RICERCA ITALIANA     

THE ENHANCEMENT OF MECHANICAL TRANSMISSIONS FOR THE REDUCTION OF THE ENERGY CONSUMPTION

Politecnico di Bari

Abstract

Automobile powertrain systems and components are continuously subjected to innovation to obtain better comfort, safety, reliability, handling and a reduced environmental impact. A great deal of electromechanical systems are being developed with those aims as, for example, innovative engine control, electro-power steering, vehicle dynamics control Electronic Stability Program (ESP), the automatically controlled gearbox. Among these innovative solutions one of the most interesting are the Continuous Variable Transmission (CVT ), the Infinite Variable Transmission (IVT), the Dual clutch systems, the Gearbox Robotized. A large number of international researchs are devoted to the improvement and enhancement of these systems. Benefits due to save energy and performance increasing can be really relevant in comparison with traditional manual gearbox or automatic ones. Chassis control systems devoted to increase suspensions, brakes, steer, tires performance require a more accurate research finalized to integrate each single stand-alone system. This is also true in the case of the Hybrid Electric Vehicles (HEV), that because of their double power source (usually a thermal engine and an electric one), have the problem to manage the power supplied by the two engines. Thus power has to be appropriately combined or split by the variator and transferred to the wheels or reconverted into electric energy stored by the electric batteries.
The research units of this program have been working together for a long period of time, thanks to the funds of COFIN 2000 and COFIN 2002, on the power train systems with excellent results, as demonstrated by their scientific publications. The promising results achieved have led the research units to collaborate once again. During the last years the main activity of the Research Units was concerned with the theoretical analysis of the mechanical behaviour of the powertrain systems. Thus during this program the Research Units will undertake an experimental activity that will be used to validate the theoretical model already available, to single out their defects and to improve the models if possible. The numerical models, once validated, will be utilized as part of more complex simulation systems, the so called Hardware in the Loop Systems (HIL) that will be used to simulate the behavior of the entire vehicle and choose the best control strategies in order to achieve the principal aim: the fuel consumption and polluting emission reduction, and a better comfort. At the same time some of the Research Units will undertake some theoretical studies concerned with some aspects of the mechanical transmission not enough investigated. This theoretical activity together with the experimental investigations will enable for an enhancement of the numerical model already proposed. <<<

