Turbocharger surge behavior for sudden valve closing downstream the compressor and effect of actuating variable nozzle turbine

Surge is an unstable phenomenon appearing when a valve closing reduces the compressor flow rate. This phenomenon is avoided for automotive turbochargers by defining a surge margin during powertrain system design. This surge margin established with measurements in steady state testing regime limits the maximal engine torque at low levels of output. An active control of the compressor could reduce the surge margin and facilitate a transient compressor operation for a short time in surge zone. In this paper, an experimental study of the transient operation of a turbocharger compressor entering the surge zone is performed. Control of the turbocharger speed is sought to avoid unsteady operation using the variable geometry turbine (VGT) nozzle actuator. From a given stable operating point, surge is induced by reducing the opening of a valve located downstream of the compressor air circuit. The effect of reducing the speed of rotation by controlling the VGT valve is investigated, as this should lead to more stable compressor operation. The rotation speed of the turbocharger is controlled to avoid an unstable operating point using servo-actuator of variable geometry turbine. From a stable operating point, the surge appearance is caused by closing a butterfly valve downstream the air circuit of the compressor. The effect on the compressor rotation speed when the opening of variable geometry turbocharger valve is modified is studied. Measurements have been conducted for different control profiles of the VGT valve placed downstream the compressor. This article presents the means used to carry out these tests as well as the results of the measurements of the instantaneous signals of pressure, temperature, flow rate and rotation speed, allowing the analysis of the surge phenomenon.

Numerical Simulation of Two-Stage Variable Geometry Turbine

The modern internal combustion engine (ICE) has to meet several requirements. It has to be reliable with the reduced emission of pollutant gasses and low maintenance requirements. What is more, it has to be efficient both at low-load and high-load operating conditions. For this purpose, a variable turbine geometry (VTG) turbocharger is used to provide proper engine acceleration of exhaust gases at low-load operating conditions. Such a solution is also efficient at high-load engine operating conditions. In this paper, the result of an unsteady, three-dimensional (3D) simulation of the variable two-stage turbine system is discussed. Three different VTG positions were considered for those simulations, along with three different turbine speeds. The turbine inlet was modeled as six equally placed exhaust pipes for each cylinder to eliminate the interference of pressure waves. The flow field at the outlet of the 1st stage nozzle vane and 2nd stage rotor was investigated. The simulations showed that the variable technologies significantly improve the efficiency of the two-stage turbine system. The highest overall efficiency of the two-stage system was achieved at 60,000 rpm and 11o VTG position.

Advances in aerodynamic, structural design and test technology of variable geometry turbines

The variable geometry turbine is one of the technical means to effectively improve the part-load performance, part-load condition stability and manoeuvrability of gas turbines, aeroengines or even turbochargers. However, the design of the variable geometry turbine is very difficult, and the decrease in efficiency offsets some of the engine cycle benefits caused by turbine variable geometry. Therefore, it is very necessary to carry out research on variable geometry turbine technology so that the technology can be successfully applied to various types of gas turbine engines as soon as possible. This paper summarizes and analyzes the recent advances in the field of aerodynamic, structural design and test of variable geometry turbines. This review covers the following topics that are important for variable geometry turbine designs: (1) flow mechanisms and aerodynamic characteristics, (2) wide-condition aerodynamic design method for turbine blades, (3) variable vane turning design method, (4) structural design technology of variable vane system and (5) aerodynamic characteristics and reliability test technology for variable geometry turbines. The emphasis is placed on the variable vane turning design method. We also present our own insights regarding the current research trends and the prospects for future developments.

HCF Optimization of a High Speed Variable Geometry Turbine

This paper describes the structural optimization of a high speed, 35mm tip diameter radial turbine wheel in a Variable Geometry Turbine (VGT) system, subjected to the wide range of aerodynamic loads experienced during the full operating cycle. VGTs exhibit a wide range of unsteady flow features, which vary as the nozzle vanes rotate through different positions during operation, as do the magnitudes and frequencies of the resulting pressure fluctuations experienced by the downstream turbine blades. The turbine wheel typically passes through a number of blade natural frequencies over their operating cycle, and there are a number of potential conditions where these unsteady aerodynamic loads can lead to resonant blade vibration. The focus of this work is on the development of a pragmatic design approach to improve the structural characteristics of a radial turbine blade with respect to High Cycle Fatigue (HCF), informed by detailed time-accurate Computational Fluid Dynamics (CFD) prediction of the unsteady pressure loads, coupled with FE vibration analysis to quantify the resulting blade vibration magnitudes. Unsteady CFD simulations are performed to determine the time-accurate pressure loads on the blades, and the results are used as input to forced response analysis to determine the peak alternating stress amplitudes. The detailed analysis results are then used to guide a subsequent parametric study in order to investigate the influence of key geometric parameters on the structural performance of the blade, with the optimum design identified through the use of a Goodman Diagram. The results quantify the influence of both blade thickness distribution and hub fillet details on the vibration characteristics of radial turbines.

