Browse Topic: Torque converters
This paper explores the application of a modeled torque converter in the real-time control of a hybrid electric powertrain. The study aims to determine the optimal gear selection and engine speed target required to meet driver demands. It also delves into the concept of torque converter input inertia compensation, particularly during open, open-to-close, and close-to-open states. The primary objective is to achieve the intended driver torque while minimizing torque sag and bumps during these transitions. This approach ensures improved powertrain response and maintains system integrity within the operational limits of the battery, motors, and engine.
In torque converters, a lockup clutch is used for direct torque transfer from the engine to the gearbox. Nowadays, earlier lockup engagement is necessary to reduce fuel consumption. It introduces noise and vibration issues in the transmission that are solved by clutch slipping. However, the clutch experiences much heat because of earlier engagement, which needs to be adequately dissipated by ATF oil. To overcome this issue, multi-plate clutches are commonly used for efficient torque transfer and clutch slipping. On the other side, packaging space for torque converters is reducing at the vehicle level, especially in hybrid vehicles, which reduces the efficient cooling of clutches. So, accurate modeling of clutch slipping is necessary to improve the clutch performance and durability of the product. Clutch slipping is a transient phenomenon that involves conjugate heat transfer and rotational flow modeling. There are different ways to model clutch slipping in CFD simulations. One of the modeling methods is applying the power loss as heat flux in clutch facings. However, heat flux is a vector quantity, and its direction is defined. That means the model introduces the approximation in heat transfer direction and reduces calculation accuracy. In this paper, power loss is applied as an energy source, which is a scalar quantity, and modeling methodology is explained. With this methodology, CFD calculation results are correlated well with test measurement, allowing to match the clutch design with the challenging packaging constraints.
Toyota developed a new hybrid unit “L4A0” for the new Tundra, which creates both good drivability and environmental performance. To ensure off-road, towing performance and typical truck driving characteristics, the unit is based on a transmission with a torque converter and a multi-plate lock up clutch, with a motor-generator and K0 clutch installed between the engine and transmission. The motor-generator and K0 clutch are built into a module, making it possible to create new hybrid units by combining the module with various transmissions. The unit features many different motor controls. For example, in the case of step-in acceleration input, in order to achieve the desired output torque, typically a kick-down shift is necessary [1]; however, by utilizing “L4A0” both high response and high power output is achieved even without a kick-down shift. This is accomplished by assisting the engine with the motor-generator even when the engine torque is delayed at low engine speeds. Simultaneously, this contributed to better fuel economy by allowing the engine to work at optimum speeds [1-3]. Another feature is the high-response and shock-less engine start that is achieved by cranking the engine with the motor-generator via the K0 clutch.
Traditionally, the controls system in production vehicles with automatic transmission interprets the driver’s accelerator pedal position as a demand for transmission input torque. However, with the advent of electrified vehicles, where actuators are located at different positions in the drivetrain, and of autonomous vehicles, which are self-driving, it is more convenient to interpret the demand (either human or virtual) in vehicle acceleration or wheel torque domain. To this end, a Wheel Torque-based longitudinal Control (WTC) framework was developed, wherein demands can be converted accurately between the vehicle acceleration or wheel torque domain and the transmission assembly input torque domain. For powertrains with a step-ratio transmission and a torque converter (TC), a key challenge of this conversion is the determination of the Inertia Compensation Torque (ICT), which is the torque required to accelerate or decelerate the TC’s impeller when the TC operates in the slipping or open conditions. In the current work, the proposed system computes a target impeller speed, taking into account the states of both the TC (e.g., open, slipping, etc.) and the transmission (i.e., fixed gear or shifting), which is then used to determine this ICT. The developed strategy was baselined against the traditional strategy and was found to be less calibration-intensive and, at the same time, effective in delivering the vehicle acceleration targets.
A Torque converter is a type of hydro-mechanical device, vastly utilized in the automatic transmission of vehicles and other machines. It is a critical component of the transmission system, having a direct impact on the fuel economy and vehicle´s performance. Computational Fluid Dynamics (CFD) has been employed by many authors and engineers to better understand the complex behavior of fluids inside of torque converters, in a way that it provides design improvements and increases model accuracy. This article presents a methodology that applies CFD as a tool in the design process of automobile torque converters. Therefore, this paper performs an extensive review of CFD associated with torque converters, and the principal concepts are stated and used to have a better understanding of the system’s dynamic behavior. Additionally, this article details some of the work done to develop an automotive torque converter model using the commercial software ANSYS CFX.
