Understanding Engine Tribology - Performing Reciprocating Tests of a Piston Ring's Interaction with the Cylinder Liner

Tribological evaluation of the interaction between a piston ring and the cylinder liner is a key step toward understanding engine tribology. This kind of test helps automotive manufacturers to comply with the stringent specifications on emission, fuel efficiency, safety, and durability of automobiles.

Traditionally, this type of test is developed for a specific engine, and the results obtained are not comparable from one test to the other. However, mechanical testing technology has reached the stage where standard tests can now be performed for cylinder liner and piston ring frictional properties through high-speed reciprocating motion.

This can help lubricant manufacturers, materials scientists, and automotive engineers worldwide in the development of cost-effective lubricants and superior engine components.

Engine Tribology

There is a rapid growth of friction and wear studies of a wide range of machine components in internal combustion engines, and this is a direct response to the ever increasing rigorous specifications for fuel efficiency, emissions, safety and general automobile durability in the automotive industry.

In particular, engine tribology research has seen incredible changes over the last 10 years, with piston assemblies, valve train components, and internal bearings receiving considerable attention as they are the key frictional components in automotive engines.

Among these, the piston assembly contributes the most friction and accounts for nearly 45% of the overall frictional losses in a standard engine.¹ If such frictional losses are reduced, the brake thermal efficiency of an engine can be considerably improved.

In order to improve engine performance, a considerable amount of effort was made to study the tribological characteristics of the piston ring's interaction with the cylinder liner. Unlike the engine bearings¸ the mechanism of cylinder liners and piston rings requires a wide variation of service speed over an operating cycle.

The speed of the piston ring is effectively zero for a short period of time at both ends of the stroke - at the bottom dead center (BDC) and the top dead center (TDC). This condition provides virtually no chance of the lubricant entering into the contact. As a result, a boundary lubrication regime is more relevant at BDC and TDC.

However, the fluid-film lubrication or hydrodynamic condition prevails at the mid-stroke, where the speed is the highest. Near to the dead centers, where there is low velocity, elastohydrodynamic conditions of lubrication and squeeze film lubrication are expected to have a greater role in engine tribology. Essentially, engine tribology encompasses all of the regimes of lubrication.

It is inherently complicated to perform tribological testing of a piston ring assembly. For example, floating liners have been used to study piston ring frictional characteristics², but such application-oriented test fixtures need a separate design for each type of engine.

These unique test rigs are not only costly, but also unable of accommodating standard test specimens. Also, there is a limited scope of application for data obtained from such specially designed test methodologies. Therefore, such data has no universal applicability and cannot be effectively compared from one test to another.

Now, the technology has been developed so that standard tests³ can be performed on a cylinder liner and piston-ring. Most advanced mechanical testers available today can be configured for these tests and they also have the ability to compare results from one test to another.

In addition to contributing to improved engine components, such comparative testing can also be employed to develop cost-effective engine lubricants and to perform quality control of these oils. For the comparative studies described in this article, a UMT TriboLab™ (Bruker Nano Surfaces, San Jose, California) was used to carry out high-speed reciprocating tests. These were performed by examining and recording the tribological characteristics of piston rings against a cylinder liner segment, in accordance with the ASTM G181 document³, with different combinations of test parameters, such as frequency, load, and stroke in lubricated conditions.

Performing the Piston-Ring Test

To measure the frictional force (Fx), the piston ring was tested with a reciprocating motion under a normal load (Fz). The UMT TriboLab has servo control of normal load, and can measure and record a number of test parameters such as Fz, Fx, and Stroke position (lvdt) as a function of time.

Figure 1 depicts representative plots of lvdt, Fx, and coefficient of friction (COF) as a function of time over four successive cycles during a reciprocating test. In order to obtain this data, the piston ring test was performed with a normal load of 200 N at a frequency of 10 Hz.

The lvdt plot reveals that the test was carried out over a stroke length of 25 mm. To some extent, the Fx plot imitates a square wave as a result of the change in direction of motion during every half cycle at the BDC and TDC positions. COF is reduced at the mid stroke because of the change in lubrication condition at higher speed, while Fx is the highest at the dead centers due to the prevalence of boundary lubrication regime.

The COF was automatically calculated as a ratio of absolute value of Fx to Fz. The static COF at the dead centers is approximately 0.11, while the average COF is about 0.06.

Figure 1. Plot of friction force (Fx), stroke position (lvdt), and coefficient of friction (COF) as a function of time for a test of piston ring on a cylinder liner segment with a normal load of 200 N in lubricated condition.

