Harold Sun

Understanding of the Interaction between Clearance Leakage Flow and Main Passage Flow in a VGT Turbine

The clearance flow between the nozzle and endwall in a variable geometry turbine (VGT) has been numerically investigated to understand the clearance effect on the VGT performance and internal flow. It was found that the flow rate through turbine increases but the turbine efficiency decreases with height of clearance. Detailed flow field analyses indicated that most of the efficiency loss resulting from the leakage flow occurs at the upstream of the rotor area, that is, in the nozzle endwall clearance and between the nozzle vanes. There are two main mechanisms associated with this efficiency loss. One is due to the formation of the local vortex flow structure between the clearance flow and the main flow. The other is due to the impact of the clearance flow on the main flow after the nozzle throat. This impact reduces the span of shockwave with increased shockwave magnitude by changing the trajectory of the main flow.

Numerical and experimental investigation of a compressor with active self-recirculation casing treatment for a wide operation range

A turbocharger compressor with a wide flow range and a high efficiency is important to the application of advanced clean combustion technologies, such as homogeneous charge compression ignition and low-temperature combustion, in diesel engines. Self-recirculation casing treatment is one of the techniques that can extend the compressor surge margin without much efficiency penalty. The underlying physics of the self-recirculation casing treatment technology were investigated with computational fluid dyamics modeling and bench testing in this study. It is identified that, if the bleed slot of the self-recirculation casing treatment is located upstream of the impeller passage’s throat area, self-recirculation casing treatment improves the surge margin but the throat still limits the maximum flow capacity of the compressor. On the other hand, if the bleed slot of the self-recirculation casing treatment is located at the impeller passage’s throat area, the self-recirculation casing treatment improves the maximum flow capacity but results in a significant compressor efficiency penalty in the low-flow range. An active self-recirculation casing treatment design was proposed. The active self-recirculation casing treatment design extends the compressor flow capacity and improves the surge margin without an efficiency penalty through dual bleed slots with one upstream and the other downstream of the leading edge of the splitter blades. In the choke condition, the upstream bleed slot will be closed; near the surge condition, the downstream bleed slot will be closed. In the middle flow range, both bleed slots are closed. Both the numerical data and the bench testing results show that the maximum flow rate could be extended by about 15% and the surge margin by about 20% without an efficiency penalty. The mechanism of the performance improvement is also numerically studied.

Steady State Engine Test Demonstration of Performance Improvement With an Advanced Turbocharger

Heavy EGR required on diesel engines for future emission regulation compliance has posed a big challenge to conventional turbocharger technology for high efficiency and wide operation range. This study, as part of the U.S. Department of Energy sponsored research program, is focused on advanced turbocharger technologies that can improve turbocharger efficiency on customer driving cycles while extending the operation range significantly, compared to a production turbocharger. The production turbocharger for a medium-duty truck application was selected as a donor turbo. Design optimizations were focused on the compressor impeller and turbine wheel. On the compressor side, advanced impeller design with arbitrary surface can improve the efficiency and surge margin at the low end while extending the flow capacity, while a so-called active casing treatment can provide additional operation range extension without compromising compressor efficiency. On the turbine side, mixed flow turbine technology was revisited with renewed interest due to its performance characteristics, i.e., high efficiency at low-speed ratio, relative to the base conventional radial flow turbine, which is relevant to heavy EGR operation for future diesel applications. The engine dynamometer test shows that the advanced turbocharger technology enables over 3% BSFC improvement at part-load as well as full-load condition, in addition to an increase in rated power. The performance improvement demonstrated on an engine dynamometer seems to be more than what would typically be translated from the turbocharger flow bench data, indicating that mixed flow turbine may provide additional performance benefits under pulsed exhaust flow on an internal combustion engine and in the low-speed ratio areas that are typically not covered by steady state flow bench tests.