A hybrid transmission with more than 10 times speed ratio is introduced in this paper. The transmission consists of a electric torque converter module (eTC) and a dual input-shaft gearbox (DIG). The configuration structure and operation mode of the hybrid system based on eTC-DIG are analyzed in detail. The hybrid module comprises a motor, a planetary gear set (PGS), and a clutch. The rotating elements of the PGS are connected to engine shaft, motor shaft, and two input shafts of DIG, respectively, in such a way, that a new speed ratio is created between each odd-numbered gearset and an adjacent even-numbered gearset. The transmission has twice as many speed ratios for the engine as the number of the speed-changing gear sets. The hybrid system can realize a variety of working modes and eliminate the dual clutch of DCT, which greatly reduces the cost and risk. The economic simulation of the hybrid system is carried out for a Pickup truck. The results show that multi-gear is conducive to optimizing the engine working area, and the fuel consumption of WLTC cycle is reduced by 28%, which greatly improves the fuel economy of the vehicle and has a good market application value.
The emission norms around the world are continuously changing and getting stringent with every revision. India is on its way to make its emission norms at par with that prevailing in the developed nations. The cold-start condition is an important factor affecting vehicle emissions from gasoline direct injection (GDI) and port fuel injection (PFI) vehicles. In this paper, the effects of change in torque converter losses on emissions are experimentally investigated in a TGDI AT vehicle. The instant engagement of the torque converter puts a sudden load on the engine and thus affects its stability. Thus, to overcome the stability issue, Engine Torque has to be simultaneously increased for smooth engagement. As a result, the likelihood of the slightly leaner air-fuel mixture in the cylinder, which results in higher NOx formation, is much greater in an AT vehicle than that of a similar MT vehicle. Additionally, the temporary ineffectiveness of motor vehicle emission controls at startup causes emission to be much higher for a short period after starting than during fully warmed, or stabilized vehicle operation. The experimental result shows that although the decrease in the torque converter losses improves NOx emission, It also affects the engine stability. Thus, To calibrate the Torque converter losses, a tradeoff between engine stability and emission has to be made. Moreover, a comparative analysis between AT and similar variant MT vehicle to understand the sole effect of torque converter on the increased NOx emission suggests that a drop of 6-8% in the NOx margin from the base is due to engagement of the torque converter.
Dynamic Skip Fire (DSF) is an advanced cylinder deactivation technology to reduce fuel consumption and emissions of internal combustion engines. The firing sequence may vary dynamically depending on driver demanded torque with all cylinders capable of deactivation. This creates a challenge for managing noise vibration and harshness (NVH) caused by the low frequency excitation in the engine’s torque profile, especially in smaller engines with 3 or 4 cylinders. Due to the varying nature of firing sequences, the excitation is not limited to one or two engine orders and can vary with time, requiring broadband mitigation of the driveline. This work proposes the optimization of flywheel inertia combined with careful control of torque converter slip to overcome this challenge. Four different flywheel configurations and varying levels of torque converter slip were tested on a VW Jetta fitted with a 1.8L 4-cylinder engine with DSF control capability. For each configuration, DSF flyzone maps were generated and fuel consumption benefits were estimated. This paper covers a theoretical understanding of DSF excitations, vehicle NVH test results including torsional and seat vibration for various firing fractions with the different configurations, and fuel economy results with the optimized configuration. The methods developed here are extendable to other engines, platforms, and powertrain technologies like downsized, boosted engines with or without DSF/cylinder deactivation.
A torque converter is a type of fluid coupling device used to transfer engine power to the gearbox and driveline. A bypass clutch equipped in a torque converter assembly is a friction element which when fully engaged, can directly connect the engine to the gearbox. The torque converter is an important launch device in an automatic transmission which decouples engine speed from gearbox input speed while providing torque multiplication to drive the vehicle. During partial pedal launch, it is desired to engage the bypass clutch early and reduce the converter slippage in order to reduce power loss and achieve better fuel economy. However, engaging the bypass clutch early and aggressively may disturb the wheel torque and cause unpleasant driving experiences. This paper describes a multi-input multi-output (MIMO) control method to coordinate both engine and converter bypass clutch to simultaneously deliver desired wheel torque and reduce converter slippage. The proposed control method leverages a feedforward control derived from a standard converter hydrodynamic model and a Linear-quadratic regulator (LQR) feedback control. It considers the desired gearbox input torque and converter slippage as two control targets and engine torque and bypass clutch capacity as two control inputs to achieve the targets. This method enables aggressive converter lock-up without degrading driveability and will potentially improve calibration efficiency.
