Turbo Charger Research Articles

Topology Optimization in 3D Printed Piston

Abstract: Piston is the main alternating mass of a single cylinder 4 stroke engine and hence contain most of the inertial forces, also modern advancement in the automotive industry for the sake of pollution control and fuel economy many of the techniques such as Downsizing of engine (turbo charging), advance spark ignition, port injection, VVA and many more are adapted and hence results in high specific loading, including thermal and mechanical loads on the several parts of engine. This paper presents the re-designing of the piston, the topology optimization techniques have been used for increasing strength and decreasing mass of the piston which is our main objective. Traditionally pistons were manufactured by casting of aluminum and grey cast iron, aluminum was used in performance oriented vehicles as it was light in weight and have addition thermal properties and also it was easy to manage the inertial forces, whereas cast iron pistons were used in industrial purpose engine (heavy duty engine) today methods are replaced with 3-D printing, as a topology optimized piston cant be manufactured with traditional methods. The procedures in the paper are done for a generalized case and not for a particular so that the same methodology can be implemented in specific cases. Also it will cover a lot about the Material selection and additive manufacturing process used for this specific application.

Variable nozzle turbocharger: gas-dynamic calculation, 3D modeling, CFD analysis, characteristics

Introduction (problem statement and relevance). The environmental requirements for vehicle engines becoming more stringent require development and introduction of high-performance turbo chargers. Transition from external to internal varying, namely to the variable nozzles of the turbine, is caused by the necessity to reduce the transient process time during operation of the engine and the vehicle itself.The purpose of the study is development and confi rmation of the methodology (general strategy) for designing and modernization of radial-axial turbines with variable nozzles.Methodology and research methods. The study used the combination of calculation models for the engine itself with preliminary verifi cation based on the results of tests on the engine test bench (AVL BOOST, CRUISE M), gas-dynamic calculation of the turbine stage with determination of geometric parameters of the blade ring, housing (volute) and blade drive mechanism, their geometric modeling (Siemens NX) followed by 3D models export, CFD simulation of the stage fl ow including taking into account the transfer element (with impeller rotation) and generation of the stage curves (AVL FIRE).Scientific novelty and results. Combination (synthesis) of calculation models of different levels makes it possible to determine geometric parameters of the turbocharger turbine with variable nozzles providing high effi ciency (performance factor) of the turbine stage with a simultaneous shift of the maximum torque mode of the engine itself to the area of lower crankshaft rotation rate (along the full-load curve).Practical signifi cance of the paper consists in reduction of the time of design and subsequent refi nement (experimental) works during creation and modernization of turbines with variable nozzles of small-sized turbochargers. The obtained values of relative geometric parameters can be used at the design stage

Impact of Whirl and Axial Motion on Ball Bearing Turbocharger Dynamics

Ball bearing turbochargers (TCs) incorporate an angular contact ball bearing cartridge to reduce friction and oil consumption, improving the efficiency of internal combustion engines. In this investigation, axial and radial TC rotor motion was experimentally measured and used to develop a model of the TC rotor-bearing system, allowing for a detailed analysis of the TC bearing under varying operating conditions. In order to achieve the experimental objectives of this investigation, eddy-current proximity probes were used to measure the radial motion at the turbine and compressor sides of the TC. The measured radial motion shows whirl with subharmonics at low speed. The measured axial motion of the rotor was found to increase with TC speed. In order to analytically investigate the bearing dynamics, a complete TC model was developed that includes the mass distribution and flexibility of the TC compressor, shaft and turbine, and squeeze-film dampers (SFDs) that support the bearing cartridge. The model was used to replicate the whirl and axial motion of the test rig and then to determine which model parameters could be adjusted to minimize whirl without negatively affecting the bearing dynamics. Simulation results revealed that centrifugal effects cause a change in the bearing internal geometry, with a small clearance becoming a preload at higher speeds. When this effect is coupled with less compliant SFDs, the subharmonics are reduced and the overall sliding and number of large load cycles at the ball–race contacts are minimized, contributing to lower bearing friction and improved overall life.

