A. Tiseira

Importance of Mechanical Losses Modeling in the Performance Prediction of Radial Turbochargers under Pulsating Flow Conditions

This work presents a study to characterize and quantify the mechanical losses in small automotive turbocharging systems. An experimental methodology to obtain the losses in the power transmission between the turbine and the compressor is presented. The experimental methodology is used during a measurement campaign of three different automotive turbochargers for petrol and diesel engines with displacements ranging from 1.2 l to 2.0 l and the results are presented. With this experimental data, a fast computational model is fitted and used to predict the behaviour of mechanical losses during stationary and pulsating flow conditions, showing good agreement with the experimental results. During pulsating flow conditions, the delay between compressor and turbine makes the mechanical efficiency to fluctuate. These fluctuations are shown to be critical in order to predict the turbocharger behaviour.

Assessment of a sequentially turbocharged diesel engine on real-life driving cycles

The article presents the development of the control manager of a parallel sequential turbocharger system. This control manager must decide the transition between the different operation modes of the boosting system. The control manager must protect the system from surge and over-speed risks.The methodology followed in the development process was based on the use of concurrence survey data from a similar engine, the simulation with a 1D code and finally the assessment on engine test bench. Real-life driving cycles were used during the development process, which were acquired in three different driving scenarios (city, road and mountain road).

Solution of the turbocompressor boundary condition for one-dimensional gas-dynamic codes

Nowadays, turbocharged engines are widely used in cars and trucks. Gas-dynamic codes are an important tool in design and optimization of these types of engines. These codes solve the one-dimensional governing equations in ducts for compressible, unsteady and non-homoentropic flow. The ducts are generally solved using finite difference schemes, the volumes are solved by means of filling and emptying models and the connections represent the boundary conditions of the ducts. One important boundary condition is the compressor which connects two ducts. In this junction an increment of momentum and energy is undergone by the flow but depending on its sense the behaviour is different. This paper presents the mathematical base of a compressor model which solves this complex boundary condition. The governing equations of the model have been presented in detail. The solution involves a non-linear equation system that has to be solved iteratively. The Newton-Raphson root-finding method has been chosen to get its solution. Finally, some results of the model have been compared to measurements focusing in surge prediction.

On-engine measurement of turbocharger surge limit

In this article a new experimental technique is presented to measure the turbocharger surge limit in a regular engine test bench. It is known that the surge margin on engine tests may be very different from that obtained in a steady-flow gas-stand. In particular, surge is very dependent on the flow pattern produced by the compressor inlet duct and also on the piping upstream and downstream the compressor. The proposed technique that is based on the injection of pressurized air into the intake manifold is compared with the other ways of measuring the compressor map on engine. Some results with different compressor arrangements are presented and discussed. It is demonstrated that this technique allows for measuring not only the actual surge line but also the complete compressor performance map.

Coupling methodology of 1D finite difference and 3D finite volume CFD codes based on the Method of Characteristics

This paper describes the methodology followed to perform a co-simulation between 1D (OpenWAM) and 3D (FLUENT) CFD codes. The Method of Characteristics (MoC) has been chosen to transfer the information between the two domains by properly updating the boundary condition at the shared interface. A short explanation of the MoC is provided, including the modifications needed by the Riemann invariants when dealing with non-homentropic flow. The implementation of the coupling is explained, focusing on the particular approach required by FLUENT in order to obtain the Riemann invariants. Two validation tests have been performed. The Sods problem has been used to test the numerical accuracy of the coupling methodology. On the other hand, an impulse test rig configuration has been simulated to show the potential capability of a co-simulation in terms of reducing the computational cost. In both cases a good agreement in the solution is found.

