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

摘要

As worldwide automotive ownership is set to exceed 2 billion vehicles by 2030 (Dargay et al., 2007 [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 23 Hz frequency, 4 MPa nominal contact pressure and 25 mm stroke length against a conformal honed Alusil cylinder liner segment. Regular velocity interruptions at 60 s 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 anti-wear 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.