Reducing Loss of Energy from Friction through a Better Understanding of Tribological Interactions

For a diesel engine, friction loss accounts for approximately 10% of total energy in fuel [1]. Moreover, 40-55% of the friction loss stems from the power cylinder system. However, this loss of energy caused by friction can be diminished by better understanding the tribological interactions occurring in the power cylinder system.

A majority of the loss of friction in the power cylinder system arises because of the contact between the cylinder liner and piston skirt. Due to the constant changes in force, temperature, and speed in a real life engine, the interactions between the piston skirt, lubricant, and cylinder interfaces are quite complex in nature.

The key is to optimize each factor in order to obtain optimal performance of the engine. The purpose of this study is to focus on replicating the mechanisms that cause friction forces and wear at the interfaces between piston skirt, lubricant, and cylinder liner (P-L-C).

Schematic of power cylinder system and piston skirt-lubricant-cylinder liner interfaces.

Figure 1: Schematic of power cylinder system and piston skirt-lubricant-cylinder liner interfaces.

Importance of Testing Pistons with Tribometers

An important lubricant that is well-designed for its application is motor oil. To improve the performance of motor oil, additives such as detergents, dispersants, viscosity improver (VI), anti-wear/anti-friction agents, and corrosion inhibitors are added, in addition to the base oil.

The presence of these additives a­ffects how the oil behaves under various operating conditions. Moreover, the behavior of oil has an effect on the P-L-C interfaces, thereby determining whether significant wear from metal-to-metal contact or hydrodynamic lubrication (i.e. very little wear) is occurring.

Without isolating the area from external variables, it is extremely difficult to comprehend the P-L-C interfaces. A more practical alternative would be to simulate the event with conditions representing its real-life application. For this purpose, the Nanovea tribometer is ideal. Thus, the Nanovea T2000 can closely mimic events occurring within an engine block and obtain valuable data to better understand the P-L-C interfaces. This is because it is equipped with multiple force sensors, depth sensor, a drop-by-drop lubricant module, and linear reciprocating stage.

Further, another crucial element for this study is the drop-by-drop module. Due to the fact that pistons can move at a very fast rate (above 3000 rpm), it is tough to create a thin film of lubricant by submerging the sample. However, it is possible to remedy this issue. The solution lies in applying a constant amount of lubricant onto the piston skirt surface, using the drop-by-drop module. Further, the application of fresh lubricant also frees from worry about dislodged wear contaminants influencing the lubricant’s properties.

Equipment Featured

NANOVEA T2000

  • Load Control, High Speed, and High Force
  • Dual Controlled Loads
  • Extreme Robust Design
  • Inline Line Sensor Capability
  • Friction, Wear, Scratch Testing

Measurement Objectives

This report will study the piston skirt-lubricant-cylinder liner interfaces. By conducting a linear reciprocating wear test with drop-by-drop lubricant module, the interfaces will be replicated. Further, the lubricant will be applied at room temperature and subsequently under heated conditions, to compare cold start and optimal operation conditions.

Sample of Piston on T2000

Sample of Piston on T2000.

Finally, the COF and wear rate will be observed for a better understanding of how the interfaces behave under real-life applications.

Measurement Parameters

Table 1: Test parameters for tribology testing on pistons.

Test Parameter Value
Load (N) 100
Test Duration (min) 30
Speed (rpm) 2000
Amplitude (mm) 10
Total Distance (m) 1200
Skirt Coating Material Moly-graphite
Pin Material Aluminum Alloy 5052
Pin Diameter (mm) 10
Lubricant Motor Oil (10W-30)
Approx. Flow Rate (mL/min) 60
Temperature Room Temperature and 90 °C

Samples

Image of Piston sample post-wear testing testing (boxed in blue).

Image of Piston sample post-wear testing testing (boxed in blue).

Linear Reciprocating Test Results

This experiment used the A5052 as the counter material. Unlike engine blocks, which are usually made of cast aluminum such as A356, the A5052 possesses mechanical properties similar to A356 in this simulative testing environment [2].

Table 1 displays the testing conditions, where signifi­cant wear was observed on the piston skirt at room temperature, as compared to at 90 °C. Further, as suggested by the deep scratches seen in Figure 1 and 2, throughout the test, there is frequent contact between the static material and the piston skirt.