A physics-based control-oriented model for compressor mass flow rate

In this paper, a control-oriented model for compressor mass flow rate is proposed. For this purpose, the compressor is approximated as an adiabatic nozzle with compressible fluid, driven by external work from the compressor wheel. The external work input is modeled using Eulers turbomachinery equations, with the main flow losses estimated via a simple slip factor model. All other flow losses, that influence the mass flow, are lumped into the discharge coefficient as a function of turbocharger speed. Consequently, the mass flow rate is estimated based on mass conservation in a compact form. Only five parameters need to be identified with clear physical interpretations. Both steady-state and transient experimental test results confirm the validity of this model in terms of estimation accuracy and extrapolation capability, making it a promising candidate for control applications.

Physics-based turbine power models for a Variable Geometry Turbocharger

Control-oriented models for Variable Geometry Turbochargers (VGT) typically calculate the turbine power based on isentropic assumptions with a fixed or a map based value for the turbocharger mechanical efficiency. While the fixed efficiency assumption is an obvious over simplification, the map based approach, on the other hand, may not be globally accurate due to the need for interpolation between varying vane positions and extrapolation when the turbocharger is operating outside the mapped region. In this paper physics-based models of turbine power as well as the power loss are developed, utilizing the VGT vane position and the shaft speed. This makes it possible to define the mechanical efficiency as a function of the vane position thereby eliminating the above mentioned uncertainties as well as allowing a smooth extension over the entire operating range. The proposed model is validated against a few sets of test data from both steady state and transient operations.

Multi-Disciplinary Multi-Point Optimization of a Turbocharger Compressor Wheel

The paper describes the application of an optimization method to the redesign of a turbocharger compressor wheel. The starting design presents quite high performance. Previous attempts to improve this design have shown it difficult to increase aerodynamic performance without compromising mechanical stress levels.The optimization methodology relies on the combination of a genetic algorithm, a neural network, a database, and user generated objective functions. The originality of the paper is that the optimization is not only coupled to a CFD solver, but also to a CSM solver, so that mechanical stresses can be included in the optimization objectives. A parametric model of the solid sector of the blade, back plate and bore zone is built and included in the optimization. The challenging turbocharger test case has allowed gaining experience with design objectives of different nature. The results show that the optimization has been able to improve the aero performance, while also decreasing the peak mechanical stress levels significantly.Copyright

REGENERATIVE HYDRAULIC ASSISTED TURBOCHARGER

Engine downsizing and down-speeding are essential in order to meet future US fuel economy mandates. Turbocharging is one technology to meet these goals. Fuel economy improvements must, however, be achieved without sacrificing performance. One significant factor impacting drivability on turbocharged engines is typically referred to as, “Turbo-Lag”. Since Turbo-lag directly impacts the driver’s torque demands, it is usually perceptible as an undesired slow transient boost response or as a sluggish torque response. High throughput turbochargers are especially susceptible to this dynamic and are often equipped with variable geometry turbines (VGT) to mitigate some of this effect. Assisted boosting techniques that add power directly to the TC shaft from a power source that is independent of the engine have been shown to significantly reduce turbo-lag. Single unit assisted turbochargers are either electrically assisted or hydraulically assisted. In this study a regenerative hydraulically assisted turbocharger (RHAT) system is evaluated. A custom designed RHAT system is coupled to a light duty diesel engine and is analyzed via vehicle and engine simulations for performance and energy requirements over standard test cycles. Supplier provided performance maps for the hydraulic turbine, hydraulic turbo pump were used. A production controller was coupled with the engine model and upgraded to control the engagement and disengagement of RHAT, with energy management strategies. Results show some interesting dynamics and shed light on system capabilities especially with regard to the energy balance between the assist and regenerative functions. Design considerations based on open loop simulations are used for sizing the high pressure accumulator. Simulation results show that the proposed RHAT turbocharger system can significantly improve engine transient response. Vehicle level simulations that include the driveline were also conducted and showed potential for up to 4% fuel economy improvement over the FTP 75 drive cycle. This study also identified some technical challenges related to optimal design and operation of the RHAT. Opportunities for additional fuel economy improvements are also discussed. INTRODUCTION The need for assisted turbocharger Engine downsizing via high throughput turbochargers (TC) is a common approach for achieving improved fuel economy (FE). However, exhaust gas TC, including VGT’s are prone to a transient response dynamic commonly referred to as Turbolag. Turbo-lag is directly related to the rotational inertia of a TC and is simply the response time to achieve the desired TC speed, NTC, starting form some initial speed. Turbo lag directly translates to slow boost response and hence slow time to torque. This delay in torque response is perceived by the driver as a sluggish engine from a lack of power. In lean burn engines tipin, turbo-lag is also the main cause for the transient smoke. The effect of turbo-lag is also perceived during tip-out events through an extended slowdown duration during which the compressor continues to pump excess air into the engine leading to oxygen rich combustion with potentially increased NOx emissions. In order to address the impact of turbo lag on transient performance of turbocharged engines, vehicle manufacturers typically resort to one or more of the following measures: 1 Copyright

