RICERCA ITALIANA     

Innovative solutions for power saving in hydraulic circuits of agricultural tractors

Università degli Studi di Modena e Reggio Emilia

Abstract

The Research Program deals with the analysis and the design of the main hydraulic circuits of mid-power agricultural tractors (90-150 kW), with the aim of determining those conceptual innovations and design developments needed to improve their global efficiency and to allow a large scale power saving. The hydraulic circuits taken into account in the research program are: the transmission system, the handling and loading system and the auxiliary actuations system.
The research activities, to be performed starting from the same point of origin and developed under a proper synergic collaboration, will lead to an integrated design enhancement of the reference agricultural tractor hydraulic architecture, characterized by stronger performances and higher global efficiency.
The study of the transmission system is divided into two parts: the first part involves the power split transmission; this part is focused on the study of a hydrostatic hybrid transmission system made by a combination of a hydrostatic unit and a planetary gear. The second part of the study involves the full power shift transmission; it deals with the optimisation of electro-hydraulically controlled multiple clutches.
The study of the handling and loading hydraulic circuit involves multiple linear actuators circuits. Some architectures of handling and loading systems will be defined, with particular attention to the analysis of the widely diffused load-sensing solutions; in particular, in order to overcome the limit actually imposed by the load-sensing architecture, the attention will be focused on the independent control of each cylinder by dedicated hydraulic circuits, and to the development of design solutions achieving the efficiency increasing through the recovery of the hydraulic power associated to the dissipative phases. The system more promising for power saving will be modelled and submitted to some typical operating cycles, in order to state the influence of their main design and operating parameters on energy dissipations.
It is worth noting that all the activities will find their finalization in a multi-objective constrained optimization problem, where the individual characteristics of hydraulic sub-systems will be contrasted in view of their ability to share the limited power available in the i.c. engine in a model based on vehicle dynamic simulation and soil interaction. <<<

Principal Investigator

Massimo Borghi Università degli Studi di MODENA e REGGIO EMILIA

Research Objectives

The research program is aimed at investigating different suitable solutions in order to improve the energy conversion efficiency of hydraulic systems equipping mid-power agricultural tractors (90-150 kW). Moreover, according to the general guidelines reported in the "Off-Highway Vehicle Technology Roadmap - U.S. Department of Energy" [www.trucks.doe.gov] which will condition the agricultural equipment technical development in the next decade, it is devoted to define possible strategies to obtain performances improvement and consumptions reduction through a generalized power systems efficiency increasing.
Medium size agricultural tractors (and other off-road machinery) considered in this research program are widely represented in the international market share. In particular, several of the most qualified agricultural tractors and associate components producers are located in Italy, hence the project presented is of significant relevance for Italy. The continuous technical and technological development, and the high quality standard level reached in the national industrial division, lead Italian hydraulic components production to hold the 13% of international production, being the 4th producer in the word. It becomes necessary that agricultural tractors and machinery manufacturers focus efforts in particular on the technological innovation of their products, especially as far as energy conversion efficiency in the hydraulic system devoted to movement actuation and control is concerned, reducing power losses and minimizing prime movers power requirements. The next generation tractors have to match their characteristics to the operational limits forced by the need to reduce environmental impact, thus allowing the industrial vehicles utilization in the agricultural field. In particular, these new tractors will have to respect the limits on internal combustion engines emissions (especially regarding Diesel engines) fixed by the regulations which will be very limiting also in the industrial vehicles and agricultural machines fields. Therefore, industrial primary movers of next generation will be distinguished by reduced fuel consumption and improved thermodynamic and global efficiency; moreover, they will be provided with all the components suited for pollution reduction and consequently they will supply a lower rate of the mechanical power needed by equipments fitted to match the demand coming from agricultural application, considering the sensitivity of the engine operatin point on the power match with implements. Because of the high-medium power rate required in the market, complex hydraulic circuits, electro-hydraulically managed, control these equipments; in these systems the power modulation is still characterized by low efficiency in power conversion and by losses, which dissipate mechanical power into heat.
In the next future, only new design approaches to fluid power transmission will make possible to realize actuation systems characterized by reduced energy consumption and high power conversion efficiency, thus allowing to keep and eventually raise the performances and quality standards of agricultural machines, with concurrent optimization of all sub-systems involved. Regarding the hydraulic systems design, the greater efforts in order to reduce power dissipation are to be focused on the circuits portions listed below: transmission system, loads holding and lifting circuit and auxiliary equipments actuation circuit, but only their simultaneous integration within a simulation environment able to catch the relevant characters of vehicle dynamics and load cycle will make optimization possible. Consequently, this research project is based both on performances optimisation of standard and well-known design solutions and on the theoretic, numeric and experimental analysis of new suitable solution. This way, all the possible solutions to develop innovative and efficient agricultural equipments will be considered, and also compatibility with alternative energy sources may be investigated. Finally, another important target of this Research Project, is to involve in a common activity different Research Groups working for the technical and scientific development and for the technological innovation of hydraulic applications in our country, in order to create a national net of skills in this field, and a reference frame where the development and optimization of on-board circuits is possible in a consistent and integrated virtual design environment. <<<