Moreover, the high viscosity observed at room temperature may cause restrictions to the oil from completely fi­lling gaps at the interfaces and creating metal-metal contact. The oil thins at higher temperatures, and can flow between the pin and the piston. Thus, significantly less wear is noted at higher temperatures. According to Figure 5, one side of the wear scan wore signifi­cantly less than the opposite side. This could be due to the location of the oil output. Uneven wearing was caused because the lubricant fi­lm thickness was thicker on one side than the other.

Linear reciprocating tribology tests’ COF can be split into a high and low pass. On the one hand, high pass refers to the sample moving in the forward, or positive, direction. Conversely, low pass refers to the sample moving in the reverse, or negative, direction. For both directions, the average COF observed for the RT oil was observed to be under 0.1. Moreover, the average COF between passes was between 0.072 and 0.080.

In addition, the average COF of the 90 °C oil was observed to be different between passes. Average COF values of 0.167 and 0.09 were observed. Thus, this difference observed in COF provides additional proof that the oil only has the potential to properly wet one side of the pin.

Due to hydrodynamic lubrication, high COF was obtained when a thick fi­lm was formed between the pin and the piston skirt. A lower COF was observed in the opposite direction, under cases of mixed lubrication.

Table 2: Results from lubricated wear test on pistons

Oil Temperature Max COF
(High/Low)
Min COF
(High/Low)
Average COF
(High/Low)
Wear Rate
(m3/ (Nm)x107)
RT 0.229/0.106 0.053/0.045 0.072/0.080 36.77
90 °C 0.345/0.161 0.008/0.021 0.167/0.094 10.77

COF graphs for room temperature oil wear test A) raw profile B) high pass C) low pass.

Figure 2: COF graphs for room temperature oil wear test A) raw profile B) high pass C) low pass.

COF graphs for 90°C wear oil test A) raw profile B) high pass C) low pass.

Figure 3: COF graphs for 90 °C wear oil test A) raw profile B) high pass C) low pass.

Optical image of wear scar from RT motor oil wear test.

Figure 4: Optical image of wear scar from RT motor oil wear test.

Profilometry image of wear scar from RT motor oil wear test.

Figure 5: Profilometry image of wear scar from RT motor oil wear test.

Volume of a hole analysis of wear scar from RT motor oil wear test.

Figure 6: Volume of a hole analysis of wear scar from RT motor oil wear test.

Optical image of wear scar from 90 °C motor oil wear test.

Figure 7: Optical image of wear scar from 90 °C motor oil wear test.

Profilometry image of wear scar from 90 °C motor oil wear test.

Figure 8: Profilometry image of wear scar from 90 °C motor oil wear test.

Volume of a hole analysis of wear scar from 90 °C motor oil wear test.

Figure 9: Volume of a hole analysis of wear scar from 90 °C motor oil wear test.

Conclusion

In an effort to simulate events occurring in a real-life operational engine, lubricated linear reciprocating wear testing was conducted on a piston. As described above, the piston skirt- lubricant-cylinder liner interfaces are crucial in an engine’s operation. Further, the lubricant thickness at the interface can cause energy loss due to friction or wear between the piston skirt and cylinder liner.

In order to optimize the engine, the film must be as thin as possible, while the piston skirt and cylinder liner should never be allowed to touch. However, the challenge is how temperature, speed, and force changes will aff­ect P-L-C interfaces.

The Nanovea T2000 tribometer can simulate diff­erent conditions possible in an engine, owing to its wide range of loading (up to 2000N) and speed (up to 5000 rpm). Potentially, future studies in this field can include how the P-L-C interfaces will behave under di­fferent constant load, oscillated load, lubricant temperature, speed, and lubricant application method.

Using the Nanovea T2000 tribometer, these parameters can be easily adjusted to provide a complete understanding of the mechanisms of the piston skirt-lubricant-cylinder liner interfaces.

References

[1] Bai, Dongfang. Modeling piston skirt lubrication in internal combustion engines. Diss. Massachusetts Institute of Technology, 2012.

[2] “5052 Aluminum vs 356.0 Aluminum.” MakeItFrom.com, www.makeitfrom.com/compare/5052-O-Aluminum/A356.0-SG70B-A13560-Cast-Aluminum.

This information has been sourced, reviewed and adapted from materials provided by Nanovea.

For more information on this source, please visit Nanovea.

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