Mobility performance prediction models for tracked vehicles are well established as seen from the literature reviews. However, these simulation models are more suitable for commercial vehicle applications than for military vehicles which operate under a wide range of terrain conditions and hostile environment. Most of the models do not take into account the effect of cooling fans, soft ground rolling resistance, and torque converter to predict mobility, and therefore using them for military vehicles would pose vital problems and not yield the expected results. This paper attempts to address these problems by using a MATLAB/SIMULINK model, which takes into account these factors for a 65 ton Main Battle Tank (MBT) as a case study. A simulation model for the above vehicle was developed incorporating effects of cooling fan and torque converter. The results were validated with published trial data for an in-service Main Battle Tank of the same weight class. The results revealed that the accuracy of the model is within 91-97% of the published data. The model was further fine-tuned to incorporate a variety of terrain conditions such as tarmac road, desert soil, clay soil, etc. to establish the maximum feasible speed for the Main Battle Tank understudy between two specific locations within a given operating theatre.
The wet clutch system (WCS) is a complex combination of friction plates, separator plates and fluid (lubricant). The basic function of the WCS is to transfer torque under various operating conditions such as slipping, shifting, start/launch and/or torque converter clutch (TCC) operation. Under these conditions the slope of the coefficient of friction (μ or COF) versus slip speed (μ-v) curve must be positive to prevent shudder of the WCS, a highly undesirable condition in the lubricated friction system. An extended durability duty cycle test procedure is required to evaluate the WCS during which the μ-v curve is monitored for a negative slope, a condition indicating the potential for shudder. The friction plates, separator plates, and lubricant must be tested together and remain together during the test to be properly evaluated as a WCS. This paper describes a new test procedure which builds on the basics of the SAE J2964 - Low Speed Continuous Slip μPVT Procedure [1] by adding a durability duty cycle to age the WCS. The test includes measurement of the μ-v curve at several intervals during the test (before break-in, after break-in, and after each 12-hour aging segment of testing). The aging segments are run at a specific continuous slip speed and constant power condition. The lubricant temperature is also controlled and the plate interface temperatures are measured. The dμ/dv slope is calculated at different speed intervals and graphed to indicate when the slope changes from positive to negative. A negative dμ/dv slope can indicate a potential for shudder in the WCS. This new procedure can be used to compare the performance of different friction material and lubricant chemistry combinations by analyzing the resulting dμ/dv change over time. It can also assess the effect of changes in the groove pattern design, friction and separator plate surface finish and separator plate materials on the dμ/dv slope.
It is common that angular velocities can be different from time to time between an engine output and transmission input, because both are connected by a damper in torque converter with flexible elements in it. When this difference occurs abruptly for some reasons, an internal impact could start between the engine-attached members (also known as driving members) and the transmission-attached members (or driven members). The resulting impact load could be several times the torque an engine’s combustion force can generate, depending on the impact energy. An impact load can be very devastating to a torque converter and other power-train members, just as to all other mechanical systems. This work presents a comprehensive and interesting study to help understand the rotational impact behavior for a system where none of bodies is stationary at the onset of impact. Using an explicit finite-element solver for case studies, the author will assess what role a friction-based clutch, placed between the engine and vibration-isolating damper, will play, by limiting the passage of engine’s kinetic energy to some degree. It is interesting to find that it is the relative, not the absolute, angular velocities of impacting bodies that dictate the outcomes of an impact event.
Determining an amount of clutch clearance for the lockup device in a torque converter is important for its being operating precisely in the intended mode. Challenges may exist for the torque converters whose nominal clearances are on purpose very small. Any potential changes in the clutch lockup system (e.g., due to the deformation of components) may make such a small clearance instantaneously diminish during the mode of open-clutch, thus leading to unwanted drag in the clutch and unnecessary loss of energy. In the open-clutch mode, the actual clutch clearance may be different from the nominal clearance anticipated, primarily because of deformation caused by the internal load acting on clutch members. It has been found that the pressure distribution in a clutch chamber also depends on the very clutch gap through which the fluid flows. This interdependence between the fluid pressure load and structural deformation is typical of two-way coupling in simulation. As an alternative approach, a simple iterative computation method is studied here to address this problem with some intuitive results. The effectiveness will be demonstrated through a typical torque converter, in a step-by-step fashion.
