Design of Piston Ring Friction Tester Apparatus

One of the major prerequisites for calculating piston ring friction is a good description of the tribological situation. Piston rings operate in three different lubrication regimes and the theoretical models should be capable to describe this. A very important condition for describing the frictional behavior of a piston ring correctly is knowledge about the amount of lubricant present. For piston rings the external load may be established by measuring the pressure distribution, i.e. the pressure drop in the piston ring package. Speed and temperature may also be established. The amount and distribution of oil present is, however, not easily determined. In theoretical analysis of piston ring lubrication it is often assumed that it operates under fully flooded conditions, but this is not the case in real life operation. These problems forms the basis for an experimental investigation. Near the top dead center the nature of the movement of the piston ring, along with the elevated pressure due to combustion, forces it to operate in the boundary lubrication regime. In the middle part of the stroke, the sliding speed is quite high and the piston ring operates in the hydrodynamic regime. Again, at the bottom dead center, the piston ring movement along the liner changes direction and due to the low speed the ring will operate in the mixed lubrication regime. In large two-stroke engines the cylinder oil is supplied to the bearing at discrete locations on the cylinder liner at a specific rate at a certain time. The shifting in lubrication regimes and the non-uniform oil distribution opens for the possibility of starved conditions for the piston ring bearing. Therefore it is important to measure the oil distribution on the liner as a function of the operating conditions. The amount of lubricant available is reflected in the friction absorbed in the bearing. The following properties will be measured: Oil fillm thickness - along liner (axial variation), oil film thickness - along piston ring (circumferential variation), piston tilt, temperature of piston rings and liner, pressure at piston lands and forces on piston rod. Since the frictional forces are small compared to the rest of the acting forces the main design idea is to fix the piston, while the cylinder liner moves. This approach makes it simple to measure the parameters mentioned above by putting the instrumentation in the piston.