Optimization of the inlet air line of an automotive turbocharger

This paper presents different aspects of air inlet behaviour near the inducer of a radial compressor and shows how the geometry can contribute to its stability and performance. Unfortunately, the space reserved for installation of an automotive turbocharger in a vehicle is constantly being reduced, so it is necessary to study the effects that elbows and abrupt changes in flow directions originate on the compressor performance. The work presented in this paper studies the effect that different 90° elbows have on the compressor with respect to its ideal, straight, no-elbow configuration, in order to obtain the best possible elbow configuration. The methodology followed has been to, initially, study different geometries in computational fluid dynamics code in order to obtain the best possible configuration. Then, several 90° elbows were constructed and characterized on a continuous flow test bench in order to validate the computational fluid dynamics results and to obtain optimum results. The elbows were then installed on a radial compressor and tested on a hot, continuous turbocharger test bench, where the compressor was characterized and maps were obtained with each different elbow. The results were compared with respect to the ideal, no-elbow configuration, which was taken as the base performance. After analysing the results obtained, it is possible to observe that in most of the cases, the elbows have a negative effect on the compression ratio, which tends to be reduced, especially at high rotor velocities and high air mass flow. On the other hand, the effect on the surge limit seems to be positive, as the surge line shifts to lower air mass flows, although the maximum mass flow allowed is reduced. It seems as if the compressor map shifts to the left with a reduction in compression ratio. From theoretical and experimental studies, it has been concluded that flow uniformity index and pressure loss are the most important factors affecting the performance of the compressor.

Effect of the numerical scheme resolution on quasi-2D simulation of an automotive radial turbine under highly pulsating flow

Automotive turbocharger turbines usually work under pulsating flow because of the sequential nature of engine breathing. However, existing turbine models are typically based on quasi-steady assumptions. In this paper a model where the volute is calculated in a quasi-2D scheme is presented. The objective of this work is to quantify and analyse the effect of the numerical resolution scheme used in the volute model. The conditions imposed upstream are isentropic pressure pulsations with different amplitude and frequency. The volute is computed using a finite volume approach considering the tangential and radial velocity components. The stator and rotor are assumed to be quasi-steady. In this paper, different integration and spatial reconstruction schemes are explored. The spatial reconstruction is based on the MUSCL method with different slope limiters fulfilling the TVD criterion. The model results are assessed against 3D U-RANS calculations. The results show that under low frequency pressure pulses all the schemes lead to similar solutions. But, for high frequency pulsation the results can be very different depending upon the selected scheme. This may have an impact in noise emission predictions.

Analysis of the influence of different real flow effects on computational fluid dynamics boundary conditions based on the method of characteristics

Abstract Nowadays, turbocharged internal combustion engines (ICEs) are very common in automotive powerplants, monopolizing the diesel sector and having a steadily increasing percentage in the gasoline one. In this frame, the interest in modeling the behavior of the turbomachinery components involved, with the ultimate goal of characterizing the performance of the turbocharged ICE, seems clear. A turbomachine can be simulated using three-dimensional computational fluid dynamics software, but its computational cost does not allow one to reproduce the whole turbocharger test rig. Moreover, the existence of long ducts requires a considerable computational time until the pressure reflections at the boundaries dissipate in order to reach a periodic solution. The use of non-reflecting boundary conditions reduces the length of ducts needed without introducing spurious wave reflections. An anechoic boundary condition (BC) based on the method of characteristics (MoC) has been previously developed, considering the case of an inviscid and adiabatic one-dimensional flow of a perfect gas. However, real flows do not behave in such an ideal manner. In this paper, the extension of the scope of the previous BC is sought. In this way, a methodology to evaluate the performance of the anechoic BC under these real flow situations is shown. The consideration of an ideal gas instead of perfect gas, the flow viscosity, and the non-homentropic flow make it necessary to modify the method of characteristics, since the Riemann invariants are not constant any more. In this frame they are referred to as Riemann variables. An additional issue that has been considered is the effect of swirl flow, such as the one in the turbine outlet, on the anechoic BC. Some improvements to be implemented in the BC are proposed in order to have a better performance in these real flow situations.