First Results

Expected results from the project are diverse in kind, and must be considered in view of the complementary role played by all research units throughout the project timeline.
During the project, a significant number of individual partial, yet significant, results will be produced by all the research units, but it will only be at the end of the activity, when all contributes will be put together that the major contribution to the advances in the field of activity are expected.
The observation of present trends in simulation of fluid power systems show a significant highlight of control theory and detail investigation. Only few examples may be found, however, where the problem of how different subsystems may interact at the vehicle level is tackled. Also the problem of the definition of an adequate representation of the machine working cycle or mission profile, is today grossly underestimated.
The interaction with the i.c. engine on one end of the power train, and with the working tool on the other have never been considered together in an integrated simulation environment, and this aspect assumes even higher relevance when the objective is the integration of optimization processes performed on portions of the fluid power transmission system onboard machines.
The main result expected will be therefore a solid methodology, to assist the global evaluation of efficiency of a machine in broad sense, combined with, on the practical side, a simulation model, with associated development guidelines, where a vehicle dynamic model efficiently interacts with fluid power systems sub-models in a co-simulation environment based on the two most popular specialized computer codes available (ADAMS-AMEsim).
In addition to these general objectives it is worth noting that each and every research unit, during its activity, will reach partial objectives, which may be briefly outlined as follows:
1. Generation of lumped parameters models of actual circuits of off road machines;
2. Evaluation, through integrated CFD tools, of detail characteristics of some metering valves used in circuits;
3. Experiences on the application of optimization techniques to sub-systems and generation of the associated know-how;
4. Development of experimentally validated numerical models of power-shift and power-split transmissions;
5. Preparation of a dynamic model of an off-road machine featuring a “virtual laboratory” suitable for investigations on power train, locomotion, control systems and their mutual interaction in specific mission profiles.
Objectives and results listed are intrinsically able to respond to specific market demands. This aspect will be boosted in the near future by ever increasing constraints coming from legislation. Technology sustainability and energy consumption will turn from added value elements to key factors to absolute needs.
When the expected results are evaluated in view of their impact on the advances in knowledge of the field of application, again they appear to have absolute relevance. At the state of the art, no cases are found where the general problem of machine efficiency evaluation is tackled entirely in a virtual environment. Once positively performed, the project will produce guidelines which will be easily generalized to allow for a wider application in the field of off-road machinery. Significant advances are also expected in the field of sub-system optimization with respect to present state of the art, where optimization is generally applied qualitatively as “improvement” .
As far as load-sensing systems are concerned, does not matter if they are applied to actuation system or remotes, the investigation of the optimal response and ability to avoid drawbacks coming from interference and saturation will be focused on:
1. Control strategy;
2. Constrained multi-objective optimization;
3. Flow coefficients and flow forces in valves.
Although these aspect are often found in scientific literature, they are very seldom treated together, and never in their totality.
Advances in knowledge are expected also when the power transmission system for locomotion is concerned. Power-split transmissions were introduced in the first half of last century, but were constantly neglected in application due to a misinterpretation in the criteria to be used to evaluate global efficiency of the machine in different applications. The possibility to establish a solid environment to contrast the widely used, yet improvable in clutch control logic, power-shift transmission with the power-split is in itself a valuable contribution to knowledge advance, and an increase of the knowledge base that a designer can use when defining the operational parameters of an off-road machine.
The integration of the lumped parameters models coming from non linear dynamic simulation of fluid power circuits within an explicit model of vehicle dynamics will also put new and stimulating problems in methodology and numerical integration. The solution of these problems will also produce new competencies. The structure of differential equations in the two cases is substantially different (stiff implicit sets in the first case and generally explicit in the second), and different are the numerical algorithm needed in the two cases. The co-simulation problem is not trivial, in spite of some claims made by commercial software developers; resources needed and strategies to be adopted are to a large extent unknown. Although not listed explicitly within the project objectives, this passage will be a crucial one in the central and final part of the project.
The reference is the agricultural (Ag) and earth moving machinery (EMM) market. Limiting to agricultural tractors (42% of total agricultural machinery market, and good index of agricultural mechanization of a country) national production accounts for some 80000 units/year, of which 60000 exported. The consolidated value of agricultural machinery is 7.2 billion Euro, and represents 0.5% of Italian gross internal product (GIP), with a positive balance of import/export above 3 billion Euros and steadily increasing. If the additional 30000 units of EMM are added, having a total value of 3.5 billion Euros and 50% exported, we get the scenery of a strategic sector of national economy which is additionally very well linked to the daily activity of research units involved in the project. The consolidated value of production is more than 1% of Italian GIP, to which the value of hydraulic and associated components should be added. Today this sector is only marginally affected by competition from emerging economies, thanks to a significant technology gap, which is however rapidly being eroded. Observation of trends in machinery in the major international exhibitions in 2006 e 2007 (Intermat, Bauma, Agritechnica, EIMA) refers. The preservation of the competitive margin must be grounded on research and innovation, exploiting at best the environmental and sustainability constraint to promote new technologies and not market protection.
The proposed project is therefore just on the cutting edge of industrial transfer, especially in some of the products expected. Application potential is high, and some preliminary hints have already raised industrial interest. Development tools for transmission control are needed to generate prototypes of control boards, and possible hardware-in-the-loop applications, although not mentioned in the project, would be immediately transferrable to industry. Constrained optimization technology in non linear dynamic simulation may lead to new circuit layouts and open new possibilities to design. Development of a numerical “virtual lab” machine and the associated methodology for its use is a potentially tremendous tool to allow a predictive evaluation of the impact of “disruptive technologies” to the field, like for instance hybrid power pack or fuel cells, avoiding the often unacceptable costs of the traditional concept-prototype approach. <<<