A New Simulation-Based Methodology to Improve Medium-Speed Diesel Engines BSFC/NOx Tradeoff With an Optimized Air Charging System

Abstract This paper aims at introducing an original, simulation based, methodology to optimize air charging system of a medium speed engine in order to improve fuel consumption/NOx tradeoff, over a given load profile. A preliminary study is performed on an engine originally equipped with fixed geometry turbocharger (TC) matched at full load. It shows that improving the TC efficiency has a positive impact at matching point but an adverse effect at lower loads. Therefore, matching the TC at part load is more promising when the whole engine operating range is considered. However, this requires using a relief system to avoid unacceptable peak firing pressure (PFP) at full load, and may alter regulated NOx emission. A new, original methodology is then proposed. Using an advanced existing algorithm, it allows for simultaneous optimization of injection timing, main TC matching and relief system design. The objective is to minimize fuel consumption averaged over several operating points (corresponding to actual load profile). At the same time, NOx emission, evaluated on regulatory cycle, is kept below a given limit and others reliability constrains are fulfilled. The methodology is assessed with several case studies: • First, various relief systems (blow-off valve, waste gate (WG) and parallel sequential turbocharger (PTC)) are compared on a given load profile. • Then, the dimensioning of a TC + WG system is performed for two different load profiles. In both cases, the proposed methodology leads to a charging system design customized for a given user profile, which enables a significant fuel consumption reduction.

A Nonlinear Rotordynamics Model for Automotive Turbochargers Coupled to a Physical Model for a (SEMI) Floating Ring Bearing System

Abstract Automotive turbochargers (TCs) use an engine oil lubricated bearing system to produce acceptable performance (as per the engine volumetric efficiency) and proven reliability. However, the bearings also cause TC rotordynamic responses that are rich in subsynchronous whirl motions through reaching stable limit cycles. The paper describes the lubrication model for a finite length semi-floating ring bearing (SFRB) system and its coupling to the rotor and ring structure models for prediction of both linear and nonlinear system responses and their characterization in terms of motion amplitudes and whirl frequency content. The SFRB model includes a thermal energy transport network for the inner and outer films in both radial bearings and the thrust bearings located on the end sides of the ring. The large temperature difference between the hot shaft and a cold housing induces a three-dimensional thermal gradient in the fluid films and the floating ring, further exacerbated by the heat generated from drag power losses in the inner films adjacent to the rotor. The temperature gradients affect the lubricant viscosity and the bearing system operating clearances. The integration of the rotor and bearing system (RBS) equations of motion accounts for the SFRB nonlinear forces and starts from static equilibrium position, if existing. The model predictions of nonlinear behavior are accurate when benchmarked to a set of measurements procured in a gas stand test rig. The analysis also investigates the influence of the bearing inner clearance and rotor mass imbalance distribution on the onset, persistence and severity of SSV.

Design and Implementation of Car Door Latch Testing Machine

Abstract: In this project, we proposed design and implementation of car door latch testing machine. Nowadays cars are a very important part of this modern world because they give luxury and comfort. Even though they are comfortable, some problems always keep arising on the safety side. After a lot of research they rectified certain problems using air bags, auto parking, turbo charger ... etc. Likewise “car door latch” is also plays a major role in safety side in automobiles now we are going to discuss about one such problem that arises on the safety side. An unexpected accident occurs when people smash their fingers in between the car doors. Due to this kind of accident around 120,000 people are injured every year. But this was not taken as a very major safety concern for the customer. To avoid this kind accident due to car doors that we make sure of testing of car door latch using latch testing machine by analysing sealing factor, release travel, full release travel, latch opening current, release effort and testing position of the screw by using proximity sensor and to check the presence of the latch cover by using non-contact sensor (photo electric sensor). The working process of the “CAR DOOR LATCH TESTING MACHINE” is, when car door latch is placed in clamp cylinder that fix work piece in place. Then using servo motor actuator, the cylinder is moved forward to lock the latch and load cell is used to measure the sealing factor of car seal (rubber type) when the cylinder is moved in reverse direction. Release travel is measured using LVDT

Influence of Fluid Film Bearings with Different Axial Groove Shapes on Automotive Turbochargers: An Experimental Study

Most commercial automotive turbochargers (TC) employ semi-floating ring bearings (SFRB) with axial groove shapes. In order to bring some insights into the role played by the axial groove geometry on the dynamics of TC, this work deals with an experimental study of the rotordynamic behavior of a stock automotive turbocharger operating on SFRB with two different groove shapes, which have the same volume and width, and with the same number of grooves. The rotating machine behavior has been evaluated under different operating conditions using a test bench specially designed to analyze turbochargers. Rotordynamic (RD) characteristics of automotive turbochargers are estimated to evaluate the influence of the axial groove geometry on the machine vibratory behavior. Frequency spectra and orbital plots of the rotor are obtained from accelerometers and proximity probes mounted on the turbocharger. The comparative analysis of the vibrational behavior of automotive turbochargers running on different supporting systems allows the identification of the role played by the axial grooves on the machine rotordynamic performance. The experimental results rendered in this work permit to classify the influence of the axial groove geometry on the turbocharger rotordynamic behavior for several speed and flow conditions.

