridge the gap between the two techniques while maintaining the overall goal of engine simulation in mind. This work deals with the frequency characterization of a complex intake element, the charge air cooler. First, a transfer matrix for a simple tube is defined and measured. Two identical versions of the previous tube serve for identifying the transfer matrix of the charge air cooler directly on an operating four-cylinder turbocharged engine. Once the transfer matrix is measured, it is coupled to GT-Power as four transfer functions coded into Simulink. The final validation comprises two tubes in GT-Power with measured boundary conditions of pressure and mass flow with the Simulink model in between. Results are presented in the time and frequency domains with future objectives and perspectives as well.

Transfer Matrix Computation for Wave Action Simulation in an Internal Combustion Engine

A new technique for simulating engine pressure waves consisting of linking pressure response and mass flow rate excitation in the frequency domain has been presented. This is achieved on the so-called “dynamic flow bench”. With this new approach, precise, fast and robust results can be obtained while taking into account all the phenomena inherent to compressible unsteady flows. The method exhibited promising results when it was incorporated in a GT-Power/Simulink coupled simulation of a naturally aspirated engine.However, today’s downsized turbocharged engines come with more stringent simulation necessities, where discontinuities such as the charge air cooler (CAC) must be correctly modeled. Simulating such engines with the transfer function methodology is quite difficult because it requires mounting the entire intake line on the bench. Modeling wave action for these engines requires an understanding in the frequency domain of the flow’s characteristics through the different elements that make up the intake line. This leads us to study the acoustic transfer matrices.In order to split the intake line into separate elements, a straight duct of 185mm length is chosen as a first reference. It is mounted on the dynamic flow bench and pressure response is measured after an impulse mass flow excitation. Transfer functions of relative pressure and mass flow rate are then identified at given points upstream and downstream of this reference tube. These functions produce the desired transfer matrix poles.The resulting matrix is validated by inserting the tube in the intake lines of two four-cylinder engines which are modeled in GT-Power. Pressure and mass flow are registered at the measurement points of the tube from the simulation. The time series data upstream of the tube is treated in the frequency domain and the transfer matrix is used to calculate the corresponding downstream values. Measured values from the native simulation and those calculated using the transfer matrix propagation are then compared.Finally, the experimental technique for identifying transfer matrices of more complex elements using two versions of the previous tube is presented.Copyright

Effect of a map width enhancement system on turbocharger centrifugal compressor performance and surge margin

Engine downsizing is potentially one of the most effective strategies being explored to improve fuel economy. A main problem of downsizing using a turbocharger is the small range of stable functioning of the turbocharger centrifugal compressor at high boost pressures. Several stabilization techniques have been studied to increase the compressor operating range without sacrificing the compressor efficiency. This paper presents an experimental study on a turbocharger compressor equipped with casing treatment (a map width enhancement (MWE) system). A KKK turbocharger was modified and tested at Ecole Centrale de Nantes. The experimental programme mainly dealt with traditional compressor performance measurements for an MWE. Whereas much work has been previously carried out to study the apparatus communicating between the annular flow passage and the impeller wheel, an apparatus with multiple holes has not been previously considered. In this paper experiments have been inducted to study the effect of an MWE structure with plurality of holes on the performance and stable functioning of a turbocharger centrifugal compressor. The effect of extending the length of the inner tubular wall has also been investigated. Two different MWE configurations have been tested and results were analysed while conducting a series of tests. The experimental studies have shown that while the MWE was advantageous in increasing pressure ratio at high compressor speeds, it failed in extending the compressor map. Moreover, extending the inner tube length was efficient in significantly shifting the surge line to low flowrates, by producing a uniform flow at inducer inlet.

Inflow boundary condition for one-dimensional gas dynamics simulation code of internal combustion engine manifolds

Abstract The modelling of pressure wave propagation into internal combustion engine manifolds implies the knowledge of multi-dimensional phenomena. However, computational times can be very important and this complex system requires reducing the order of the models. In the current paper, a new one-dimensional modelling of the inflow boundary conditions for a plain open end is proposed. This is obtained with the use of a computational fluid dynamics code and an experimental test bench under unsteady state conditions. Numerical and experimental analyses show that the pressure losses can be modelled with a coefficient that depends on the Mach number as well as geometrical parameters. Finally, numerical results are compared with experimental data in a shock tube application with an intake manifold of internal combustion engine. The code enhancements significantly improve flow calculations.

Different Measurement Techniques for Wider Small Radial Turbine Performance Maps

Engine downsizing usually requires the use of a turbocharger. This component’s related data are reduced. This is caused by the small range of stable functioning of its centrifugal compressor at high boost pressures. That is why the measurement of the data of both the compressor and the turbine is limited. Numerical simulations are used by automotive manufacturers for internal combustion engines simulations, so it is necessary to have an accurate and reliable extrapolation model of the turbine performance maps. Once an extrapolation model is established, the new performance map can be used for internal combustion engines calibration. This study presents different experimental techniques to measure the widest performance map of a small radial turbine. This turbine is a part of a turbocharger of a small diesel engine. Experiments were held on a traditional turbocharger test rig at first. The turbine inlet temperature was changed to extend the mass flow rate measurement range. Then there was the compressor force-feeding where air was blown through the compressor inlet and exit. This technique allowed the extension of the map by increasing and decreasing the power consumed by the compressor rotor and moving its surge and choking limits. After that, the compressor was replaced by another one with a reversed rotational direction. Blowing air to the new compressor exit enabled us to use it as a turbine and hereby extend the data map. We measured very low mass flow rates using a hot wire anemometer. This sensor also allowed us to measure negative mass flow rates to reach the expansion ratio of one zone. These techniques gave an almost complete mass flow rate performance map with an expansion ratio going from 1 to 6 for some rotational speeds. As for the efficiency, data were measured in adiabatic conditions. This allowed the calculation of the turbine total-to-static isentropic efficiency and the turbocharger mechanical efficiency separately. These data enabled us to calculate the turbine efficiency according to the manufacturer’s method.