Principal Investigator

Luigi MANGIALARDI Politecnico di BARI

Research Objectives

Nowadays mechanical transmissions are one of the most interesting research topic in the scientific and industrial field. These transmission are essential device in all machine systems, and their presence is diffused in every sector, from agricultural to automotive sector. High is the interest of researchers to find out new and original solutions that could improve the performance of mechanical transmissions, because their efficiency highly affects the behaviour of any mechanical system. Actually the continuously variable transmissions CVT and IVT, the former with a finite ratio range and the latter with a infinite ratio range, are capturing more and more interest in scientific and industrial research field. Investigations are being extensively carried out because these transmissions are able to optimise the performance of different mechanical systems and,by means of a suitable integrated transmission control strategy, to reduce the polluting emissions, the fuel consumption, and to improve the vehicle comfort. Moreover very simple configurations can be arranged for the continuous variator, which make it suitable for applications on low power vehicles as scooters.
The research units of this program have worked together in the past (COFIN 2000 and COFIN 2001) on the power train systems with excellent results, as demonstrated by their scientific publications. The activity consisted in producing theoretical-numerical models of the different mechanical transmissions under study. The promising results achieved have led the research units to collaborate once again and to propose a new research program, the main purpose of which is to continue the research activity on the mechanical transmission by means of a detailed experimental investigation that may enable the Research Units to compare the experimental data with the theoretical predictions. This will serve to point out the limits of the previous developed theoretical models and to single out the possible improvements. Moreover some theoretical studies will also be performed on some aspects of the mechanical transmission not enough investigated during the previous research programs. Therefore, a detailed theoretical study of the friction phenomenon at the interface between the sliding surface of the transmissions will be performed as well as the study of cam-follower mechanisms which constitute one of the main components of robotized gearboxes. This theoretical activity together with the experimental investigations will enable a further enhancement of the numerical model already proposed. The numerical models, once validated, will be utilized as part of more complex (HIL) simulation systems that will be used to simulate the behavior of the entire vehicle and choose the best control strategies in order to achieve the principal aim of a fuel consumption and polluting emission reduction and a better comfort. It is the intent of all Research Units to pursue these objectives by consolidating the collaboration among them and by join their skill and experience. The innovative mechanical transmissions, that will be investigated, are the continuously variable transmission, the split path variators. At the same time a research activity will be undertaken with the aim of improving and consolidating the knowledge achieved in the field of the HIL systems, with the aim of testing by means of an experimental activity the innovative mechanical transmission under study.
The purpose that the research program intends to obtain in the field of the split-power transmissions is to improve the performance of the CVT variator in terms of transmissible power, to match the speed variogram with the vehicle demands, to improve efficiency, and reduce pollution and fuel consumption. All performances will be analysed in the two chief operative conditions, the steady working and the speed ratio shifting. The research that will be undertaken on the pulley CVT variators consists in a experimental investigation needed to validate and improve the theoretical dynamical model of the variator, already obtained during the last years of research. After the validation, the theoretical model will be proposed as a simulation tool to be used during the design of the control strategy of the CVT variator. Moreover a deep theoretical study of the interaction at the interface between the sliding surfaces of the variator will be performed. The analysis will allow for a much more detailed comprehension of the sliding friction and wear, that, so much, influence the mechanical behaviour of the variators. Since a fundamental component of automotive mechanical systems, such as the robotized gearbox, is the cam-follower mechanism, this component will be studied both from a theoretical and an experimental point of view. The aim of the research is that of finding out what are the geometrical and mechanical quantities which influence more the cam-follower mechanical efficiency and wear. Regarding the HIL simulation, the activity will be devoted to integrate in an existing and experimentally validated vehicle dynamics model, the mathematical models, conceived by some of the Research Units, concerning these innovative transmissions. Such integration will permit to evaluate the compatibility of energy saving aspects and vehicle dynamical performances. Successively the models will be reviewed to become usable for real-time computing purposes. Such provision will permit to test, through an existing hardware-in-the-loop (HIL) test bench devoted to vehicle stability control researches, the benefits obtainable by integrating innovative transmissions with systems like Electronic Stability Program (EPS). A further activity will be carried out in order establish if some solution derived from the automotive field can be transferred, with the appropriate modification, to train technology. This request is very felt because of the strong interest in DMU (Diesel multiple units) which are a typical example of hybrid technology which are motorized through an array of Power-Pack that are derived from automotive-truck technology. <<<

