The field of use of disclosed embodiments is rail vehicle engineering. Rail vehicles normally have a self-acting, automatic compressed-air brake in which the drop in pressure in a main air line proceeding from a normal operating pressure brings about the generation of a brake cylinder pressure. The function of converting the falling pressure in the main air line to the rising brake cylinder pressure and vice versa is performed by control valves of the type of interest here.
The speed of the conversion of pressure changes in the main air line, the gradients, and the uniformity of the build-up and dissipation of the brake cylinder pressure during braking and release are significant parameters which permit the safe operation of long, heavy and fast trains without dangerous longitudinal dynamics forces. Control valves therefore have devices for the temporal control of the processes, the efficiency of which devices directly influences the safety and economy of the railway operation.
It is basically advantageous for the pressure increase in the brake cylinders, for example in the event of a sudden full or rapid braking operation, to take place with so-called uniform action. Uniform action is to be understood to mean that the brake cylinder charging time in the event of rapid braking is independent of the size of the brake cylinder volume or of the level of the maximum brake cylinder pressure in a certain value range. The charging of the brake cylinders must not take place too quickly in order to avoid dangerous longitudinal forces in the train; unnecessarily slow charging would, by contrast, entail an unnecessarily long braking travel, and is therefore likewise undesirable.
WO 2012/038283 A2 discloses a control valve for a compressed-air brake of a rail vehicle, the control valve being composed substantially of a pneumatic relay valve for controlling a brake cylinder pressure, a so-called three-pressure element, and a pilot-control circuit for the generation of a pilot-control pressure for the proportional activation of the relay valve. In order, with the control valve, to attain the uniform action explained above, a variable nozzle is arranged on the path for the release, by deaeration, of the pressure from a control chamber of the pilot-control circuit to the environment, the cross section of which variable nozzle can be varied, by actuation means, from a closed state via a minimum cross section to a maximum cross section. Here, the variation may take place in either stepped or continuous form. In addition to at least one nozzle, it is also possible for multiple nozzles to be connected in parallel or in series, wherein at least one of the multiple nozzles is in the form of a variable nozzle.
By means of the variable cross section, it is possible, in the event of a rapid drop in the main air line pressure, to attain an adequately slow drop in the reference pressure in the pilot-control circuit, which can thus be used as a reference for a brake cylinder charging time with uniform action. In the case of a slow drop in the main air line pressure, the nozzle can be increased in size to such an extent that an air volume required for the acceleration of the braking action can flow out. As a result of the accelerating effect on the drop in the main air line pressure, the control valve can, even at the end of long trains, be optimally adjusted through the selection of a suitable maximum cross section of the throttle. Associated with this is the possibility, through a suitable selection of the minimum cross section for the head of the train, of setting an adequately slow build-up of the brake cylinder pressure, which serves to prevent dangerous longitudinal forces. Both effects ultimately permit higher speeds or greater lengths of the trains. Owing to the effect of the nozzle cross sections on the speed of release, by deaeration, of a reference pressure from a constant volume, there is the further resulting advantage that the gradients that arise are independent of the number and size of the brake cylinders and of the brake cylinder maximum pressure, such that a unified control valve can be expediently produced in an economical manner.
In the case of the control valves of interest here with uniform action for rail vehicles, the times for the build-up and dissipation of the brake pressures must comply with certain predefined standards. For example, the braking and release times are defined in the data sheet UIC 540 or in the standard EN 15355.
In the case of control valves of the conventional type, the generation of the braking and release times is realized by means of a device positioned upstream of the pilot-control circuit of the control valve in the compressed-air supply and deaeration arrangement. As a compressed-air supply, the device in the pilot-control circuit provides a constant volume flow, that is to say ΔV/Δt=constant. As a de-aeration arrangement, the device in turn permits a defined volume flow to the environment. By means of special nozzles with certain diameters for the compressed-air supply and deaeration of a pilot-control volume of known size, the desired speeds of the pressure changes of the pilot-control pressure, and thus the demanded braking and release times, are attained.