Down-speeding diesel engines with two-stage turbochargers: Analysis and control considerations

Diesel engines continue to be used in truck applications, so reducing fuel use and hence CO2 emissions, is a priority. Single-stage turbocharged diesel engines are known to be fuel efficient under steady load at low speeds. However, the engine’s ability to track load transients becomes limited by emission constraints due to the rate of production values for smoke and the resulting higher nitrogen oxides (NO x). Modern air-path solutions including a variable geometry turbine (VGT) and high pressure exhaust gas recirculation (EGR) can be used to improve dynamic response without increasing NO x emissions, but lead to complex interactions that can be difficult to control. This paper develops a two-stage, in-series, air-path configuration, which improves the typical part-load performance at low engine-speeds through adjustments to the turbine expansion ratios. Better EGR rates (for NO x reduction) at low engine speeds can be achieved whilst the engine transient response is maintained. The air-path system is simulated using Ricardo Wave software and analysed for steady-state and transient behaviour in order to identify the relationships, constraints and performance measures for different operating regions that specify the controller requirements.

Behavior of a Variable Geometry Turbine Wheel to High Cycle Fatigue

Maximization of the turbocharger efficiency is fundamental to the reduction of the internal combustion engine back-pressure. Specifically, in turbochargers with a variable geometry turbine (VGT), energy losses can be induced by the aerodynamic profile of both the nozzle vanes and the turbine blades. Although appropriate considerations on material limits and structural performance of the turbine wheel are monitored in the design and aero-mechanical optimization phases, in these stages, fatigue phenomena might be ignored. Fatigue occurrence in VGT wheels can be categorized into low and high cycle behaviors. The former would be induced by the change in turbine rotational speed in time, while the latter would be caused by the interaction between the aerodynamic excitation and blades resonating modes. In this paper, an optimized turbine stage, including unique nozzle vanes design and turbine blades profile, has been assessed for high cycle fatigue (HCF) behavior. To estimate the robustness of the turbine wheel under several powertrain operations, a procedure to evaluate HCF behavior has been developed. Specifically, the HCF procedure tries to identify the possible resonances between the turbine blades frequency of vibrations and the excitation order induced by the number of variable vanes. Moreover, the method evaluates the turbine design robustness by checking the stress levels in the component against the limits imposed by the Goodman law of the material selected for the turbine wheel. In conclusion, both the VGT design and the HCF approach are experimentally assessed.

Numerical Investigation on Aerodynamic Characteristics of Variable Geometry Turbine Vane Cascade for Marine Gas Turbines

Variable geometry turbines for marine gas turbines typically use variable vane technology to regulate turbine performance under variable operating conditions, but the variable geometry turbine produces additional losses as compared to the fixed geometry turbine. The method of combining experiment and numerical calculations was adopted to investigating the variable vane tip leakage loss at different vane turning angles, and its influence on the vane aerodynamic characteristics. The numerical calculations were performed using the ANSYS CFX 18.0 numerical prediction code, adopting the SST k-ω turbulence model to investigate the aerodynamic parameter distribution downstream of the variable vane under five different vane turning angles (−6°, −3°, 0°, +5° and +10°) and three different Mach Numbers (0.3, 0.5 and 0.6). The results showed that the tip leakage is the main source of aerodynamic loss of variable vanes. The tip leakage vortex and passage vortex show strong mixing characteristics in the downstream of variable vanes, especially at the 0.3Mach condition. The change of the vane turning angle alters not only the incidence angle to the vane itself, but also the outflow angle downstream of the vane. There is a linear relationship between the downstream outflow angle and the turning angle of the vane. The total pressure loss coefficient and energy loss coefficient decrease as the Mach number increases, and the changes of energy loss coefficient value from 0.3Mach to 0.5Mach are most obvious. Results from this investigation are well presented and discussed in this paper.