Timescale

24 months

National and international background

The great impulse given in last decades by the technological progress to all industrial fields allowed a strong design development and a relevant efficiency improvement of hydraulic applications for heavy duty machines, such agricultural and earthmoving vehicles. The great attention devoted to the environment, the need of limiting the internal combustion engines pollution and the increasing costs of fuels force the hydraulic designer to develop more efficient hydraulic actuation systems, in order to achieve an overall power saving without reducing the machineries performances. In this scenario must be located the design limitations coming from future standards for the control of the exhaust emissions in internal combustion engines, and in particular those imposed by the EPA-TIER IV, which will force manufacturers to reduce the energy consumption by improving the hydraulic circuits with the aim of reducing energy losses (see the cited "Off-Highway Vehicle Technology Roadmap - U.S. Department of Energy" [www.trucks.doe.gov]). Consequently, the main targets to be achieved by the mobile application designers are on one hand the operating performances optimization, with particular care devoted to the metering and the modulation of the hydraulic power addressed to the actuators; on the other hand, the increasing of the energy conversion efficiency related to the power transmission, in order to minimize both prime mover power requirement and power dissipations (limiting the subsequent heat generation). In agricultural tractors the hydraulic systems absorb a significant amount of mechanical energy supplied by an internal combustion engine, because there are many interconnected circuits for several functions, i.e. traction, load handling and auxiliary system actuation, braking, steering , suspension control. Regarding the hydraulic circuits for multiple actuations of an actual agricultural tractor (such as the load handling and the auxiliary actuations), since more than ten years the load-sensing solution represents the design approach which allows, among others, a flexible and accurate modulation of the hydraulic power.
As well known, a load-sensing supplying system is composed by a set of components which can determine their own operating condition in order to satisfy the hydraulic requests of the actuation system, both for single and for multiple actuator architectures. This concept, applied to the typical operating conditions of an actual tractor, and developed in order to simplify the user-machine interaction, can be realized adopting variable displacement pumps and motors (electro-hydraulically controlled), electro-hydraulic proportional directional control valves and safety components, to create a complex system in which the proper management of piloting signals assures the instantaneous actuators needs satisfaction. This way, the power modulation can be achieved controlling the flow rate, making the supply system almost insensitive to the external loads, and allowing the operator to control individually each hydraulic actuator. In a load-sensing actuation system a fundamental role is played both by the proportional directional control valves and by variable displacement pumps. The first ones are designed in order to minimize the pressure drop produced by the flow rate metering, and to introduce a minimal energy dissipation during power modulation. The latter ones, having a displacement regulator continuously controlled by the piloting signals coming from the actuation system, are able to generate the amount of flow-rate instantaneously requested, and to adsorb a minimum amount of power from the prime motor. To overcome the problems occurring during the non-standard operating conditions (such as the flow rate saturation or the dragging load uncontrolled movement), the actual load-sensing applications present the classical architecture of the mechatronics solutions, in which the hydro-mechanical circuit is integrated with an electronic control units network. This way, in actual agricultural tractors the energy saving and the efficiency