**4. Friction reduction for engines - a practical example**

In the following, the potential for friction reduction in the journal bearings of the crank train shall be analysed for a modern four cylinder passenger car turbodiesel engine lubricated with common multi-grade oils using the isothermal method discussed in Sec. 3.1. In particular, Styrene-Isoprene-Copolymer (SICP)-additive enhanced oils are considered in the following (in terms of shear rate dependency of the lubricant). For all variants, the friction will be calculated as sum of all five crankshaft main bearings and four big end bearings at full-load operation with a peak cylinder pressure of 190 bar, which leads to specific bearing loads of up to about 50 MPa for the main bearings and to about 90 MPa for the big end bearings. Further, the dynamic oil supply for the big end bearings is realistically represented in the simulation as oil supply network.

friction losses, widths of 21mm and 16mm are studied in addition to the original width of 18mm. Further, the original lubricant-grade 10W40 is reduced to 0W30 and for a specific case to 0W20. Two different oil temperatures are considered in the simulation because in everyday life not only warm engine conditions exist, but also low oil temperatures occur specifically with low exterior temperatures or for short driving distances. For the warm engine operation

Friction in Automotive Engines 181

**Figure 26.** Friction power losses in the journal bearings at various speeds for different bearing shell

Fig. 26 shows the summary of all results evaluated through the total friction power losses for all journal bearings at different speeds. Most notable from the results is that the oil temperature impacts the friction performance most intensely which is taken advantage of by many modern engines through a reduction of the oil volume in the oil sump and the related quicker warming-up of the engine. In particular, the friction power losses are cut in half throughout the entire speed range by increasing the lubricant temperature from 40◦C to 100◦C. Furthermore, it is shown that the different oil viscosities have a much stronger effect than the changes in bearing width. Referring to the example a savings potential is found to be in average 35% for the cold and still 20% for the hot case through changing from 10W40 to 0W30. In contrast, using more narrow bearings leads to a reduction of the losses by 9% for the cold case and a high speeds, but yields only a reduction of max 3% of the total journal bearing losses for the studied hot lubricant temperature. This minimal impact can be employed to use even lower viscosity oil for the engine and - while the load carrying capacity by utilizing wider bearings needs to be restored for this lubricant - a net reduction of friction power losses can be achieved. Fig. 26 also shows how the presented method assists in identifying potential issues of mixed lubrication: for the case of a 16mm wide bearing and lubrication with 0W30 there

widths, lubricant viscosity grades and operating temperatures.

100◦C and for the cool operation 40◦C oil temperature is considered.

**Figure 25.** Plot of a part of the inline four cylinder engine for which the calculations are carried out; it shows the locations of the main and big end bearings which are shown with examplary oil film pressure distributions shown as 3D-plot.

## **4.1. Finding a friction optimized solution**

In the following basic example, easily modifiable parameters such as bearing shell width and viscosity grade (SAE-class) of the engine oil are in the focus. The savings potential derived from the reduction of the bearing shell width is set with the reduction of the oil-filled volume and the use of low viscosity oils directly influences the viscosity losses. Both measures reduce the load capacity of the bearings. Therefore, it is crucial to identify occurring mixed lubrication in order to find a low friction solution which does not impair the bearing lifetime through emerging mixed lubrication. To illustrate the influence of the bearing shell width on the friction losses, widths of 21mm and 16mm are studied in addition to the original width of 18mm. Further, the original lubricant-grade 10W40 is reduced to 0W30 and for a specific case to 0W20. Two different oil temperatures are considered in the simulation because in everyday life not only warm engine conditions exist, but also low oil temperatures occur specifically with low exterior temperatures or for short driving distances. For the warm engine operation 100◦C and for the cool operation 40◦C oil temperature is considered.

32 Will-be-set-by-IN-TECH

In the following, the potential for friction reduction in the journal bearings of the crank train shall be analysed for a modern four cylinder passenger car turbodiesel engine lubricated with common multi-grade oils using the isothermal method discussed in Sec. 3.1. In particular, Styrene-Isoprene-Copolymer (SICP)-additive enhanced oils are considered in the following (in terms of shear rate dependency of the lubricant). For all variants, the friction will be calculated as sum of all five crankshaft main bearings and four big end bearings at full-load operation with a peak cylinder pressure of 190 bar, which leads to specific bearing loads of up to about 50 MPa for the main bearings and to about 90 MPa for the big end bearings. Further, the dynamic oil supply for the big end bearings is realistically represented in the simulation

**Figure 25.** Plot of a part of the inline four cylinder engine for which the calculations are carried out; it shows the locations of the main and big end bearings which are shown with examplary oil film pressure

In the following basic example, easily modifiable parameters such as bearing shell width and viscosity grade (SAE-class) of the engine oil are in the focus. The savings potential derived from the reduction of the bearing shell width is set with the reduction of the oil-filled volume and the use of low viscosity oils directly influences the viscosity losses. Both measures reduce the load capacity of the bearings. Therefore, it is crucial to identify occurring mixed lubrication in order to find a low friction solution which does not impair the bearing lifetime through emerging mixed lubrication. To illustrate the influence of the bearing shell width on the

**4. Friction reduction for engines - a practical example**

as oil supply network.

distributions shown as 3D-plot.

