Analysis of the Impact of Production Lubricant Composition and Fuel Dilution on Stochastic Pre-Ignition in Turbocharged, Direct-Injection Gasoline Engines 2019-01-0256

The occurrence of abnormal combustion events leading to high peak pressures and severe knock can be considered to be one of the main challenges for modern turbocharged, direct-injected gasoline engines. These abnormal combustion events have been referred to as Stochastic Pre-Ignition (SPI) or Low-Speed Pre-Ignition (LSPI). The events are characterized by an undesired, early start of combustion of the cylinder charge which occurs before or in parallel to the intended flame kernel development from the spark plug. Early SPI events can subsequently lead to violent auto-ignitions that are often referred to as Mega- or Super-Knock. These heavy knock events lead to strong pressure oscillations which can destroy production engines within a few occurrences. SPI occurs mainly at low engine speed and high engine load, thus limiting the engine operating area that is in particular important to achieve good drivability in downsized engines. Recent experimental SPI studies have linked this phenomenon strongly to engine oils.

While numerous studies have been published using target blended oils, the presented study focuses on the impact of lubricants with production level formulations on SPI event occurrence. The utilized test engine is a production, turbocharged, direct-injection gasoline (GTDI) engine with a homogeneous common rail high pressure injection system and side mounted multi-hole injectors. All experiments were conducted on a steady state engine test bench with intake air, coolant, oil and fuel conditioning. The engine was equipped with a prototype engine controller that allowed negating the influence of vehicle, knock mitigation or balancing algorithms on combustion properties. 11 market relevant production engine oils for sale in the US in 2017 were analyzed regarding their composition using Inductively Coupled Plasma Mass Spectrometry (ICP-MS). All 11 oils were then tested for their impact on SPI occurrence, severity and characteristics. The experimental results were subsequently correlated to the oil analysis results. In addition, one oil was analyzed regarding the impact of fuel dilution and ageing on SPI occurrence using the same test setup.