A standard load profile during increasing and decreasing load cycles in an engine test³ is shown in Figure 2. Using a high-speed reciprocating drive, the test was carried out after a step-load profile at a frequency of 10 Hz over a stroke length of 25 mm.

From each constant load step, COF data was collected for increasing and decreasing load cycles (Table 1). As the normal load was increased, the COF data show an increasing trend. This frictional behavior was due to the change of lubrication regimes toward mixed, or boundary regions at higher load, where the COF is likely to be higher.

From Table 1 it can be seen that there was some offset in COF in the same constant load steps between decreasing and increasing load cycles. This offset in COF was likely due to the inadequate run-in of the piston ring before the test, as recommended in the ASTM standard. ³

Figure 2. Normal load profile (Fz) as a function of time for an engine test.

Table 1. COF values at various loads during increasing and decreasing loading cycle

In order to ascertain the comparative surface roughness parameter, a Bruker ContourGT^® optical profiler was used to evaluate the surface topography of the piston ring segment before and after the engine test. The representative topography of the piston ring surface before and after the test is shown in Figures 3 and 4.

Figure 3. Surface topograph of the piston ring before performing the engine test, obtained with a Bruker ContourGT interferometer.

Figure 4. Surface topograph of the piston ring after performing the engine test, obtained with a Bruker ContourGT interferometer.

Surface parameters, including root mean square height (S_q), arithmetical mean height (S_a), minimum valley depth (S_v), maximum peak-to-valley height (S_z), and maximum peak height (S_p), were acquired with the interferometric profiler (Table 2).

It was found that the parameters S_a, S_p, S_q, S_v, and S_z have decreased following the test. This could be due to the initial run-in step before the actual engine test of the piston ring, which may have made the original surface smoother by reducing the surface asperities.

Such reduction in surface asperity height can also be attributed to the boundary lubrication condition at the dead centers, where the piston ring came in direct asperity contact with the cylinder liner segment.

Table 2. Surface parameters of the piston ring before and after the engine test

The reciprocating test results can be compared among different cylinder liners, piston rings, and lubricant materials. Such studies can be used to evaluate the tribological characteristics of cylinder liner and piston ring segments across an array of engine types.

They can also be used to develop and test novel materials for engine components, and help the development of more cost-effective and efficient lubricants.

Conclusion

Engine tribology plays a key role in the research, development, and quality control of a wide range of machine elements for internal combustion engines. As piston assemblies are the biggest contributor toward frictional losses in an engine, tribological evaluation of cylinder liner and piston ring components in the presence of lubricants is considered to be the most beneficial activity to improve an automobile’s energy efficiency.

The UMT TriboLab test system from Bruker can perform such tribological evaluation to assist lubricant manufacturers, materials scientists, and automotive engineers toward achieving their emission, fuel efficiency, and durability objectives.

About UMT TriboLab

Bruker’s UMT TriboLab system is built on the company’s historic Universal Mechanical Test (UMT) platform and its precision control of speed, load, and positioning. The TriboLab has a modular design that ensures the flexibility to enable a wide range of stroke length, speed, force, and temperature test capabilities.

It is easy to reconfigure the UMT TriboLab for almost any tribological test, usually within minutes. The instrument is versatile, user-friendly and highly productive, thanks to the integrated “intelligent” hardware and software interfaces, such as TriboID™ and TriboScript™.

TriboID automatically detects the numerous components connected to the main system required for its proper functioning, and also configures them for operation. TriboScript provides a secured and enhanced scripting interface to enable easy compilation of test sequences of the already created test blocks. Equipped with real-time control and data analysis software, the TriboLab system ensures the highest accuracy and repeatability.

References

1. Dowson, D., Piston Assemblies: Background and Lubrication Analysis, Tribology Series Volume 26: Engine Tribology, Taylor C. M. (Editor), (Elsevier Science Publishers BV, Amsterdam, 1993) 213.

2. Hamatake, T., Kitahara, T., Wakuri, Y., Soejima, M., Friction Characteristics of Piston Rings in a Reciprocating Engine, Lubrication Science, 6 (1) (1993) 21.

3. ASTM G181-11, Standard Test Method for Conducting Friction Tests of Piston Ring and Cylinder Liner Materials Under Lubricated Conditions, ASTM International, West Conshohocken, PA, 2011, www.astm.org.

This information has been sourced, reviewed and adapted from materials provided by Bruker Nano Surfaces.

For more information on this source, please visit Bruker Nano Surfaces.