Test rig for investigating Poiseuille flow in narrow gaps
The development of combustion engines is increasingly specified by the overall objectives to reduce the fuel consumption and exhaust emissions. For achieving the limit values, it is necessary to increase the pressure in the common-rail (CR) system up to 3000 bar. The design of the piston-cylinder contact in the CR pump is thereby of particular relevance.
Increasing the pressure leads to a more distinctive throttling in the piston-cylinder gap and causes a significant heating up of the lubrication film. Due to the high pressure of 3000 bar, the elevated temperature and the relative low diesel viscosity, the gap dimensions between the piston and the cylinder have to be dimensioned as tight as possible to minimize leakage.
As part of the project “Developing the fundamentals for the design of piston-cylinder contacts of future CR-pumps”, funded by AiF 18193 and processed by the two research institutes IFAS of the RWTH Aachen University and iaf of the University Kassel, at IFAS a high pressure gap test rig to determine the local temperatures and pressures in high pressure loaded condition is designed.
FEM-calculations predict a temperature increase in the piston-cylinder gap of about 140 °C due to throttling the fuel from 3000 bar to environmental pressure. The gap’s thermal expansion is expected to have a greater influence on the resulting gap heigth than the expansion due to the deformation under high pressure. New approaches of TEHD simulations consider, besides the interaction between structural deformation and hydrodynamic pressure build up, the thermal degrees of freedom and are capable to simulate leakage and friction more precisely. To perform verification tests of such simulation results, the precise narrow gap dimensions have to be known.
The gap test rig shall map the piston-cylinder gap geometry as exact and reliable as possible and at the same time shall be able to measure the gap’s local temperatures and pressures, as well as the flow rate through the gap. The difficulty lies in the precise adjustment of the typical gap heights of only a few micro meters, a fraction of the human hair diameter, and the high pressures up to 3000 bar. Specific seal solutions are required to provide absolute tightness and, at the same time, don’t affect the constant pre-defined gap heigth. The required working packages are illustrated in the picture below.
The designed test rig allows for a deeper understanding of the Poiseuille flow characteristics in narrow gaps under high pressure and high temperature conditions. Comparable test rigs are limited to the usage of entire components, for example piston-cylinder units to measure the gap flow. Focusing solely on the narrow gap and extracting this specific element from the entire system makes it possible to adjust the test conditions exactly and repeatedly. As well as the gap dimensions, a wide range of operating conditions can be configured. In addition to the piston-cylinder contact in next generation CR pumps, examined in this project, similar boundary conditions also can be found in others sectors, so that the procedures and methods developed in this project can be transferred to further applications. Suitable examples are the contacts with distinct pressure flow, typically be found in hydrostatic piston units.