A study of exhaust valve and seat insert wear depending on cycle numbers

Abstract

This study investigated the wear of the seating faces of the exhaust valve and seat insert that have influence on an engine's performance depending on mileage (cycle numbers). Exhaust valves and seat inserts that are commonly used for commercial vehicles were used as test specimens. The cycle numbers for tests were 2 × 10 6, 4 × 10 6, 6 × 10 6 and 8 × 10 6, and the test speeds were 10 and 25 Hz. All other parameters such as temperature (350 °C), fuel (LPG) and load (1960 N) were fixed. The 10 Hz tests indicated that the average maximum roughness (Rmax) of the valve increased at the rate of 7.76 μm/10 6 cycles starting from 29.42 μm at the 2 × 10 6 cycles, and that the average Rmax of the seat insert increased at the rate of 8.57 μm/10 6 cycles starting from 34.19 μm at the 2 × 10 6 cycles. The 25 Hz tests indicated that the average Rmax of the valve increased at the rate of 1.58 μm/10 6 cycles starting from 74.2 μm at the 2 × 10 6 cycles, and that the average Rmax of the seat insert increased at the rate of 1.25 μm/10 6 cycles starting from 83.95 μm at the 2 × 10 6 cycles. The tribochemical reaction product covered the base metals of the valve and seat insert, preventing the wear of the base metals, and included O, V, S and Al, which are the compositions of the two base metals. The tribochemical reaction product observed on the valve had cracks, and it was also noticed that the seat inserts were laid one upon another. As cycle numbers became greater, the Rmax of the seating faces of the valve and seat insert also increased, and the wear rate rose significantly depending on the Hz (RPM). The wear mechanism of the valve and seat insert was investigated through the tribochemical reaction.