Article citation info: (*) Tekst artykułu w polskiej wersji językowej dostępny w elektronicznym wydaniu kwartalnika na stronie www.ein.org.pl KAmińsKi Z, KuliKowsKi K. Determination of the functional and service characteristics of the pneumatic system of an agricultural tractor with mechanical brakes using simulation methods. Eksploatacja i Niezawodnosc -maintenance and Reliability 2015; 17 (3): 355-364, http:// dx.doi.org/10.17531/ein.2015.3.5. Zbigniew KAmińsKi Krzysztof KuliKowsKiDetermination of the functional anD service characteristics of the pneumatic system of an agricultural tractor with mechanical brakes using simulation methoDs wyznaczanie metoDami symulacyjnymi właściwości funkcjonalnoużytkowych pneumatycznej instalacji ciągnika rolniczego z hamulcami mechanicznymi* IntroductionIn agricultural tractors, a variety of friction braking mechanisms are used, including band brakes, drum brakes, and dry or wet multidisc brakes [5]. To transfer the energy needed to run the service brakes of a tractor, a mechanical, hydraulic or an air drive is used. Selection of the drive type and the energy source depends on the design and weight of the tractor. In low-and medium-power tractors, manually operated hydraulic brake systems are used, which are relatively inexpensive and simple. Because of their cost, mechanically actuated brakes are still attractive in low-power tractors.Agricultural tractors with mechanical or hydraulic brakes are equipped with a pneumatic system designed to run the air braking systems of towed trailers and agricultural machines. So called combined systems are currently used [32], which have the capability to operate with the single-and dual-line braking systems of towed vehicles.A typical combined pneumatic system of a farm tractor consists of two major parts: an energy supply unit and a control device. The function of the energy supply unit is to compress and purify air and to maintain the adequate pressure in the tractor and trailer reservoirs in order to ensure the required trailer braking performance. The role of the control device is to provide the follow-up control of the single-or dual-line braking system of a towed vehicle in a manner that enables the synchronous braking of the both vehicles. Control devices differ mainly in the type of the trailer brake control valve, which can be actuated either mechanically, hydraulically or pneumatically, deEksploatacja i NiEzawodNosc -MaiNtENaNcE aNd REliability Vol.17, No. 3, 2015 356 sciENcE aNd tEchNology pending on the tractor brakes [32]. A schematic diagram of a tractor's pneumatic system with a mechanically actuated brake valve is shown in Figure 1.Because of the road safety, agricultural vehicles braking systems must meet several specific requirements [8,9] for braking performance, high operation speed during rapid braking (the response time should not exceed 0.6 s) and the compatibility of the tractor and towed vehicle braking systems [27] (the synchrony of operation of individual circuits). Research by Scarlett [28] reveals that ...

Farm tractors are provided with pneumatic systems to control and operate air braking systems as well as pneumatic suspension systems, the latter being increasingly often applied to high-capacity trailers. The excessive consumption of compressed air may adversely affect the braking effectiveness of a tractor-trailer unit; therefore, the impact of performance of the air supply equipment of the tractor on the transient processes taking place in the air braking system of the trailer should be taken into consideration as early as at the design stage. A mathematical model of the air supply equipment, consisting of functional and structural models of individual equipment components such as compressor, pressure regulator, and compressed air reservoir, has been presented in this paper. An example has been included, where the computer model of the supply system, prepared in the Matlab-Simulink program, was used to assess the correctness of compressor selection to the pneumatic system of the Pronar 5110 tractor. The adequacy of the implemented computer model of the air supply equipment, experimentally confirmed, was evaluated with statistical methods, with the use of the Kolmogorov-Smirnov test. The computer model developed may be used as a tool to assess the correctness of selection of parameters of the air supply equipment within the designing process and as a subsystem in order to evaluate, with the use of simulation methods, the transient processes taking place in the air braking systems of agricultural vehicles.

Pipes are widely used in hydraulic and pneumatic subsystems for transferring energy or signals. Accurate prediction of pressure transients is very important in the drive and control circuits of complex fluid-line systems. Based on the approximation of Navier-Stokes equations for one-dimensional flow, a mathematical model of the pneumatic pipe with lumped parameters was developed using ordinary differential equations, which can be easily implemented in most computer programs for the simulation of complex heterogeneous engineering systems. Implemented in Matlab-Simulink software, the computer model of the pipe makes it possible to determine the influence of capacitance, inertance, resistance and heat exchange on the dynamic characteristics of the control and power circuits of pneumatic systems. An advantage of the model is that various functions can be selected to describe linear resistances and local resistances are taken into account, particularly at the inlet and outlet. Such resistances largely affect flow resistances in short tubes (up to 10 m) that can be found, e.g., in pneumatic brake systems of road vehicles. Confirmed by Kolmogorov-Smirnov test results, the consistency of the pressure curves obtained in experimental and simulation tests proves the implemented tube model to be useful for the calculations of pneumatic system dynamics.

A mathematical model of the trailer relay emergency valve used in the pneumatic braking systems of agricultural trailers has been presented. The differential equations of the model allow for the inertia of control elements, static and kinetic friction, and heat exchange in individual valve chambers. The developed mathematical valve model was used for simulation of the transient processes in the dual-line braking system of a single-axle agricultural trailer. The computer models for individual components of the braking system were made within the program MATLAB/Simulink in the form of S-function-type functional structural blocks based on original algorithms. The positive results of validation of the trailer braking system computer model show a potential for a wider use of dynamic computations in the computer-aided process of designing pneumatic vehicle braking systems.

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