Influence of Low Inlet Pressure and Temperature on the Compressor Map Limits of Electrical Turbo Chargers for Airborne Fuel Cell Applications

For the optimal high-efficiency operation of a PEM fuel cell system, the temperature, pressure, humidity and mass flow of the supplied air must be tuned to the fuel cell stack requirements. Especially for aircraft applications, this requires a thorough understanding of the fuel cell air supply system behavior and how it changes when the ambient pressure is below 1 bar(a) during flight. This work investigates the influence of low inlet pressures and varying inlet temperatures on the compression map of an electrical turbo charger. This is especially relevant in airborne fuel cell application and not much literature can be found on that topic. Compressor limits are evaluated experimentally and theoretically. The theory of mass flow and speed correction is compared to experimental findings and found to be applicable for the surge and speed limit of the investigated turbo chargers as long as the compressor map is not limited by the power of the electric motor and inverter. Based on this, a prediction of the compressor map for altitudes up to 10,000 m is made with the help of a developed software tool.

3D CFD Simulation of Hydraulic Test of an Engine Coolant System

<div class="section abstract"><div class="htmlview paragraph">Designing an efficient vehicle coolant system depends on meeting target coolant flow rate to different components with minimum energy consumption by coolant pump. The flow resistance across different components and hoses dictates the flow supplied to that branch which can affect the effectiveness of the coolant system. Hydraulic tests are conducted to understand the system design for component flow delivery and pressure drops and assess necessary changes to better distribute the coolant flow from the pump. The current study highlights the ability of a complete 3D Computational Fluid Dynamics (CFD) simulation to effectively mimic a hydraulic test. The coolant circuit modeled in this simulation consists of an engine water-jacket, a thermostat valve, bypass valve, a coolant pump, a radiator, and flow path to certain auxiliary components like turbo charger, rear transmission oil cooler etc. A commercial CFD software, Simerics-MP+®, is used to simulate the hydraulic test for two different positions of the poppet valve of the thermostat, viz. a closed position and an 50% opening, at different speeds of the engine. In the CFD model, the complete geometrical details of water-jacket, thermostat, and pump are considered. The remaining components are approximated as pipes with flow resistance models to account for flow and pressure drop at different engine speeds. Firstly, the standalone pump performance is validated in the operating regime of interest, followed by the calibration of the resistance models for the simplified components. At the end, complete system level 3D simulations are conducted and validated for the above mentioned two positions of the poppet valve. The flow distribution and pressure drop across different components show good comparison with the hydraulic test data within 7% error band.</div></div>

Визначення причин, попередження і усунення нестійких режимів газотурбокомпресорів суднових дизелів

Annotation – With commissioning of the “Socofl Star” ship’s series, negative occurrences relative to surging of the Main Engine (ME) «Hanshin Diesel» 6LF46 turbochargers (TC) VTR 401-2. To elimination of a surging, it was necessary reduce loading of ME to the safe level. This action caused the ship’s speed to fall from 11 – 10 to 4 knots which resulted in worsening of the ship’s maneuverability characteristics and lead to the failure to provide the ship’s service speed stipulated in the contractual arrangements. Existence of this problem instigated the shipowner to charge us as experts with the mission of carrying out appropriate investigations and working out recommendations as to how to prevent and eliminate surging of TC. This task was solved on the m/v “Socofl Star”. Based on results the ME shop test and trial test of the vessel and also the saved-up data of work of ME in various conditions of swimming, the analysis of the causes of a surge of the TC was made. It is established that small values of safety factor of stability of the compressor of TC on a surging – KCT which are not allowing to ensure effective functioning of TC on the main modes of loading of ME are its reason. For increase in area of steady work of TC it is necessary to reduce the hydraulic resistance of components of the Air-Gas Path (AGP) of the ME which can be realized by changes in a design of units of air supply and gas exchange or reduction of productivity and extent of increase in pressure of air in the compressor of TC. Under operating conditions vessels an optimal solution an objective is removal of a part of blowing-off air after compressor of the TC. The air can be discharged into the flue gas header after the waste heat recovery boiler or directly into the atmosphere. This allowed the shipowner not to make constructive changes to the components of the AGP of ME and TC. The description of the operated unloading device controlled remotely on removal of air which ensures effective functioning of TC and ME that is confirmed by results of natural tests and the subsequent operating experience of vessels of the “Socofl Star” series is provided.

