Measurement and Simulation of the Oil Supply at the Piston Assembly

The investigation of the lubricating oil supply at the piston assembly and its influencing factors are an important contribution to optimise future engine concepts with regard to fuel consumption and emissions. Within the FVV research project series Piston Ring Oil Transport, a comprehensive simulation program is developed at several universities and research institutes, with which the tribology conditions at the piston assembly can be designed.

AUTHORS

Dipl.-Ing. Benedict Uhlig

is Research Assistant at the Institute of Internal

Combustion Engines at the Technical University of

Munich (Germany).

Ann-Christin Preuß, M. Sc

is Research Assistant at the Institute of Analytical

Measurement Tech­nology  Hamburg e. V.

(IAM­Hamburg) (Germany).

Dipl.-Ing. Johann Grafis Research Assistant at

the Institute for Powertrain and Vehicle Technology at

the University of Kassel (Germany).

Matthias Neben, M. Sc.is Research Assistant

at the Department of Aerodynamics and

Fluid Mechanics of the Brandenburg University of

Technology Cottbus­ Senftenberg in Cottbus

(Germany).

© TU Munich

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1 INTRODUCTION

The main focus of the project series “Piston Ring Oil Transport” is the investigation of the tribological conditions in the piston assembly. Within the first part of the project, a 0.5-l single-cyl-inder research engine was built up [1, 2], which is equipped with numerous sensors to measure the tribological circum-stances. In the second project part [3] presented here, addi-tional simulation models are developed, which gain accuracy compared to the recorded measurement data and contribute significantly to an understanding of the oil supply at the piston assembly.

2 MEASUREMENT TECHNOLOGY AND RESULTS

2.1 OIL FILM THICKNESS MEASUREMENTS BY LASER-INDUCED FLUORESCENCEFor measuring the oil film thicknesses at the piston assembly, optical fibres are inserted in the liner at several positions of the piston stroke. The oil film thickness between the piston and the liner can be determined using the principle of LIF. The measure-ment method described in detail in [1, 2] was improved with opti-cal fibres with a smaller core diameter of 100 μm instead of 400 μm, as well as an increase in the selected sampling rate from 0.1 to 0.05 °CA, so that the narrow ring edges of the oil control ring can be identified. FIGURE 1 (left) shows the installation of the optical fibres. FIGURE 1 (right) visualise the resulting signal when the oil control ring passes the measuring point during the exhaust stroke. The local minima of the lubricating oil film thickness at the edges of the oil control ring can be seen clearly.

In the project Piston Ring Oil Transport II components of the piston assembly were varied to evaluate their influence on the oil film thicknesses. The minimum lubricating film thickness between the piston skirt and the cylinder liner is measured during four times per stroke. The result is formed from the average of the four min-imal oil film thicknesses of the four strokes and of the measured 100 combustion cycles. FIGURE 2 shows the effect of the different components on the lubricating film thicknesses on the piston skirt. A reduction in preload of the oil control ring only has a slight effect on the resulting lubricating oil film thickness. The tangential force of 14.4 N was reduced to 6.7 N. Overall, slightly lower oil film thicknesses remain on the piston skirt, which can be explained by reduced scraping of the oil from the cylinder-liner downwards. The use of a three-piece oil control ring shows a better scraping effect at high engine speeds compared to the two-piece standard ring, so that more oil reaches the piston skirt. Disabling the piston cooling jet, as is done to prevent oil emissions at low loads, results in sig-nificantly reduced supply of oil, especially at higher speeds. Some of this effect can also be attributed to the reduced clearance between the piston and the liner due to the thermal expansion.

1 INTRODUCTION

2 MEASUREMENT TECHNOLOGY AND RESULTS

3 SIMULATION

4 CFD MODELLING OF A PISTON RING

5 SUMMARY AND OUTLOOK

FIGURE 1 Magnified view of the cylinder liner with embedded LIF-measuring point (left) and resulting signal with the oil control ring sliding over the measuring point (right) (© TU Munich)

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As a result of the lower piston clearance (reduction from 90 µm to 55 µm) the lubrication gaps are reduced, which leads to lower oil film thicknesses at the piston skirt in the entire engine map.

