Turbocharger Dynamics Research Articles

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.

Predictive capability of various linearization approaches for floating-ring bearings in nonlinear dynamics of turbochargers

A floating-ring bearing (FRB) is composed of a journal, a floating ring and a housing which are separated by two thin oil films. This system is inherently nonlinear and if it is lightly loaded or operated at high speeds, it is prone to the fluid-induced instability. The threshold speed for the instability of a journal bearing with one oil film can be estimated using linearized forces acting in the film. However, the linear analysis of FRB might be problematic because results of such an analysis are usually difficult to interpret. Nevertheless, the linear analysis can provide some fundamental insights into system behaviour. This work aims to evaluate in detail the use of linear theory in the stability analysis of rotating systems supported on FRBs. Several approaches for the linearization of the forces acting in FRB are proposed and analysed. The results are visualized in the form of a holistic Campbell diagram which together depicts natural frequencies, whirl frequencies, modal damping and precession of mode shapes. Moreover, it is demonstrated that the holistic Campbell diagram can be used to interpret – and to some extent, also to predict – results of nonlinear analysis.

Thrust bearing coupling effects on the lateral dynamics of turbochargers

Nowadays tendency is the practice of engine downsizing with a turbocharger, to increase fuel economy and decrease emissions. For both design and analysis, a reliable rotor-bearing model is mandatory. In turbochargers, floating-ring bearings support the rotating shaft, with a double-acting thrust bearing, to support axial force imbalances from the compressor and turbine gas flows. The typical turbocharger is highly nonlinear due to the high rotational speeds achieved. Therefore, a turbocharger model, supported by floating-ring and thrust bearings, is developed, considering both bearings nonlinear behaviour, including temperature variations in oil films. The thrust bearing model contemplates a full thermo-hydrodynamic evaluation, while the floating-ring bearing investigation is somewhat simplified. The analysis focuses on the thrust bearing influence in the turbocharger lateral vibrations.

A Comparison of Optimal Gear Shifts for Stiff and Flexible Driveshafts During Accelerations

Reducing the fuel consumption is important and much development work is on engine optimization for best stationary fuel consumption. Here, a solution is developed for the transient operation to get fuel optimal accelerations, considering the actuation of fuel injection, wastegate control and gear utilization. The transient acceleration scenario studied is; a truck is approaching a red light at slow rolling speed, the light turns green and the truck shall be accelerated to 50 km/h with minimum fuel. Optimal control is used to find the fuel optimal control strategies. By using a dynamic engine model, taking the turbocharger dynamics into consideration, the engine air fuel ratio is taken into account. The differences and similarities between a stiff and flexible driveline model, are analyzed. The results show that the most dominating effect is the turbocharger dynamics of the engine. The two drivelines have similar gear changing strategies while the finer details differ due to the additional degrees of freedom that are present in the flexible driveline.

A Nonlinear Feedforward Controller Design Taking Account of Dynamics of Turbocharger and Manifolds for Diesel Engine Air-Path System

Abstract In this study, we propose a design method of a model-based nonlinear feedforward controller for a diesel engine air-path system. First, a physical air-path model of the diesel engine was developed. Then, a feedforward controller was developed based on the inverse characteristics of the developed physical model. To improve the transient responses of the boost pressure and EGR ratio, the response delay of the turbocharger and manifolds dynamics were considered while designing the feedforward controller. The effectiveness of the proposed method was evaluated by conducting experiments using a real diesel engine. The experimental results indicated a good tracking performance for both the boost pressure and EGR ratio.

Effect of various analytical descriptions of hydrodynamic forces on dynamics of turbochargers supported by floating ring bearings

Abstract The scope of this paper is to provide a general model of a turbocharger using approximate analytical solutions of the hydrodynamic forces in oil films improved by correction polynomials. This approach brings two main advantages: the considered analytical solutions hold for wide range of bearing's length-to-diameter ratios and it is not restricted to infinitely short or infinitely long journal bearing approximations whose assumptions are often not satisfied. The usage of the closed-form approximate analytical solution of the Reynolds equation is more time efficient compared to the usage of common numerical methods and enables to effectively solve complex problems like parametric studies. The influence of various models of hydrodynamic forces on the turbochargers behaviour is studied on two different model systems.