**4.1. Finding a friction optimized solution**

**Figure 26.** Friction power losses in the journal bearings at various speeds for different bearing shell widths, lubricant viscosity grades and operating temperatures.

Fig. 26 shows the summary of all results evaluated through the total friction power losses for all journal bearings at different speeds. Most notable from the results is that the oil temperature impacts the friction performance most intensely which is taken advantage of by many modern engines through a reduction of the oil volume in the oil sump and the related quicker warming-up of the engine. In particular, the friction power losses are cut in half throughout the entire speed range by increasing the lubricant temperature from 40◦C to 100◦C. Furthermore, it is shown that the different oil viscosities have a much stronger effect than the changes in bearing width. Referring to the example a savings potential is found to be in average 35% for the cold and still 20% for the hot case through changing from 10W40 to 0W30. In contrast, using more narrow bearings leads to a reduction of the losses by 9% for the cold case and a high speeds, but yields only a reduction of max 3% of the total journal bearing losses for the studied hot lubricant temperature. This minimal impact can be employed to use even lower viscosity oil for the engine and - while the load carrying capacity by utilizing wider bearings needs to be restored for this lubricant - a net reduction of friction power losses can be achieved. Fig. 26 also shows how the presented method assists in identifying potential issues of mixed lubrication: for the case of a 16mm wide bearing and lubrication with 0W30 there occurs for a lubricant temperature of 100◦C already significant metal-metal contact at 2000rpm which leads to a significant rise in friction for this engine and potentially to problems in the operating reliability. However, with an enlarged bearing width even lower viscosity oil can be used; for the case presented the optimum is a low viscosity 0W20 oil combined with a broader bearing shell, in this case 21mm. Thereby, in comparison to the original configuration with 18mm bearings and 10W40 oil, the journal bearing losses can be reduced by 10% at 2000rpm and by approximately 30% at 4600rpm despite the significantly wider bearing shells.

**5. References**

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insights from IEA indicator analysis.

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Friction in Automotive Engines 183

[2] Allmaier, H., Priestner, C., Reich, F., Priebsch, H., Forstner, C. & Novotny-Farkas, F. [2012b]. Predicting friction reliably and accurately in journal bearings - the importance

[3] Allmaier, H., Priestner, C., Six, C., Priebsch, H., Forstner, C. & Novotny-Farkas, F. [2011]. Predicting friction reliably and accurately in journal bearings–a systematic validation of simulation results with experimental measurements, *Tribology International*

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[7] Fatu, A., Hajjam, M. & Bonneau, D. [2006]. A new model of thermoelastohydrodynamic lubrication in dynamically loaded journal bearings, *Journal of tribology* 128: 85. [8] Fontaras, G. & Samaras, Z. [2010]. On the way to 130g CO2/km–estimating the future characteristics of the average european passenger car, *Energy Policy* 38(4): 1826–1833. [9] Greenwood, J. & Williamson, J. [1966]. Contact of nominally flat surfaces, *Proceedings of the Royal Society of London. Series A, Mathematical and Physical Sciences* 295: 300–319. [10] Hall, D. & Moreland, J. [2001]. Fundamentals of rolling resistance, *Rubber chemistry and*

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## **4.2. Conclusion**

The results show that small changes in the bearing geometry bear no significant impact on the friction losses in the journal bearings. However, the use of a low viscosity lubricant holds obvious advantages in regards to a reduction of these losses, despite the need of wider bearings to retain the bearing load capacity. In the presented example this combination of low viscosity lubricants with wider bearings revealed itself as optimal and proves approximately 10-30% decreased losses in comparison to the initial situation. Alternatively, if more complex in design, the increase in size of the journal bearing diameter and the therefore necessary larger journal diameter brings advantages also in regards to the NVH performance due to the increased stiffness of the crankshaft. Further measures for friction reduction like an on-demand oil supply could potentially also attain significant savings and be analysed through the presented model.

While this basic example of friction reduction in engines displays the efficiency of various measures, it is important to emphasise that the choice of the optimum lubricant affects the whole engine and the other major source of mechanical losses, namely the piston assembly, challenges with (partly) opposing requirements to the lubricant. In this sense, the optimum choice of the lubricant in terms of friction reduction shall only be taken under consideration of the complete system.