An Engine System Approach to Improve Turbocharger Fatigue Life

The ultimate goal of an advanced turbocharger development is to have a superior aerodynamic performance while having the turbocharger survive various real world customer applications. Due to the uncertainty of customer usage and driving pattern, the fatigue life prediction is considered one of the most ambiguous analyses in the entire design and analyses processes of the turbocharger. The turbocharger system may have various resonant frequencies, which may be within the range of turbocharger operation for automotive applications. A turbocharger may operate with excessive stresses when running near resonant frequencies. The turbocharger may experience fatigue failures if the accumulative cycles of the turbocharger running across the resonant frequencies exceeds a certain limit.In this study, the authors propose an alternative approach to mitigate this kind of fatigue issues: i.e. engine system approach to improve turbocharger fatigue life via avoiding operating the turbocharger near resonant speeds for extended period of time. A preliminary numerical study was made and presented in this paper to assess the feasibility of such an engine system approach, which is followed by an engine dynamometer test for engine performance sensitivity evaluation when the turbocharger operation condition was adjusted to improve the high cycle fatigue life. The study shows that for a modern diesel engine equipped with electrically controlled variable geometry turbine and EGR for emission control, through the engine calibration and control upgrade, turbocharger operation speed can be altered to stay away from certain critical speeds if necessary.The combined 1D and 3D numerical simulation shows the bandwidth of the turbine “risk zone” near one of the resonant speeds and the potential impact on engine performances if the turbocharger speed has to be shifted out of the “risk zone.”© 2014 ASME

Fault Diagnosis and Failure Prediction by Thrust Load Analysis for a Turbocharger Thrust Bearing

The axial thrust load of a turbocharger is generated due to the pressure differential between the compressor and turbine. Changes in compressor and turbine geometry and variations in test conditions can influence the thrust load. Once the axial force exceeds the loading capacity limit of a thrust bearing, the balance of the thrust bearing system cannot be maintained, which may lead to a catastrophic failure of the turbocharger. In this paper, a fault diagnosis of a turbocharger that experienced a catastrophic failure during flow bench testing is analyzed. A detailed analysis of a turbocharger thrust load, based on empirical formulae and CFD verification, is presented. The thrust prediction at high rotation speed is helpful for further flow bench testing and to avoid the future turbocharger failure. NOMENCLATURE c C_a1 Axial velocity at compressor inlet c T_a6 Axial velocity at turbine exit Cpa Specific heat of air at constant pressure Cpe Specific heat of Exhaust gas at constant pressure Fc Resultant force of compressor side Fc1 Axial force at compressor wheel inlet Fc2 Axial force at compressor wheel shroud Fc3 Axial force at compressor wheel back face Ft Resultant force of turbine side Ft1 Axial force at turbine wheel shroud Ft2 Axial force at turbine wheel exit Ft3 Axial force at turbine wheel back face FTC Resultant force of turbocharger k Ratio of specific heats mC Compressor mass flow rate mT Turbine mass flow rate pC00 Total pressure at compressor inlet pC1 Static pressure at compressor inlet pC2 Static pressure at compressor impeller exit pC5 Static pressure at compressor diffuser exit pC_shroud(r ) Static pressure distribution along the compressor wheel shroud pC_backface(r ) Static pressure distribution along compressor wheel back face pColl Total pressure at compressor exit pT00 Total pressure at turbine inlet Proceedings of ASME Turbo Expo 2010: Power for Land, Sea and Air GT2010 June 14-18, 2010, Glasgow, UK