Combined Effects of Engine and Oil Age on Low Speed Pre-Ignition 2019-01-0033
Low-speed pre-ignition (LSPI) is a well-established phenomenon that occurs in boosted, direct injection, spark ignition engines. The impact of lubricant formulation has arguably received the most attention, leading to the introduction of the Sequence IX test for LSPI as part of the API SN PLUS lubricant service category. This test, as with most other LSPI evaluations, considers the performance of the fresh oil. A handful of papers have started addressing the effect of both engine and oil age on LSPI during both test-stand and in-vehicle studies. The current paper adds to this body of knowledge by analyzing results from multiple test-bench installations of the GM LHU engine platform. For each engine, multiple tests, each comprising of multiple segments of a high-load, low-speed test point, known to amplify the occurrence of LSPI, were analyzed to investigate the combined effect of oil and engine age on LSPI activity. Three different baseline oils, which were repeated multiple times, as well as several candidate oils, were included in the data set. Since the baseline LSPI activities showed an overall downward trend attributed to engine aging, bracketing adjustments of candidate oil results were considered most appropriate. Once the individual segment results were corrected for engine age, there was no consistent, residual pattern of activity change due to oil aging. Since early LSPI segment activities were noticeably different for some tests, caution should be taken when selecting a test for statistically comparing means of tests oils. Selected oils were subject to extended LSPI tests, but no correlations could be found between LSPI activity and used oil analyses results.