Timescale

24 months

National and international background

Automobile powertrain systems and components are continuously subjected to innovation to obtain better comfort, safety, reliability, handling and lower environmental impact. A great deal of electromechanical systems are developed with those aims like, for example, the engine control, the hybrid transmissions, the adaptive cruise control systems (ACC) the antilock braking system (ABS), the electro-power steering, the automatic stability control (ASC), the electronic traction system (ETS), the traction control system (TCS), the electronic throttle control (ETC), the electronic stability program (ESP), the gearbox automatically controlled.
Every car manufacturer of automobile carried out vehicles complete with those kind of systems known as active systems or x-by-wire systems. Innumerable are the researches managed on those subjects. Fundamental, to carry out the systems indicated formerly, is the know-how about powertrain mechanical components like engine, clutch, gearbox, axles, belts, tires and so on. That knowledge was due to the necessity to implement in the Electronic Control Unit whatever information useful to consider how a drivetrain component characteristics depends on phenomena like, e.g., wear, thermal dependence, operating conditions, backlash.
From this point of view it is important to evaluate the mechanical features of friction clutches and belts, because they are essential elements of the transmission system. At the same time the development of an integrated control strategy of the transmission will be attained in order to reduce pollution and fuel consumption (see Kyoto conference) and to increase the vehicle comfort and driveability. Each research unit of this research program has been developing an extensive activity on the mechanical transmissions, and each of them attained excellent results. But the constant evolution of mechanical transmissions lead to new functional solutions, which, now, request the strict integration of all the transmission components, and, unfortunately, each research unit cannot have, as stand-alone, the skill necessary to face each problem. Hence the principal task of this research's program is to enforce the collaboration among all the groups in order to reach a synergic action, necessary to develop innovative solution and to improve the performance of mechanical transmissions.
The research units of this program have worked together in the past (COFIN 2000 and COFIN 2002) on the power train systems with excellent results, as demonstrated by their scientific publications. The promising results achieved have led the research units to collaborate once again and to propose a new research program. The research activity carried out in the last years has been concerned with the study of the dynamic behavior of the IVT and CVT transmissions with expanding pulleys during the phases of shifting, with the assessment of performance and of the capacities of toroidal transmissions to transmit torque and power. Thanks to the improvement in the mechanical properties of their constituting materials, the toroidal transmissions have aroused the interest of car producers that, in some cases propose them as an alternative to Van Doorne or to Luk chain CVT (see the example of Nissan). From the other hand, utilizing the results obtained in the previous investigations on the functional design of IVT variators and on the dynamic response of the metal belt CVT transmissions, the behavior of the entire vehicle system considering the whole kinematic chain "engine, motion transmission, mechanisms-road" has been studied, and the optimal IVT or CVT transmission configurations, from the perspective of the management of power, of energy saving, of the reduction of polluting emissions and of the comfort improvement, have been detected. The possibility offered by the CVT transmissions to achieve a wider range of transmission ratio is of fundamental importance for many reasons. First of all, the engine has the possibility to work within the regions of regimes characterized by the maximum efficiency. In fact, a larger range of transmission ratio makes it easier to adapt the loads and speeds (on the wheels) to the engine characteristic curve. This is also useful for the limitation of the polluting emissions. Instead, in the traditional transmissions, the engine almost never works near the ideal condition, since the produced power must be adapted to many different speed and load conditions depending on the path. Nowadays, the ideal point for an engine is possible only in hybrid engine systems, which uses also electric power for transmission. These hybrid system may have benefits in adopting the split path CVT transmission containing belt variators and epicyclic gears, where two- or multi-mode operative conditions are allowed. In these drives, the speed ratio range is divided into partial sub-ranges, to be realized by different mechanical configurations of the system (modes), where the passage from the one to the other mode is obtained by means of friction clutch connection/disconnection in synchronism of the shaft angular speeds. Since, in the automotive field, and in particular in the case of robotized gearboxes, the cam-follower mechanism are fundamental components these need to be further investigated.
During the last years the Research Units have developed numerical models to simulate the mechanical behavior of different vehicle components with very good results, but now their experimental validation is indispensable.
But it is well know that the experimental validation on vehicles may be very expensive, but can be improved thanks to HIL technology. In particular, in order to reduce the cost of validation (and/or certification) tests for components and mechatronic subsystems of common use in a given field, such as railways or automotive, it is nowadays possible to build Hardware-In-the-Loop (HIL) simulation systems which conjugate high degree of fidelity and easy reconfigurability. HIL simulation consists in letting the component "feel" the operational conditions which it would experiment in a real mission. A peculiar type of HIL systems are virtual reality systems where one of the "hardware components" is a human being: examples are flight simulators or vehicle ride simulators. The concept is not new and the test rigs used for many decades in the field of automotive engins, give a partial answer to this need. The new thing is that powerful computation means allow to create an environment (partly hardware and partly real-time software) which encircles the component to be tested (even the component can be partly hardware and partly real-time software), which reproduces in a Hi-Fi fashion the remaining part of the dynamical systems in which the component has to be embedded. This could be helpful also for the experimental investigation of the continuously variable transmissions, of which the main properties can be identified by the power capacity, the aperture (ratio of maximum to minimum speed ratio), the efficiency and the class (overall geometrical sizes), and by its behavior during the transient phases of shifting. <<<