Optimal Powertrain Lock-Up Transients for a Heavy Duty Series Hybrid Electric Vehicle

Fuel optimal lock-up transients for a heavy duty series hybrid electric vehicle are studied. A mean value engine model is used together with numerical optimal control to investigate the interplay between electric machine, gearbox and engine with its turbocharger dynamics in particular how they influence the manner and rate at which the engine should be controlled in order to reach a synchronized speed with the gear-box, enabling lock-up. This is studied both for prescribed gear-box speeds, simulating a mechanical transmission, and with gear-box speed an optimization variable, simulating a continuously variable transmission. The optimal engine transients and their duration are seen to be dictated by the stationary efficiency of the different drivetrain modes, showing that the ratio between the efficiencies of the electric and mechanical path dominates the dynamics and have a greater effect than the engine and turbocharger dynamics. In particular the transition between the modes is as fast as possible when the conventional powertrain is the most efficient and as slow as possible when the engine-generator set is more efficient. This points out that the stationary efficiency maps can be used in a central way for the control design of lock-up transients.

Investigation of Turbocharger Dynamics Using a Combined Explicit Finite and Discrete Element Method Rotor–Cartridge Model

The objectives of this investigation were to develop a coupled dynamic model for turbocharger ball bearing rotor systems, correlate the simulated shaft motion with experimental results, and analyze the corresponding bearing dynamics. A high-speed turbocharger test rig was designed and developed in order to measure the dynamic response of a rotor under various operating conditions. Displacement sensors were used to record shaft motion over a range of operating speeds. To achieve the objectives of the analytical investigation, a discrete element angular contact ball bearing cartridge model was coupled with an explicit finite element shaft to simulate the dynamics of the turbocharger test rig. The bearing cartridge consists of a common outer ring, a pair of split inner races, and a row of balls on each end of the cartridge. The dynamic cartridge model utilizes the discrete element method in which each of the bearing components (i.e., races, balls, and cages) has six degrees-of-freedom. The rotor is modeled using the explicit finite element method. The cartridge and rotor models are coupled such that the motion of the flexible rotor is transmitted to the inner races of the cartridge with the corresponding reaction forces and moments from the bearings being applied to the rotor. The coupled rotor–cartridge model was used to investigate the shaft motion and bearing dynamics as the system traverses critical speeds. A comparison of the analytical and experimental shaft motion results resulted in minimal correlation but showed similarity through the critical speeds. The cartridge model allowed for thorough investigation of bearing component dynamics. Effects of ball material properties were found to have a significant impact on turbocharger rotor and bearing dynamics.

Investigation of bearing clearance effects in dynamics of turbochargers

Turbochargers are modern and very interesting dynamical systems used in various engines in order to increase their power. They are operated at hundreds of thousands revolutions per minute and tend to be fatigue and stability prone. For this reason, a turbocharger rotor has to be designed carefully with respect to its dynamic properties. The following article deals with effects of radial bearing clearances on the dynamical response of the turbocharger rotor. The influence of a bearing clearance on stiffness and damping of a single-film journal bearing is well known and documented. Turbochargers, however, are often supported by floating ring bearings. Such bearings have two bearing clearances—between a journal and a floating ring and between a floating ring and a housing—which are determined by different temperatures of oil films. Turbocharger analysed in the article is modelled by means of flexible multibody dynamics approaches. Bearings’ behaviour is described using Reynolds equation, which is solved numerically. It is shown, but not mathematically proved, how the outer clearance and the ratio between the inner and the outer clearance affect amplitudes of sub-synchronous components in rotor's response.