2.2 MASS SPECTROMETRIC OIL EMISSION MEASUREMENTFor metrological support of the simulation models mass spectro-metric oil emission measurements of different piston ring config-urations have been performed. The results, measured according to the method described in [2, 3], are shown in FIGURE 3. When using series piston rings, FIGURE 3 (a), the oil emission with a max-imum of 8 g/h is noncritical. If the ring gap of the first compres-sion ring is extended by 2 mm, FIGURE 3 (b), oil emission increases up to 12 g/h at 4000 rpm and maximum load. If the ring gap of the second compression ring is extended by 2 mm, FIGURE 3 (c), the emission values in the lower load/speed area are at the same level as for the series piston rings. In the upper area, the emission is slightly increased, but with a maximum of 10 g/h it is below the values in case, FIGURE 3 (b). With a reduced pretension of the oil control ring, FIGURE 3 (d), up to 11 g/h are emitted.

2.3 TRACER INJECTION FOR OIL TRANSPORT ANALYSISAn injection system, which allows the defined injection of a com-posite of fluorescent tracer and oil into the piston assembly, was developed by IAM. By means of the LIF measuring technology of LVK, as well as the capillaries coupled to the piston via a measur-ing link, the transport behaviour of the injected composite can be detected along the piston and liner. The objective of this measure-ment procedure is to determine the transport routes, quantities and velocities.

FIGURE 4 shows a cyclically dissolved fluorescence signal (uncalibrated), which was recorded with the optical fibre at top land. This optical fibre samples the opposing liner by the move-ment of the piston. About 50 combustion cycles after the start of the injection strong fluorescence at the position where the first piston ring reverses at top dead centre (TDC) occurs. The signal continuously increases over a period of about 80 combustion cycles, widens by 2 mm above the piston ring (oil collar) and then decreases. The signal intensity is related to the film thickness. In the region of the TDC of the first compression ring (oil collar) the maximum is at least a factor of 10 greater than along the liner. The signal intensity in stroke 2 and 3 at TDC is lower than in stroke 1 and 4 at gas-exchange TDC. This may be due to the piston tilt. It must be assumed that in future inves-tigations of the oil transport processes in the piston assembly essential knowledge will be gained by this oil film measurement technology.

3 SIMULATION

At iaf-mt simulation models were built for the single-cylinder research engine and calculations were carried out with Kori3D, a simulation program that determines the three-dimensional dynamic piston ring motion as well as blow-by and ring land pres-sures. Ring models are based on finite element (FE) beam theory, blow-by and ring land pressures result from simplified gas dynam-ics. Input variables for the calculation are: geometry of the crank-shaft drive (stroke, compression, cylinder distortion, ring and groove geometry and preload of the rings), engine speed, the tran-sient combustion chamber pressure, the piston secondary move-ment and the physical properties of oil and fuel.

FIGURE 2 Engine map of minimal oil film thicknesses between piston skirt and liner with various component options (a to d) (© TU Munich)

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Validation of the simulation model is based on measured ring movement, integral blow-by and transient pressure between the rings. The gas flow through the individual piston rings of the pis-ton ring pack determines ring land pressures, which form impor-

tant boundary conditions for the lubrication analysis and – in the low speed range – almost exclusively determine the ring motion. The successful validation of the blow-by is a prerequisite for the subsequent validation of the ring movement. The gas flows

FIGURE 3 Oil emission of research engine with series piston and various component options (a to d) (© IAM-Hamburg)

FIGURE 4 Fluorescence signal of the oil film at the liner (1000 rpm, 2,6 bar IMEP, area: TDC) (© IAM-Hamburg/TU Munich)

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between the ring contour and the piston and through the ring gap can be calculated in the simulation program with two different models: orifice (1-D) and the more advanced flow thread (2-D) model, which was used here. To account for circumferential

geometry variation, ring gap effects and flow contraction and losses, flow coefficients are used to match measured and predicted blow-by. In the present simulation model, twelve mutu-ally influencing coefficients are to be determined. FIGURE 5

FIGURE 5 Ring land pressures (1/2 between first and second ring, 2/3 between second and third ring) (© IAM-Hamburg/University of Kassel)

FIGURE 6 Modelled fluid domain with dis-tribution of lubricant (top left), piston and ring displacement (top right), identified lubri-cant consumption mechanisms and the velocity field of the cavity flow (bottom) (© BTU CS)

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shows the intermediate ring pressures for a rotational speed of 2000 rpm and an indicated mean effective pressure of 5 bar. The curves show deviations during intake with values in the neg-ative pressure range. During the pressure build-up and the sub-sequent expansion with gas ejection, the curves are met very well. An integral blow-by of 3.7 l/min was measured in the exper-iment, the simulation yielded 3.9 l/min (deviation < 8 %). In upcoming work, an optimisation process (simulated annealing) will be used for an even more precise matching of the interme-diate ring pressures.

