Experimental and Numerical Low-Speed Pre-ignition Analysis and Mechanism Synthesis on a Turbocharged Gasoline Engine with Direct Injection

Abstract

The concept of downsizing is a successful approach to improve efficiency for passenger car spark-ignition (SI) engines. This leads to highly charged gasoline engines with direct injection and high specific power densities, promoting a combustion anomaly known as low-speed pre-ignition (LSPI). This unpredictable occurring and multi-cycle phenomenon is not yet fully understood and thus limits the achievable in-cylinder pressure and further efficiency gains. To achieve a comprehensive understanding of the entire PI initiating mechanism, thermodynamic and optical experiments were conducted, as well as accompanying numerical investigations. For this purpose, the influence of various engine parameters is investigated on the testbed in a modern three-cylinder gasoline engine. In particular, an increased spray/liner interaction was found to promote PI frequencies vigorously. Based on these results, optical characterization of the emerging premature ignitions is performed by utilizing a minimally invasive endoscopic high-speed imaging testbed setup. It could be observed that visibly glowing flying objects and deposits at specific regions initiate premature ignitions. The use of a computational fluid dynamics (CFD) engine model incorporating a multicomponent fuel surrogate revealed that those deposit-prone regions coincide with areas of strong fuel wall wetting on the liner and the piston crevice region. This observation could be validated experimentally by in-cylinder recordings of fluorescence tracer-doped fuel. Based on the optical results, a numerical study on the thermal behavior of detached inert particles over three consecutive combustion cycles is performed. A stochastic particle release from deposition hotspots could reveal that inert particles do not reach a sufficiently high surface temperature to initiate a premature ignition. Advanced ex-situ analysis of naturally and artificially generated combustion chamber deposits showed that inorganic substances originating from lubricating oil enhance oxidation reactivity and enable deposits to ignite the surrounding mixture. Through a final synthesis step of all results, a multidimensional reactive particle-driven LSPI mechanism is established.