Modelling the thermal state of a turbocharger bearing housing when calculating the rotor dynamics at transient modes

PurposeThe reliability of various mechanisms and machines is determined by the durability of tribo-units, which must ensure operation at high temperatures and an extended range of rotor shaft speeds. The best performance of the bearing assembly is achieved with hydrodynamic lubrication, which depends on optimal operating conditions and temperature conditions. The purpose of this paper is determining the thermal state of the turbocharger (TCR) bearings.Design/methodology/approachThe simulation was carried out in the ANSYS Fluent software package. The boundary conditions for the calculation were obtained from experimental data. The experiments were carried out at a specialized stand created at the scientific and production association “Turbotekhnika”.FindingsThe result of the simulation was the determination of temperatures and thermal fields in the TCR housing. The data obtained testify to the uneven thermal loading of the bearings. When calculating the dynamics of the rotor, transient modes are considered. The results are the trajectories of the rotor in the space of the bearing clearance. The thickness of the lubricating layer was calculated as a parameter that determines the hydrodynamic friction regime. The thermal state of the TCR elements was evaluated at all the considered rotor speeds. The flexible axis of the rotor was obtained at different speeds.Originality/valueThe paper presents a model of heat transfer in a TCR housing and rotor dynamics, based on numerical methods, which will help in the design of TCRs and journal bearings.

Operating lines for plug-in hybrid turbo charger systems meeting LEV III evaporative gas regulations

Operating lines for plug-in hybrid turbo charger systems meeting LEV III evaporative gas regulations

Studying Surge Phenomena by Improving the Construction of a Turbocharger Testing Bench

It is common knowledge that diesel engines are frequently used in agricultural processes. Uprating a diesel engine by means of turbocharging is widely used nowadays. Turbocharging easily enables the increase in the power of the engine by 10-50%. However, the turbine creating a high effectiveness of a diesel engine became an essential problem connected with the reduction of reliability of farming machinery. One of the vital issues here refers to surge phenomena in a turbocharger. Thus, in routine maintenance of turbocharged engines the stochasticity of load leads to the changes in the geometry of the pump and turbine wheels of a turbocharger. The complex work to improve the construction of the turbocharger test bench was connected with re-creating the real operational modes of a turbocharger (TCR) on the bench and studying the limits of a surge during tests.

A Thermo-Hydrodynamic Model for Thermal Energy Flow Management in A (Semi) Floating Ring Bearing System for Automotive Turbochargers

Abstract Oil-engine lubricated turbochargers (TCs) operate at high temperature and must withstand large temperature gradients that produce severe thermomechanical stresses in the TC mechanical components. Thus, an insight into the thermal energy flows and an effective thermal management are paramount to ensure reliable TC operation. The paper analyzes the transport of energy and heat flows in semifloating ring bearings (SFRBs) for automotive TCs with integrated heat and fluid flow models for both (turbine and compressor sides) radial bearings and thrust bearings to produce a complete thermohydrodynamic analysis predictive tool. The model couples the energy transport equations and the lubrication Reynolds equations in the inner and outer films of an SFRB and the adjacent thrust films to a three-dimensional heat conduction in the floating ring and along with thermal soaking into the TC casing. Cold lubricant, supplied at a specific temperature and pressure, flows to fill the films of the radial bearings, and then the thrust bearings. The lubricated bearings, radial and axial, support shaft loads, static and dynamic, and produce drag power losses. The streams of lubricant warm up as they take away a sizable portion of the heat flow from the hot shaft plus that due to viscous shear drag. Another fraction of thermal energy flow sinks into the floating ring, which presents a distinct temperature field varying along the radial, circumferential, and axial directions. The computational analysis contemplates a TC operating at shaft speeds (Ω) ranging from 30 krpm to 240 krpm (4 kHz) and an SFRB supplied with engine oil at PSUP = 3.0 bar and TSUP = 120 °C. The analysis focuses on a brass-made turbine bearing (TB) as it is the one that disposes most thermal energy flow since the shaft surface is hot at Ts = 213 °C (just below the lubricant flash point temperature at 230 °C) while the casing temperature is TC = TSUP. The ring with length/diameter = 1.6 has radial bearings with four equally spaced feed holes and four axial grooves, and the ratio outer film clearance/inner film clearance equals 5.3 at room temperature. As shaft speed increases (= 100 m/s max. surface speed), the inner film temperature increases proportionally; albeit the heat flow from the shaft into the inner film decreases while the viscous drag power raises rapidly. The outer film heats to just a few degrees above TSUP since the nonspinning ring does not generate viscous shear drag. The ring heats unevenly, radially with a ∼20 °C temperature gradient from its inner to outer diameters (ID and OD), and axially with up to a ∼50 °C difference from the thrust bearing side that also produces a drag power loss. At a low shaft speed (45 krpm), heat flowing from the shaft overwhelms the drag power loss induced by shearing the inner film; whereas as shaft speed increases (240 krpm), the contribution from the drag power loss to the total energy flow disposed increases significantly, from 3% to 63%. The lubricant flows, inner plus outer, advect most of the thermal energy flow, 74% to 81%, over the range of shaft speeds, low to high. The floating ring conducts a sizeable portion of thermal energy flow, 39% to 49% of the total, though varying little with shaft speed. Similarly does the fraction of heat, 9% to 13% of the total, conducted into the TC casing. A more conductive ring material or an outer film with a longer length conduct more heat into the ring although the lubricant flows still carry most of the thermal energy flow generated by viscous drag losses and heat from the shaft. The results demonstrate the importance of designing an SFRB system with adequate clearances and proper materials to offer an adequate thermal management and avoiding too high temperatures that could varnish, even flash and burn, the engine oil. The improvements in the energy transport and heat flow modeling of an SFRB system will produce significant savings in TC performance.