A Procedure for the Unsteady Characterization of Turbochargers in Reciprocating Internal Combustion Engines

In actual scenario of turbocharged automotive engines a precise knowledge of unsteady effects on turbocharger performance, such as pulsating flow influence, is needed. The paper exposes several methodologies to characterize unsteady phenomena in both compressor and turbine of the turbocharger. The experimental work goes along with compressor and turbine models that account for the unsteady effects. These models, implemented in a 1D gas-dynamic code, become a powerful tool in turbocharging system development. A methodology mixing calculation and experiments is presented that allows for measuring and modeling the pulsating effects in turbocharger performance at close to engines conditions and calculating the average turbine isentropic efficiency, under pulsating flow.

Characterization of EGR Cooler Response for a Range of Engine Conditions

Fouling phenomenon is a key issue for EGR cooler operation. In spite of the fact that soot deposition is imposed by the characteristics of the exhaust gases flow, the design of the EGR cooler has a significant impact for effect on the engine. New combustion modes corresponding to new engine developments and combination of EGR system with other post-treatment devices make that fouling conditions for future generations of EGR coolers can be significantly different from previous applications from Euro 3 to Euro 5. An investigation has been performed in order to characterize the response of different EGR coolers designs for different conditions of the exhaust gases. As for the design, the technology selected has been tube-and-fin heat exchanger, which is a high performance technology that fits Euro 6 customer specifications. The variations in design have been made through modifications in fin characteristics, both in configuration and geometric dimensions. As for engine operation conditions, the exhaust gases characteristics have been modified from a standard calibration to get more severe fouling conditions, in terms of HC content, opacity, exhaust gas temperature and flow. The degradation of performance has been characterized through measurements of thermal efficiency and permeability. It can be concluded that HC content and opacity have a significant influence on fouling phenomena, and that EGR cooler optimum design is highly dependent on these exhaust gas conditions. INTRODUCTION Nowadays refrigerated EGR is a solution for NOx reduction that has been generalized following continuous tightening of environmental regulations. Customer requirements have imposed a roadmap for EGR coolers development that has resulted in a significant increase of thermal efficiency balanced with the permitted pressure drop for these heat exchangers. It has been achieved without a significant penalty on space. Thus, compact technologies for EGR coolers are being implemented in current and future engines. Another challenge for EGR coolers is the requirement for durability of anti-pollutant devices. That is, function of the device must be maintained at a certain level after a period of use. In particular, for European emission standards, the verification condition for durability has been fixed in terms of a number of kilometers, varying from 80000 to 160000 km from Euro 3 to Euro5/6, or a number of years, in particular 5, whichever occurs first [1]. However, there is no a clear criteria to define acceptance of the degradation, so application to EGR Cooler component design cannot be done. There are also specifications coming from some customers regarding fouling impact on the degradation of performance for the EGR cooler, both for thermal efficiency and pressure drop. The exposure conditions for that degradation are not always defined. In some cases a given engine test is specified or a fouling factor is considered. In some others degradation is not linked to any specification, and supplier experience is taken into account to evaluate performance change. The specifications for fouling impact on EGR Coolers are much more generalized for Euro 6 applications than for previous engine generations. Two main concerns affect engine developers. On one hand, decrease in thermal efficiency and on the other hand increase in pressure drop. Decrease in thermal efficiency is directly linked to quantity of recirculated exhaust gas, since it means an increase in temperature and therefore a decrease in mass, so it will affect to NOx formation. Pressure drop increase can be assumed by EGR valve regulation up to certain level. However, if increase is very high it could compromise the position of air inlet valve, therefore decreasing engine performance. EGR Cooler designer must have the knowledge to answer customer demands, anticipating to incidences that may occur during engine tuning. For that, characterization of EGR Coolers in engine with a wide range of operation conditions is needed. Past experiences are based on conventional combustion modes not representing possible use conditions of the heat exchanger for next generation of engines.