improvement can be obtained only optimizing the interaction between the mechanical, the hydraulic and the electronic design aspects. Nowadays, a first field of interest for technological advancement is focused on the optimisation of variable displacement pumps behaviour, mainly through the "electronic" development of electro-hydraulic displacement regulator able to replace the use of proportional directional control valves for the power modulation. This would allow to avoid localised losses introduced by the valves themselves but, on the other hand, would introduce the incapacity to manage critical operational conditions, such as the presence of dragging loads, sudden growths of the loads themselves, saturation of flow rate or sudden failures. The second research field widely interesting the fluid power design regards the overall reduction of the power dissipation of the hydraulic circuit, by managing the evolution of the classical load-sensing system to an integrated mechatronics solution in which the development of electronic controls must be associated both with the circuit functionalities optimization, and the design improvement of the single hydraulic component. In this case efforts have to be focused on different tasks, such as the design of innovative proportional directional control valves, the development of new compensated actuation systems and the conceptual tailoring of a "new generation" of hydraulic power supply systems. The conceptual development of new trasmission units, combined with the performance optimization and the design evolution of a new generation of transmission systems,
could represent another fundamental step forward in the meaning of the efficiency improvement of agricultural tractors. A direction potentially useful in achieving the first aspect is represented by the hybrid transmission, which is now being applied in the automotive field and urban transportation. The improvement of drive efficiency can be reached by optimizing the performances of the transmission drive system, and the development of "power split" drives or "power shift" drives represent the high-level solutions for agricultural tractors. The power split drive is given by a combination of a hydrostatic unit and a planetary gear. As well known, the hydrostatic transmission has a higher flexibility than the one showed by the mechanical gear drive, but it presents a lower efficiency(typically, 0.78-0.85 vs. 0.93-0.95 ). The power split drive combines the positive aspects of the two transmissions: efficiency and flexibility. Accordingly with the design choices, only a part of the transmitted power is transferred via the hydrostatic unit; the amount of this part can be easily controlled by the variable displacement unit; in the same way, the output shaft speed can be continuously varied. Since the planetary train has two degrees of freedom, it is possible to vary independently the speed of the two shafts. This property allows the ICE to operate close to the minimum specific consumption speed. The power shift drive controls and manages to the vehicle movement equipments through the electro-hydraulic modulation of the power transmission. The application of this type of transmission to the agricultural tractor, and in particular to the mid-power segment (90-150 kW), is strongly affected by the dynamic interaction between hydraulic and mechanical components. The management of the power shift drive is hardened by the mismatch actually existing between the "linear" electro-hydraulic control and the non-linear behaviour both of the mechanical (clutches, suspensions, tires, chassis) and the hydraulic components, and its design optimization is complicated by the needs of introducing into the project both the environmental and the human factors. At the present time, only few applications of the optimization parametric approach of the single sub-systems (transmission system, loads holding and lifting circuit, auxiliary equipments actuation circuit) into the whole system, represented in this case by the agricultural tractor, are available in literature and mainly concerning the automotive and military field. <<<