Optimal Transient Control Trajectories in Diesel–Electric Systems—Part I: Modeling, Problem Formulation, and Engine Properties

A nonlinear four state-three input mean value engine model (MVEM), incorporating the important turbocharger dynamics, is used to study optimal control of a diesel–electric powertrain during transients. The optimization is conducted for the two criteria, minimum time and fuel, where both engine speed and engine power are considered free variables in the optimization. First, steps from idle to a target power are studied and for steps to higher powers the controls for both criteria follow a similar structure, dictated by the maximum torque line and the smoke-limiter. The end operating point, and how it is approached is, however, different. Then, the power transients are extended to driving missions, defined as, that a certain power has to be met as well as a certain energy has to be produced. This is done both with fixed output profiles and with the output power being a free variable. The time optimal control follows the fixed output profile even when the output power is free. These solutions are found to be almost fuel optimal despite being substantially faster than the minimum fuel solution with variable output power. The discussed control strategies are also seen to hold for sequences of power and energy steps.

Turbocharger Dynamics Influence on Optimal Control of Diesel Engine Powered Systems

The importance of including turbocharger dynamics in diesel engine models are studied, especially when optimization techniques are to be used to derive the optimal controls. This is done for two ap ...

Optimal and real-time control potential of a diesel-electric powertrain

Real-time control strategies and their performance related to the optimal control trajectories for a diesel-electric powertrain in transient operation are studied. The considered transients are steps from idle to target power. A non-linear four state-three input mean value engine model, incorporating the important turbocharger dynamics, is used for this study. The strategies are implemented using the SAE J1939-standard for engine control and evaluated compared to both the optimal solution and the solution when the engine is restricted to follow its stationary optimal line. It is shown that with the control parameters tuned for a specific criteria both engine control strategies in the SAE J1939-standard, speed control and load control, can achieve almost optimal results, where engine load controlled shows a better trade-off between fuel economy and duration. The controllers are then extended and it is shown that it is possible to control the powertrain in a close to optimal way using the SAE J1939-standard, both with the engine speed and load controlled. However the mode where the engine is load controlled is seen to be more robust.

Time and Fuel Optimal Power Response of a Diesel-Electric Powertrain

Optimal control policies for a diesel-electric powertrain in transient operation are studied. In order to fully utilize the extra degree of freedom available in a diesel-electric powertrain, compared to a conventional powertrain, the engine-speed is allowed to vary freely. The considered transients are steps from idle to target power. A non-linear four state-three input mean value engine model, incorporating the important turbocharger dynamics, is used for this study. The study is conducted for two different criteria, fuel optimal control and time optimal control. The results from the optimization show that the optimal controls for each criteria can be divided into two categories, one for high requested powers and one for low requested powers. For high power transients the controls for both criteria follow a similar structure, a structure given by the maximum torque line and the smoke-limiter. The main difference between the criteria is the end point and how it is approached. The fuel optimal control builds more kinetic energy in the turbocharger, reducing the necessary amount of kinetic energy in the system to produce the requested power. For low power transients the optimal controls deal with the turbocharger dynamics in a fundamentally different way.

Turbocharger seal as a decisive enabler for downsizing concepts

This brief feature article provides details of a gas-lubricated turbocharger seal that is now ready to go into series production. On one side it reduces blow-by at high boost pressure and drastically reduces oil leakages at low pressure. It enables a wider operating pressure range in both directions – vacuum and over-pressure. It therefore creates the basic pre-conditions for additional engine measures that help bring about a sustainable improvement in turbocharger dynamics, engine response and oil consumption. The seal's design also gives designers more freedom in deciding where the turbocharger is positioned in the engine compartment.

Global Airpath Control for a Turbocharged Diesel HCCI Engine

A motion planning based control strategy is proposed for the airpath control of turbochargedDiesel engines using Exhaust Gas Recirculation (EGR). The considered model uses simple balance equations. The fully actuated dynamics are easily inverted, yielding straightforward open-loop control laws. This approach is complemented by experimentally derived look-up tables to cast the drivers requests into transients between operating points. Moreover, a turbocharging control strategy is developed to provide the required amount of fresh air taking into account the turbocharger dynamics. Experimental tests are reported on a 4-cylinder engine in Homogeneous Charge Compression Ignition (HCCI) mode. Conclusions stress the possibility of taking into account the non minimum phase effects of this system by a simple, yet efficient, control law. Realized transients are fast and accurate.

