Experimental investigation of plateau honed cylinder bore tribological characteristics

Decreasing in fossil fuel reserves and increasing in environmental pollution day by day cause to fluctation in oil prices and also tightening up on emission regulations. These situations lead to whole automotive industry and researchers to design internal combustion engines which are more efficient and environmental friendly. These design studies split into division which are namely increasing thermal efficiency and reducing the frictional losses in general. The notable shareholders of friction in an internal combustion engine are mainly piston rings, piston, bearings and valve train. Among them the piston ring package and power cylinder interaction has share approximately 50% of total friction due to high mechanical and thermal loads and also highly corrosive chemical environment. Up to today, there are numerious researches have been done to reduce these frictional losses in this region. The fundamental strategies which have been chased in these studies are improving the design of components, formulation of lube oils, lube oil filtration and circulation, coatings on surfaces of components and changing the surface roughness values or surface texture patterns. This brief summary of strategies is important to understand the studies that have been done so far in literature. The last two strategies are about surface texturing. Surface texturing term was adapted to internal combustion engine manufacturing process in 1940s with honing of diesel engine cylinders due to prevent to scuffing failure. Since then, the honing process is an inseperable step of the engine manufacturing. Honing is a final machining operation of cylinder bores that increases the diameter of liner to its exact dimension with fixing cylindricality and gives the desired surface profile and roughness. Honing tool (stone) which includes small abrasive particles with chemical binder makes rotational and reciprocating motion inside the cylinder bore and engrave the surface with crosshatch pattern. Surface roughness is able to be adjusted within the desired values with choosing the proper honing tool grits and honing time. Surface crosshatch pattern geometry is also able to be adjusted at desired shape with choosing the rotational and reciprocating speeds proper. Honing process generally includes 3 steps. The first step is called as "rough honing" in which the cylinder liner is machined to its designed dimension. The following step is "finish honing" in which the crosshatched surface pattern is machined. The third and final step is "plateau honing" in which the final surface roughness is formed. This step gives the whole process its own name "plateau honing". This plateau honing process is used almost every engine cylinder liner today. Plateau honed surfaces are characterized with surface peaks and valleys when the surface roughness is main issue. There are many experimental and numerical researches about this topic. Peaks are the contact points of the surfaces in mixed and boundary lubrication regimes. Therefore, wear properties are affected negatively with their increasing heights due to abrasive wear mechanism. On the contrary, the surface peaks enhance the load bearing capacity at their specific values. Low peak heights are not only affect negatively to load bearing capacity but also lead to possibility of scuffing failure. Besides these, surface valleys have crucial role on lubrication. They act as lube oil reservoir and feed the contact region. The shallow valleys lead to change the lubrication regime from hydrodinamic to mixed or boundary sooner than expected. But, they have also a potential to decrease the lube oil consumption. Excessive deep valleys have effects on hydrodynamic pressure development and oil consumption. According to this literature knowledge it is obviously seen that a surface roughness optimization is needed with varying temperature, speed and load conditions in an engine. Plateau honing marks the cylinder liner with its idiosyncratic crosshatch pattern. This crosshatch pattern has been tried to explain with geometrical parameters such as angle, width, depth/width ratio, aspect ratio, surface density etc. by researchers. These parameters have been examined according to their tribological performance in both experimental and numerical way. This thesis study aims to understand the plateau honing tribological performance and to testify the test rig's reliability and measurement accuracy. It also contains the manufacturing of the specimens that have different honing angles as the first step of the planned forward study. It is planned to focusing on the effects of honing angle on tribological performance afterwards the test bench testifying. Honing angle is an important parameter that is able to change the lubrication regime individually. It is stated in almost every study which focuses on this parameter that decreasing honing angle leads to increase in oil film thickness and reduce the friction. The reason behind this tendency is local pressure growing due to flow blockage hence the local viscosity increasing leads to hydrodynamic regime. However, excessive oil film thickness cause increasing in oil consumption potential. Contrary to this, increasing honing angle is better for oil consumption. But this time, the friction performance drastically reduces and scuffing risk gains importance. In literature, there are numerous experimental and numerical study about this topic. Researchers still endeavour to enlight the theory behind the effects of geometrical parameters on tribological performance and optimize them at varying running conditions. In this experimental study, a reciprocating high speed and engine like stroke bench test is used. The bench test is capable to measure the coefficient of friction and oil film thickness continuously and simultaneously by means of its reciprocating cylinder liner and stationary piston ring configuration. The friction coefficient is measured via full bridge Wheatstone configurated strain-gage. Laser induced fluorescence method is used for oil film thickness measurements. A 445 nm wavelength laser source is directed to oil and fluorescent dye mixture via fiber optic setup and the 495 nm wavelength emitted beam is collected and directed to photomultipliers via the same setup. Laser induced fluorescence signal is collected from 2 different channels to enhance measurement accuracy. The system calibration is done with a special manufactured cylinder liner specimen while the bench test is running. The specimen has 6 dimples which width and depth values are measured on its surface. The LIF signal is calibrated according to known depths. A plateau honed cylinder specimen is used in experiments. A large number of experiments are performed with varying lube oil temperature, engine speed and normal load to understand the characteristics of plateau honing texture in different lubrication regimes and to testify the test bench accuracy. The friction coefficient and oil film thickness signals are collected and saved according to crank angles. In this study, three different cast iron liner samples are manufactured. The cylinder liner specimens have different honing angles (30, 45, 60°) and close surface roughness. The experiment results are overlapped with theory and the test bench's accuracy and reliability are testified. Increasing in speed leads to increasing in film thickness and decreasing in friction coefficient. On the contrary, increasing in lube oil temperature and load separately cause to decrease in oil film thickness hence the friction coefficient tends to increase. In experiments, squeeze film effect is observed in the dead centers. It is realised that the piston ring movement affects the tribological performance. The experiment results can be used in numerical model verification. This thesis provides a large and qualitative data to literature about plateau honed liner surface tribological performance under various working conditions. Continuous oil film thickness measurements in an engine like bench test enable to examine and verify the variety of designs with respect to tribological performance.