Elias Chebli

Development of a turbocharger compressor with variable geometry for heavy-duty engines

This article describes the first development phase of a centrifugal compressor with variable geometry which is designed to match the needs of future heavy-duty engines. Requirements of truck engines are analyzed, and their impact on the properties of the compressor map is evaluated in order to identify the most suitable kind of variable geometry. Our approach utilizes the transformation of engine data into pressure ratio and mass flow coordinates that can be displayed and interpreted using compressor maps. One-dimensional and three-dimensional computational fluid dynamics fluid flow calculations are used to identify loss mechanisms and constraints of fixed geometry compressors. Linking engine goals and aerodynamic objectives yields specific recommendations on the implementation of the variable geometry compressor.

Innovative Variable Turbine Concept for Turbochargers

New concepts for the optimisation of supercharging systems have been analysed to improve fuel consumption, emissions and transient diesel engine response. In addition to the conventional VTG (Variable Turbine Geometry) where the variability takes place upstream of the turbine impeller, a new innovative variable turbine geometry called VOT (Variable Outlet Turbine) is investigated in this paper where the variability takes place at impeller exit. The flow variability is achieved by variation of the flow cross-section at the turbine outlet using an axial displacement of a sliding sleeve over the exducer and provides a simple solution for flow variability. The flow field of the VOT is calculated by means of steady state 3D-CFD simulations to predict the aerodynamic performance as well as to analyse the loss mechanisms. The VOT design is optimised by finding a good balance between clearance and outlet losses to improve the turbine efficiency. Furthermore, experimental results of the VOT are presented and compared to a turbine equipped with a waste gate (WG) that verify the efficiency advantage of the VOT. In general, it is found that the use of the VOT at high specific speed is important to reduce the outlet losses and to improve the turbine efficiency over a wide operating range.Copyright

The Variable Outlet Turbine Concept for Turbochargers

A variable geometry concept for advanced turbocharger (TC) systems is presented. The variability of the device is based on outlet area changes as opposed to the more common systems that are based on inlet turbine geometry changes. In addition to the conventional variable turbine geometry (VTG), the new variable turbine type is termed variable outlet turbine (VOT). The flow variability is achieved by variation of the flow cross section at the turbine outlet using an axial displacement of a sliding sleeve over the exducer and provides a simple solution for flow variability. In order to predict the aerodynamic performance and to analyze the loss mechanisms of this new turbine, the flow field of the VOT is calculated by means of steady state 3D-CFD (computational fluid dynamics) simulations. The VOT design is optimized by finding a good balance between clearance and outlet losses. Furthermore, experimental results of the VOT are presented and compared to a turbine equipped with a waste gate (WG) that demonstrates an efficiency advantage of 5%. Additionally, engine performance measurements were carried out to investigate the influence of the VOT on fuel consumption and to asses the functionality of the new pneumatic actuating system. The VOT engine tests show also performance advantage in comparison to a WG turbine especially toward high engine loads. It is found that the use of the VOT at this condition shows a turbine efficiency advantage of 6% related to a reduction in engine fuel consumption of 1.4%. The behavior at part load is neutral and the peak turbine efficiency of the VOT is comparable with a fix turbine geometry.

Investigation on the Impact of Circumferential Grooves on Aerodynamic Centrifugal Compressor Performance

Casing treatments are widely used in compressors for increasing range, stability and aerodynamic performance. However, applications in centrifugal compressors, as commonly used in turbochargers, are rare and mostly applied at the inlet region in terms of bleed slots. This paper presents the application of circumferential grooves, which are known to increase stability and performance in axial compressors, to the rear part of the impeller shroud casing in a centrifugal compressor. Experimental and numerical investigations of three different configurations have been performed and compared with the initial geometry. Experiments were conducted on a hot gas test facility where static pressure and temperature measurements up- and downstream of the compressor were acquired. The results indicate only small changes in operating range except for one speedline, where a considerable improvement is observed. Efficiency remains nearly unaffected for all configurations whereas the pressure ratio is increased at some operating points. For detailed examination of the compressor flow field, CFD analyses were conducted using steady-state RANS calculations. Structured meshes with node to node connections were used to suspend any possible influences stemming from interfaces in regions of interest. Validation with test data yields good agreement concerning choke margin and gradient trends. CFD results confirm that the investigated configurations of circumferential grooves have only small impact on efficiency and pressure ratio. Investigations on the mechanism which balances the additional losses due to increased groove surface friction and increases pressure rise are presented.Copyright

Enhancing the Centrifugal Compressor Performance Map Measurements Through a Developed One–Dimensional Calculation Tool to Analyse Local Flow Phenomena

Centrifugal compressors for automotive turbochargers have large influence on the operation characteristic of combustion engines. Especially the improvement of the surge margin is one of the most important development targets. Thereby, a reliable detection of local flow phenomena within the compressor stage is necessary and a procedure to gain this information from standard measurement data is discussed in this paper.A one–dimensional calculation methodology for a single-stage centrifugal compressor with a vaneless diffuser and casing treatment is presented. The tool calculates the flow properties at the impeller inlet and exit as well as at diffuser exit, based on the measured inlet and outlet data and the geometry information of the compressor. The calculated flow characteristics are plotted within the measured compressor performance map to show local flow parameters. The unsteady recirculation flow within the casing treatment, the inflow angle and the total pressure losses are considered.The tool is validated on different compressor sizes. Thereby the compressor is equipped with static pressure measuring points at the impeller inlet and exit as well as at the diffuser exit. The calculated static pressure correlated well with the measured data with an accuracy of 2 % to 5 % on 95 % of the operating range.In this paper an experimental parameter study is executed in order to improve the surge margin. Thereby the geometry of the diffuser and the casing treatment is varied and the compressor performance is measured on a turbocharger test rig. The calculation of the flow angles and other flow characteristics within the diffuser enables one to find out whether surge is triggered through the diffuser or the impeller.Copyright

Variable Asymmetric Turbine for Heavy Duty Truck Engines

Turbochargers with variable turbine geometry (VGT) are established in diesel engines for passenger cars because of the beneficial effect on transient operation. The variability permits the reduction of exhaust back pressure, resulting in lower fuel consumption. There are only a few applications in heavy duty truck engines due to increased mechanical complexity and vulnerability to failure.This paper presents a turbine concept with a simple variability developed for a heavy duty engine. The variability is achieved upstream of the rotor by changing the sectional area of the volute. This can be done through a rotationally movable ring which shifts the circumferential position of the volute tongues. These separate both scrolls of a double segment turbine and can be rotated by an electric actuator.The performance maps measured at the hot gas test stand show the large variability of the flow parameter and the high efficiency levels over the operating range of the variable asymmetric turbine (VAT).The flow field is computed by the use of 3D-CFD simulations in order to analyze the loss-generating mechanisms that occur within the machine.Test runs on an engine test stand demonstrate the high potential of the concept concerning reduction of fuel consumption and a wide scope of realizable EGR rates in order to reduce NOx emissions in a cost-effective way. The resultant large mass flow variability allows the deletion of the waste gate and enables efficiency improvements.© 2013 ASME