As part of the U.S. Environmental Protection Agency’s (EPA’s) continuing assessment of advanced light-duty automotive technologies in support of regulatory and compliance programs, a 2018 Toyota Camry front wheel drive eight-speed automatic transmission was benchmarked. The benchmarking data were used as inputs to EPA’s Advanced Light-duty Powertrain and Hybrid Analysis (ALPHA) vehicle simulation model to estimate GHG emissions from light-duty vehicles. ALPHA requires both detailed engine fuel consumption maps and transmission torque loss maps. EPA’s National Vehicle and Fuels Emissions Laboratory has developed a streamlined, cost-effective in-house method of transmission testing, capable of gathering a dataset sufficient to characterize transmissions within ALPHA. This testing methodology targets the range of transmission operation observed during vehicle testing over EPA’s city and highway drive cycles. With this method, the transmission is tested as a complete system, as opposed to disassembling the transmission components and testing each separately. This paper describes the benchmarking process used to gather transmission data and the test results obtained. A UB80E eight-speed automatic transmission from a 2018 Toyota Camry was installed in an engine dynamometer test cell along with a 4-cylinder 2.5L A25A-FKS engine from the same vehicle. The test dataset collected from the transmission includes gear efficiencies, torque converter slippage and K factors, spin losses, oil temperature and pressure, and CAN bus data. The transmission data collected with this benchmarking method were used as inputs to the ALPHA full vehicle simulation model. ALPHA simulation results were validated using vehicle chassis dynamometer test data from the 2018 Toyota Camry containing this engine and transmission. The ALPHA simulation also allowed the Toyota UB80E transmission to be compared to other benchmarked transmissions.
The constant growth of the automotive market demands for comfort to the user and energy efficiency have caused the intensification of the industry researches and development of the automatic transmissions (AT). However, vehicles equipped with these gearboxes entails in higher fuel consumption levels than the one required by vehicles equipped with manual transmission. In the automotive industry due to the advantages offered using computer simulations, such as fast evaluation an optimization, many researchers are using virtual models for optimization of dynamic behavior of systems and fuel consumption. Aiming to study the dynamic behavior of an AT and the influence of its components on that behavior, this paper presents an AT dynamic model developed in MATLAB® / Simulink®. The AT model has three main subsystems: a torque converter model, which includes the dynamic of both the forward flow mode and the reverse flow mode; a Lepelletier gearbox model, composed by a set of three planetary gearsets in parallel, resulting in a six forward speeds gearbox; and a gear-shift schedule, which has the vehicle speed and accelerator pedal position as inputs of the model and the gear that should be selected in that condition as output of the model. The torque converter subsystem considers the transient and steady-state dynamic and their mainly operation dynamic characteristic: the conversion range, in which occurs a torque amplification and the stator is held; the coupling range, in which the stator freely rotates; and the transition periods from the forward to the reverse flow mode and vice-versa. The axial volume flow of the fluid, the speed and the torque of the three wheels (impeller, stator and turbine) were verified for the dynamic analysis of the system. In addition, the AT model was integrated into a MATLAB® / Simulink® vehicular dynamics and fuel consumption model in order to be analyzed under the ABNT 7024 standard speed profile.
The automobile manufacturers are currently facing a double challenge. While they must meet tight vehicle emission regulations established by the authorities, they also have to achieve the current market demands, which look towards fuel efficient vehicles for city driving, but still delivering high performance for unproblematic highway cycles. The purpose of this study is to evaluate the influence of different axle ratios in the conflicting fuel economy versus acceleration performance trade-off. The article will present the modeling and simulation of a four-wheel-drive light-duty vehicle with six-speed automatic transmission subjected to three drive cycles: the FTP-72 (Federal Test Procedure) cycle, the Highway Fuel Economy Test (HWFET) cycle, and the 0-100 km/h acceleration cycle. The simulations were performed in MATLAB/Simulink® environment by using system modeling that incorporates powertrain components such as engine, transmission, torque converter, axle ratio, wheels, driveshaft, etc. The auto driver was implemented by the approximation of the commanded speed curves to the desired speed curves (driving cycles) using a controller subsystem. The vehicle model results aim to analyze the improvements generated by the optimal differential ratio in the maximum reduction of fuel consumption while keeping a minimum performance threshold of 7 seconds for the 0-100 km/h acceleration drive cycle.
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