Control of Charge Dilution in Turbocharged Diesel Engines via Exhaust Valve Timing

Abstract In this paper we extend an existing crank angle resolved dynamic nonlinear model of a six-cylinder 12 l turbocharged (TC) Diesel engine with exhaust valve closing (EVC) variability. Early EVC achieves a high level of internal exhaust gas recirculation (iEGR) or charge dilution in Diesel engines, and thus reduces generated oxides of nitrogen (NOx). This model is validated in steady-state conventional (fixed EVC) engine operating points. It is expected to capture the transient interactions between EVC actuation, the turbocharger dynamics, and the cylinder-to-cylinder breathing characteristics, although this has not been explicitly validated due to lack of hardware implementation. A nominal low order linear multi-input multi-output model is then identified using cycle-sampled or cycle-averaged data from the higher order nonlinear simulation model. Various low-order controllers that vary EVC to maximize the steady-state iEGR under air-to-fuel ratio (AFR) constraints during transient fueling demands are suggested based on different sensor sets. The difficulty in the control tuning arises from the fact that the EVC affects both the AFR and engine torque requiring coordination of fueling and EVC. Simulation results are shown on the full order model.

Dynamic simulation of a high-performance sequentially turbocharged marine diesel engine

The sequential turbocharging technique is used to improve the performance of highly rated diese engines in particular at part loads. However, the transient behaviour of the sequential turbocharging connection/disconnection phases may be difficult to calibrate and requires an accurate study and development. This may be accomplished, in addition to the necessary experimentation, by means of dynamic simulation techniques. In this paper a model for the dynamic simulation of a sequentially turbocharged diesel engine is presented. A two-zone, non-adiabatic, actual cycle approach is used for the chemical and thermodynamic phenomena simulation in the cylinder. Fluid mass and energy accumulation in the engine volumes are evaluated by means of a filling and emptying method. The simulation of the turbocharger dynamics combines the use of the compressor and turbine maps with a model of the sequential turbocharging connection/disconnection valves and of their governor system. The procedure is applied to the simulation of the Wärtsilä 18V 26X engine, a highly rated, recently developed, sequentially turbocharged marine diesel engine, whose experimental results are used for the steady state and transient validation of the simulation code with particular reference to the sequential turbocharging connection/disconnection phases. The presented results show the time histories of some important variables during typical engine load variations.

“p Booster” and combined EGR and A/F control for turbocharged gasoline engines

Turbocharger dynamics and control of the EGR rate are major problems of supercharged gasoline engines. Two solutions were developed at Lucerne University of Applied Sciences and tested on a “smart” 3-cylinder engine. To boost inlet manifold pressure during fast load demands, compressed air from a small pressure vessel is temporarily applied to the compressor inlet. The EGR rate is controlled by reducing the injected fuel, while the EGR valve serves to control the A/F ratio.

Hardware-in-the-Loop Simulation for the Design and Testing of Engine-Control Systems

For the design, implementation and testing of control systems increasingly hardware-in-the-loop (HIL) simulation is required, where parts of the control loop components are real hardware and parts are simulated. Usually, the process is simulated because it is not available (simultaneous engineering), or experiments with the real process are too costly or require too much time. The sensors and actuators may be simulated or real and the controller is real hardware and software. The real-time requirements for the simulation depends on the time scale of the process and the simulated components. The contribution gives first an overview of the various kinds of real-time and HIL simulation. Then, two cases are considered. First the HIL simulation for relatively slow processes, like in basic industries or heating systems. Here, the simulation-speed may be limited either by the complexity of the processes or by the real controller hardware. Then, the HIL simulation of combustion engines both with transputers and digital signal processors is shown in detail. The required models for 6- and 8-cylinder diesel engines are described, including fuel injection and burning, pressure development, torque generation at the crankshaft, exhaust turbocharger dynamics and the vehicle dynamics. The HIL-simulator test bench consísts of a real-time computer system, sensor-interface, actuator interface, real injection pumps and the real control unit. Comparisons of real-time simulation with measurements on real diesel engines and trucks are shown. The goal of the HIL system is to develop new control algorithms and to investigate the effect of faults in sensors and actuators and the engine itself.