Friction Response of Piston Rings for Application-like Starvation and Benefit of Amorphous Carbon Coatings

Abstract

The oil supply at the interface between the top ring and the cylinder liner (TRCL) plays a major role in an internal combustion engines efficiency. In particular, the interface forms a trade-off between the serving of enough lubricant for sufficient lubrication conditions and emissions through subsequent combustion. This can lead to deficient top ring lubrication conditions. In this study, a new developed reciprocating long-stroke tribometer, enabling the variation of oil supply, is used to investigate such application-like starved lubrication conditions of the TRCL interface. With the simulative investigations, a comparison with the fired engine is possible. The performance of diamond-like carbon coatings is compared to standard nitrided piston rings. It was found that the tetrahedral amorphous carbon (ta-C) coatings exhibit up to 31% reduced friction as well as a lower wear under starved lubrication conditions. Simulative investigations show a good correlation between engine friction and tribometer measurements for selected oil supply conditions.

Positive Effect of Periodic Micropatterns on Compression Ring Friction

“…At this point, the maximum combustion chamber pressure (about 383° crank angle) is present, pressing the piston ring against the cylinder wall. [ 22,30 ] In sum, the a‐C + DLIP ring generates 20% less friction power, as mentioned before.…”

“…A large part of the compression ring friction is generally located at the fired top dead center (from approximately 270 to 450° crank angle). [ 22–24 ]…”

“…The obtained results show that the measured friction force was lower for the DLIP structured compression ring (compared to the untextured reference) for approximately 15% corresponding to the compression stroke (190°–330° crank angle) and a smaller part of the combustion stroke (400°–530° crank angle). Between 330° and 400° crank angle (FTDC), when the compression ring is dominated by gas pressure‐induced loads, [ 22 ] the DLIP‐treated compression ring caused more friction. The measurement data of the intake and the exhaust stroke were not taken into account further in this investigation.…”

“…This could be traced back to the a‐C coating, due to the high hardness of the deposited (hardness of 3000 HV) and complementary wear resistance. [ 22 ] Nevertheless, the worn DLIP microtexture also appears flattened after the tribological test and the apparent contact area of the DLIP peaks became wider.…”

“…It was calibrated for each first compression ring cylinder liner contact using the reciprocating long‐stroke tribometer, [ 32 ] testing starved lubrication conditions as described in ref. [22]. Further details about the friction simulation of the piston assembly can be found in ref.…”