In modern heavy duty diesel engines, the valve system plays an important part. The intake valves open to let air flow in to the combustion chamber, the combustion takes place and then the exhaust valves open the let the combustion products flow out of the chamber. This is repeated as the engine drives the vehicle forward and at all other time, the valves must be closed to avoid any leakage of the combustion chamber which would reduce the power output. This process takes place at high temperature, high chamber pressure and high frequency and the wear rate of the valve surfaces must be extremely low to allow them to operate satisfactory during the life-time of the engine.
In today’s engines, a tribofilm is formed from elements from the engine oil, which lowers the wear rate and thus gives the valves the wanted long life-time. However, little is known about the mechanisms of how this tribofilm is formed and how it behaves to lower the wear rate. When analyzing field samples, it appears as if additive elements from the oil have been trapped between the valve and valve seat insert surfaces and then as the valve closes, is smeared out into a smooth layer protecting the metal surfaces. The question is how fast this process takes place and if the tribofilm can function without continuous addition of new tribofilm forming elements.
In this work, the build-up phase of these tribofilm has been studied by running real valves and valve seat inserts in an in-house rig which allows the addition of oil into a hot air stream which passes the opening and closing valve. The rig allows stopping a test at any point, taking the valve and valve seat insert out to analyze the surfaces and then re-starting the test. To see the build-up phase of the tribofilm, tests have been run for 10; 100; 1000 and 10000 cycles with the addition of engine oil. Also, test have been continued after 1000 and 10000 cycles, without the addition of engine oil, for 1000 cycles to see if the tribofilm can sustain the wear rate without the addition of new tribofilm forming elements.
The results will be presented and discussed regarding their effect on future work to develop the valve system as they will see less and less tribofilm forming elements due to harder legislation on exhaust limits—e.g. the euro class which is implemented in Europe—which may reduce the amount of additives that can be put into future engine oils.
The 16th Nordic Symposium on Tribology - NORDTRIB 2014, Aarhus, June 10-13, 2014