4 CFD MODELLING OF A PISTON RING

To obtain the predominant local effects of the lubricant transport the integral values of the experiment are analysed in detail by CFD-simulations. The focus of the 2-D simulations is the fluid flow around a single compression ring. The particular challenge lies in high pressure gradients coupled with high velocities and divergent geometric scales. The inertia force on the fluid due to piston motion is realised by a displacement of the piston and the piston ring according to the load cycle. A full compressible and multipha-sic solver based on OpenFoam with a moving and deformable mesh consisting of 100,000 cells was developed.

The pressure profiles of the top land and second land as well as the displacement data of the piston and the piston ring was adopted from the experimental data, FIGURE 6 (top right). The minimal gap width between ring and liner varies between 1.5 and 2 µm. However, the mesh resolution in this area is 0.2 µm, which increases the computational effort significantly. Numerical details are published in the final report of Piston Ring Oil Trans-port II [3].

The solver was tested with two geometries. The following results refer to the preliminary described Munich research engine, where the mean effective pressure is measured at 10 bar and 2000 rpm. With the minimal gap width of 2 µm we observed dry running between liner and ring. Consequently, the gas velocity arises 500 m/s just as the combustion chamber pressure reaches 50 bar. Reducing the gap width to 1.5 µm leads to a stable lubricant film and a maximum velocity of about 100 m/s. FIGURE 6 (below) shows the identified thrown off lubricant due to inertia force at 370 °CA and reverse blow-by between 500 to 600 °CA. Surprisingly, the differential velocity between liner, ring and top land respectively induces a lid-driven cavity flow between 640 to 670 °CA. This newly investigated transport mechanism pushes lubricant upwards along the top land. Further simulations need to be done to check whether the lubricant arrives the combustion chamber or flows back to the ring pack along the liner.

5 SUMMARY AND OUTLOOK

Within the FVV research project series Piston Ring Oil Transport, a comprehensive simulation program is developed, with which the tribology conditions at the piston assembly can be designed. Based on a proposed follow-up project, the newly created simula-tion models may be validated by further engine tests.

As an addition to the individual measurement spots with optical fibres, the integration of a glass window in the liner enables area specific, though time discreet measurements. Such an extension to the engine is being developed in the FVV project 1210 (Piston Ring Oil Transport – Glass Liner).

REFERENCES[1] Uhlig, B. et al.: Investigation of the Lubricating Oil Management on the Piston Assembly. In: MTZworldwide 77 (2016), no. 4, pp. 62­69[2] Kirner, C. et al.: Kolbenring­Öltransport: Öltransport durch die Kolbenringe. In: FVV Abschlussberichte (2015), no. 1072[3] Uhlig, B. et al.: Kolbenring­Öltransport II: Öltransport durch die Kolbenringe. FVV Herbsttagung 2016, Informationstagung Motoren (2016), Issue R576

THANKSThe research project Piston Ring Oil Transport II (project no. 1197) was encour-

aged by the Research Association for Combustion Engines e. V. (FVV). The

authors thank for the allowance and for the support of the user committee, which

was leaded by the chairman Dr.-Ing. A. Robota (Federal Mogul Burscheid

GmbH). Furthermore, the authors thank Prof. Dr.-Ing. Georg Wachtmeister

(Institute of Internal Combustion Engines, Technical University of Munich), Prof.

Dr.-Ing. Gerhard Matz (IAM-Hamburg e. V.), Prof. Dr.-Ing. Adrian Rienäcker

(Institute for Powertrain and Vehicle Technology, University of Kassel) and Prof.

Dr.-Ing. Christoph Egbers (Department of Aerodynamics and Fluid Mechanics

(LAS), Brandenburg University of Technology Cottbus-Senftenberg) for their

support. In addition, the thanks go to the additional author of this paper, Dipl.-

Ing. Claus Kirner.

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