Investigation of the independent lubrication system of modern turbocharged vehicles

It was revealed that in modern operating conditions turbocharger failures account for up to 27% of the total number of failures of internal combustion engines (ICE). Such a high failure rate is due to difficult operating conditions, heavy-duty operation, and limited load-bearing capacity of oils and structural materials. To solve the problem of improving the reliability of a turbocharger (TC), it is proposed to introduce an independent lubrication system with an installed hydraulic accumulator. For this purpose, a test bench with systems for monitoring and evaluating the output parameters of oil was developed. The following parameters were monitored: TC rotor speed, oil pressure in front of the bearing of the TC without a hydraulic accumulator and with fluid makeup using a hydraulic accumulator, oil consumption, oil temperature, oil effusion time, oil pressure reduction time. As a result of experimental studies, we established the limits of TC bearings’ performance with constant fluid makeup from the hydraulic accumulator. We also investigated the discharge characteristics of the hydraulic accumulator at oil operating temperatures. Experimental results for different clearances in TC bearings were obtained. The results of the study can be used by auto manufacturers, industrial enterprises, as well as educational institutions in the analysis and modification of modern TC assemblies.

Research of Post Injection Strategy of an EGR Diesel Engine to Improve Combustion and Particulate Emissions Performance: Application on the Transient Operation

Mobile source emissions have already accounted for a large proportion of environmental pollution, which seriously affect the symmetric characteristics of atmosphere, and automobile emissions have extremely serious deterioration of emissions under transient operation, especially particulate emissions. These factors exacerbate the asymmetry of the environment. So, the paper reports an experiment about the improvement of post injection strategy on combustion, regulated emissions (HC, CO, and NOx), and particle number emissions especially the emissions of different size particles in the transient process of an EGR diesel engine, meanwhile, the effects of post injection on the combustion of mixture are further analyzed by numerical simulation method. The test speed was 1600 r/min, and the torque increased from 5% of the maximum torque to 100%. The results indicated that the shorter the instantaneous loading time, the more severe the deterioration of particulate emissions, HC and CO emissions, but loading time has little effect on NOx emissions. The particles with the size range of 50–100 nm, 23–50 nm, and >100 nm are greatly affected by the loading process and post injection. In comparison, it has little effect on ultrafine particles with particle size of 15–23 nm and <15 nm. With the amount of post injection increased, the in-cylinder disturbance increased, and the oxygen-rich area in cylinder increased, the particle number concentration first decreased and then slightly increased. When the amount of post injection fuel is 2 mg and the main-post injection interval is 2000 us, the effects of suppressing particulate emissions are the best, for the 50–100 nm and >100 nm particles, the peak number concentration can be reduced by 25% and 50%, respectively. Due to the turbo charging lag, the peak of NOx emissions during the unloading process were slightly larger than the loading process.

