One of the key conjunctions in the IC engine is that made by the top compression ring to the cylinder liner. Although the compression ring has seen considerable improvements since its introduction into steam engines by John Ramsbottom in the 1850s, its multiple and often contradictory functions still remain a challenge today. The primary aim has always been to seal the combustion chamber and guard against leakage of combustion gasses. The ring is also required to conduct some of the generated heat away. These requirements call for good ring-bore conformance, but often at the expense of increased friction.
A simplified inter-ring gas flow model, as well as the measured chamber pressure using a Kistler pressure transducer is carried out under motored conditions to obtain the net gas pressure acting behind the ring. This and the elastic pressure as the result of ring restoring tension force in fitment constitute the contact load, which is usually carried by a mixed-hydrodynamic regime of lubrication. The conjunction pressures are treated as a combination of hydrodynamic generated pressures and asperity-pair interactions. The former is obtained by solution of two dimensional Reynolds equation, whilst the latter is determined assuming a Gaussian distribution of asperities on the counterfaces. Surface topography of the bounding solids; the compression ring and the liner are measured and used as appropriate statistical functions in the Greenwood and Tripp model.
Using an analytical flow model, pressure acting behind the ring (on the inner periphery of the ring) is obtained, leading to the calculation of ring-bore friction. A floating liner is used to measure friction of piston and ringpack in situ. The characteristic of the predicted variations are compared with the measured data. However, a quantitative comparison is not possible as the measured data corresponds to all the conjunctions of piston system, including the oil control rings as well as the piston skirt. The results show that variation of gas force behind the ring significantly changes the ring-bore contact, thus friction and the nature of interactions.