The influence of start-stop transient velocity on the friction and wear behaviour of a hyper-eutectic Al-Si automotive alloy

As worldwide automotive ownership is set to exceed 2 billion vehicles by 2030 [1], environmental and economical pressures on the automotive industry mean there is a trend to reduce harmful greenhouse gas emissions whilst simultaneously improving fuel economy. The use of electrical hybrid, start-stop idling and materials light-weighting are some of the technological solutions which can offer efficiency savings, however their combined use in a tribological context is less well understood. Interruption of piston sliding as a result of start-stop technology will simultaneously interrupt fluid film lubrication at the ring/liner interface. This could have implications on the friction and wear behaviour of Al-Si cylinder blocks, where scuffing can be an issue. In order to determine the effect of a start-stop velocity cycles on the lubricated friction and wear behaviour of a hyper-eutectic Al-Si liner, an interrupted reciprocating laboratory tribometer test programme was developed based upon the European Urban Cycle standard. Refined cast iron piston ring segments were slid at 23Hz frequency, 4MPa nominal contact pressure and 25mm stroke length against a conformal honed Alusil cylinder liner segment. Regular velocity interruptions at 60s intervals did not significantly inhibit the dynamic friction behaviour between the liner and the cast-iron piston ring segment, indicating that lubricant additive function was not significantly inhibited. However contact potential and FIB-SIMS depth profiles indicated that antiwear tribo-film thickness was reduced as a result of start-stop cycling. Mass loss from the piston ring was also notably higher as a result of the interruption of elasto-hydrodynamic lubrication causing boundary conditions at re-start and subsequent 2-body abrasion by harder protruding Si particles. Specific wear rates for the Al-Si liner as a result of start-stop velocities were surprisingly lower compared to uninterrupted tests and was believed to be due to faster running-in of the piston ring surface. These results are discussed in terms of the viability of Al-Si as engine materials running start-stop technology.