Experimental and Analytical Investigation of Turbocharger Whirl and Dynamics

The objectives of this investigation were to experimentally and analytically investigate the dynamics of a medium-sized, relatively high-speed ball bearing–supported turbocharger (TC). In order to achieve the objectives, a TC test rig was designed, developed, and instrumented with a combination of strain gauge–based load sensors and proximity probes. The sensors were used to measure the axial and frictional loads on the TC bearing, as well as the whirl of the compressor and turbine rotors, under various operating conditions. In this TC, the compressor and turbine shaft are supported by a pair of unloaded back-to-back angular contact ball bearings. The bearing cartridge is supported by squeeze film dampers (SFDs) and is prevented from rotation by an anti-rotation pin. To achieve the analytical aspects of this investigation, an equivalent bearing model was developed to investigate the bearing dynamics and whirl of the TC rotating assembly. The TC bearing cartridge was modeled with a single deep groove ball bearing (DGBB) using the discrete element method. The rotating shaft, compressor, and turbine were considered to be rigid and integrated with the inner race of the bearing. The SFD that supports the bearing was modeled with a bilinear spring and damper. A DGBB was used because it can support axial load in both directions. The results from the analytical model correlate well with the experimentally observed loads and whirl and show that whirl can have a significant effect on the dynamics of the individual bearing components.

Application of variable geometry turbine turbochargers to gasoline engines- A review

An ideal engine today is considered to be one with low emission, low fuel consumption and high specific power output. Engine downsizing by turbocharging provides an answer toall the three. Though the traditional problems of poor low end torque and inadequate transient response have been overcome to a great extent using variable geometry turbine turbocharger in diesel engine, the application of variable geometry turbine turbocharger is in quite a nascent stage when it comes to gasoline engine. This article primarily describes all the important aspects of application of VGTs to gasoline engines right from when VGT’s were restricted to only very expensive research prototypes to the very recent application in cost sensitive production engines. Also at the same time,it highlights the need and development in the flow range of the turbocharger compressor, addressing problems of choke and surge in the turbo chargers compressor.

Fast and Integrated Chargers for Lithium Ion Batteries

Electric vehicles were introduced in the year of 1908, where they have larger battery for power supply. Now- a- days the technique for charging e-bikes is done either by using grid supply or solar power. This paper says about charging circuit for batteries which uses both solar power and power from the grid to charge the electric vehicles. The charging with solar helps to reduce the emission from power grid. Ever-increasing demand for fuel supply, rising fuel prices, and increased environmental awareness among masses are paving the way for electric vehicles (EVs). Although, in recent years EV market has seen an exponential growth, one of the major challenges faced by this automotive/vehicular market. Now the Indian government had ordered to Indian Vehicle Association to launch the electric vehicles with in 2023. But the charging of electric Batteries has taken more time, which compare to fossil fuel and gas. To overcome this issue use of turbo charger, to reduce the charging time as well as self charging by solar during running time is proposed. The charger uses the source from EB grid and solar as well as at parking time. The results proves that proposed work have reduced the charging time of the lithium ion battery compared to the conventional charging technique and all under controlled temperature raise.

Analysis of Radial Compressor Performance at Low Pressure Ratios and Compressor Choke

Abstract Strong transient engine load steps can result in low pressure ratio (ΠC) compressor operation for single stage turbocharged (TC) systems. For conventional full load TC engine matching using one-dimensional (1D)-engine process simulation, these operating points are of limited relevance and are consequently less studied. However, for the layout of sequential turbocharging systems, low pressure ratio compressor operation has to be thoroughly understood. Therefore, in this paper, three-dimensional (3D)-computational fluid dynamics (CFD) simulations will be presented, which analyze the stationary compressor behavior at low pressure ratios. Operating points at ΠC<1 are investigated by reducing the compressor outlet pressure. The simulation results are validated against measurement data acquired at a stationary hot gas test bench. The compressor performance is quantified by a corrected compressor torque. Opposed to the well-known operation at ΠC>1, the compressor generates power close to zero speed for ΠC<1 (turbine operation). At higher mass flowrates and ΠC<1, the compressor consumes power. Pressure build-up in the wheel is overcompensated by losses in the diffusor and the volute resulting in a net pressure drop across the stage. The 3D-CFD simulations also allow a speed-dependent evaluation of the choking cross section inside the compressor. At low circumferential speeds, compressor choke occurs in the volute or at the wheel outlet. At higher speeds, choking is observed at the wheel inlet. This behavior must be accounted for compressor map extrapolation methods for 1D-engine process simulations in order to correctly predict the choking mass flowrate.