TY - THES

T1 - Tribological characterization of valvetrain ball/socket-contacts based on a tribometric model testing system

AU - Denzel, Michael

N1 - embargoed until 19-06-2024

PY - 2019

Y1 - 2019

N2 - The demand for higher efficiency and less emissions in combustion engines requires the use of low-viscosity engine oils in order to reduce friction losses. In addition, certain oil additives must be reduced to meet novel environmental regulations. This leads to increased wear on engine components which were uncritical in the past. Especially at ball/socket-contacts in valvetrains, wear is critical because it affects the valve clearance and consequently the gas exchange, which can lead to efficiency losses, increase in emissions and engine damage. Currently, engine tests with a run time of several days are used to investigate the tribological performance of valvetrain components. To reduce costs and testing duration, a model testing system is demanded. Therefore, a test setup on a rotary tribometer was designed, where various ball and socket specimens cut out from engine valvetrain components can be tested within one day. With the test setup's modular structure it is possible to vary different specimens, their assembly positions, lubrication systems and motion. A suitable test program to simulate the running conditions was developed, which is based on a start-stop cycle, and an appropriate data evaluation was programmed. The results of an accompanying optical analysis and gravimetric wear measurements are compared with engine tests. Changing the axis of rotation due to machine settings requires an adjustment of speed and load to match the same mild abrasive wear mechanism as observed in engine parts. As tests with fresh oil did not cause significant wear to make meaningful statements in terms of oil effectiveness, an artificial oil aging process was performed by adding abrasive particles or a soot surrogate to fresh oil. It was shown that the geometric difference in sockets has a big influence on reproducibility as it supports the build-up of a potential load-bearing oil film. Tests with used engine oil proved that, with this test setup, causing significant wear in a moderate time is possible. Furthermore, it was shown that, depending on their concentration, abrasive particles increase the coefficient of friction, which can lead to seizure, but do not increase mild abrasive wear in the investigated components. Adding a soot surrogate significantly increases wear. According to literature, this is due to chemical interaction of the oil additive zinc dialkyldithiophoshpate with soot. For the purpose of validation, tests with various oils with well-known engine performance were conducted. It was shown that an oil with low wear in the engine also shows low wear in model tests using artificial oil aging. Consequently, the developed test procedure provides a good tool for gaining deeper understanding and tribosystem-screening of ball/socket-contacts.

AB - The demand for higher efficiency and less emissions in combustion engines requires the use of low-viscosity engine oils in order to reduce friction losses. In addition, certain oil additives must be reduced to meet novel environmental regulations. This leads to increased wear on engine components which were uncritical in the past. Especially at ball/socket-contacts in valvetrains, wear is critical because it affects the valve clearance and consequently the gas exchange, which can lead to efficiency losses, increase in emissions and engine damage. Currently, engine tests with a run time of several days are used to investigate the tribological performance of valvetrain components. To reduce costs and testing duration, a model testing system is demanded. Therefore, a test setup on a rotary tribometer was designed, where various ball and socket specimens cut out from engine valvetrain components can be tested within one day. With the test setup's modular structure it is possible to vary different specimens, their assembly positions, lubrication systems and motion. A suitable test program to simulate the running conditions was developed, which is based on a start-stop cycle, and an appropriate data evaluation was programmed. The results of an accompanying optical analysis and gravimetric wear measurements are compared with engine tests. Changing the axis of rotation due to machine settings requires an adjustment of speed and load to match the same mild abrasive wear mechanism as observed in engine parts. As tests with fresh oil did not cause significant wear to make meaningful statements in terms of oil effectiveness, an artificial oil aging process was performed by adding abrasive particles or a soot surrogate to fresh oil. It was shown that the geometric difference in sockets has a big influence on reproducibility as it supports the build-up of a potential load-bearing oil film. Tests with used engine oil proved that, with this test setup, causing significant wear in a moderate time is possible. Furthermore, it was shown that, depending on their concentration, abrasive particles increase the coefficient of friction, which can lead to seizure, but do not increase mild abrasive wear in the investigated components. Adding a soot surrogate significantly increases wear. According to literature, this is due to chemical interaction of the oil additive zinc dialkyldithiophoshpate with soot. For the purpose of validation, tests with various oils with well-known engine performance were conducted. It was shown that an oil with low wear in the engine also shows low wear in model tests using artificial oil aging. Consequently, the developed test procedure provides a good tool for gaining deeper understanding and tribosystem-screening of ball/socket-contacts.

KW - Ball

KW - Socket

KW - Valvetrain

KW - Tribometric model testing

KW - Artificial oil aging

KW - Abrasive particles

KW - Soot

KW - Kugelgelenk

KW - Ventiltrieb

KW - Tribometrisches Modellprüfsystem

KW - Künstliche Ölalterung

KW - Abrasivpartikel

KW - Ruß

M3 - Master's Thesis

ER -