Piston-ring film thickness: Theory and experiment compared

Abstract

A review of the published literature has demonstrated a large variability and discrepancies in the measured and predicted values of piston-ring lubricating film thickness in internal combustion engines. Only 2 papers have been found that compare experiments in firing engines directly with outputs from sophisticated ring-pack lubrication models. The agreement between theory and experiment in these comparisons was limited, possibly because of inadequacies in the models and/ or inaccuracies of measurement. This paper seeks to contribute to the literature by comparing accurately calibrated experimental measurements of piston-ring film thickness in a firing engine with predictions from an advanced, commercial software package alongside details of the systematic analysis of the measurement errors in this process. Suggestions on how measurement accuracy could be further improved are also given. Measurements of oil film thickness with an error (standard deviation) of +/-15% have been achieved. It is shown that this error can be reduced further, by changes in the design and installation of the sensors.Detailed experimental measurements of film thickness under the top compression ring in a firing petrol engine have been made and compared with the predictions from a commercial, state-of-the art modelling package. The agreement between theory and experiment is excellent throughout the stroke in most cases, but some significant differences are observed at the lower load conditions. These differences are as yet unexplained, but may be due to the sensor topography influencing the hydrodynamic lubrication, lubricant availability, out-of-roundness in the cylinder, or squeeze effects. This a topic that requires further study.

Experimental Measurement of the Time-Based Development of Oil Film Thickness, Lubricating Film Extent and Lubricant Transport in Crosshead Engines

“…Lubricants 2021, 9, x FOR PEER REVIEW 6 of 17 ring faces and identify machining marks on piston-rings [19]. Bespoke transducers were manufactured for this study.…”

“…Lateral resolution is difficult to define and depends on the proximity of the ring to the transducer, but approaches the size of the transducer electrode (0.16 mm) at small film thickness. This is fine enough to determine the shape of pistonring faces and identify machining marks on piston-rings [19]. Bespoke transducers were manufactured for this study.…”

Comparison of Ring-Liner Oil Film Thickness Resulting from Different Injector Designs in a Diesel Marine Engine Using an Ultrasound Measurement Method

“…Through modifications to the liner, a capacitance probe can be mounted flush with the internal surface of the liner and the ring acts as the other electrode. This approach has measured oil films ranging from 0.2 to 9 μm [6][7][8][9][10][11][12] although the methodology is prone to breakdown from excessively thin films (< 1 μm) as shorting of the circuit may occur (metal-metal contact). A major assumption in this technique is that the region between the piston ring and liner is fully flooded with oil as cavitation changes the dielectric constant.…”

Piston ring oil film thickness measurements in a four-stroke diesel engine during steady-state, start-up and shut-down

“…29–34 These methods require invasive machining of the cylinder bore such that the probe is in contact with the oil film, or that there is a transparent window to facilitate the optical measurement. A comprehensive review of instrumented engine tests using the various methods is given in Garcia-Atance Fatjo et al 33 which describes the different experimental techniques and draws comparisons with the theoretical studies.…”

“…[25][26][27][28] Electrical capacitance probes and inductance probes can be mounted flush on the internal surface of the liner and have also shown much success in measuring the minimum oil film thickness. [29][30][31][32][33][34] These methods require invasive machining of the cylinder bore such that the probe is in contact with the oil film, or that there is a transparent window to facilitate the optical measurement. A comprehensive review of instrumented engine tests using the various methods is given in Garcia-Atance Fatjo et al 33 which describes the different experimental techniques and draws comparisons with the theoretical studies.…”