Mechanism Triggering Pre-Ignition in a Turbo-Charged Engine

Abstract

Pre-ignition in modern engines is largely attributed to oil-fuel mixture droplets igniting before the spark timing. Researchers have also found pre-ignition events to be triggered by high hydrocarbon emissions from the previous cycle as well as late spark timing in the previous cycle. Additionally, an ideally scavenged engine was not found to be limited by pre-ignition. These observations point to a significant role of residuals in triggering pre-ignition events. Current work studies preignition in a probabilistic approach. The effect of residuals and incylinder thermodynamic state is studied by varying the exhaust back pressure and intake air temperature respectively. Experiments were performed with a fixed mass flow rate of air + fuel and intake air temperature while the exhaust back pressure was varied. Intake air pressure varied in response to fixed intake temperature. Pre-ignition and super-knock count increased with increasing exhaust back pressure. In the next set of experiments, mass flow rate of air + fuel and intake air pressure were fixed, while the exhaust back pressure was varied. Intake air temperature was varied to fix the intake air pressure constant. Pre-ignition counts generally increased with increasing intake temperature, although the exhaust back pressure decreased. Number of super-knock cycles correlated directly with intake air temperature. Conclusively, the current study shows that probability of a pre-ignition event relies on (a) the likelihood of precursor generation (from fuel impinging the liner), (b) the likelihood of precursors being held back in cylinder (related to exhaust back pressure) and (c) the reactivity of bulk mixture (related to in-cylinder temperature).

Pre-ignition Detection Followed by Immediate Damage Mitigation in a Spark-Ignited Engine

“…This idea gained support from experiments that varied the amount of residuals in the engine by varying the exhaust backpressure, and found pre-ignition frequency correlate with mass of residual fraction. The pre-ignition frequency followed the residual mass fraction in such experiments, as well [25]. Other researchers have shown that time-resolved detection of unusually high hydrocarbon emissions during the exhaust stroke indicates unusually high hydrocarbon content in the residual gases that may trigger a preignition event in the next cycle [19,26,27].…”

On the Relevance of Octane Sensitivity in Heavily Downsized Spark-Ignited Engines

“…A modern engine architecture based on AVL 5401 Pre-ignition engine is used for building a GT-Power model. The AVL 5401 engine has been used by the researcher in previous work and details can be found in [27,28]. The GT-Power model is used to simulate the engine with intake pressure ranging from 1 bar (abs) to 4 bar (abs).…”

Effect of Different Fluids on Injection Strategies to Suppress Pre-Ignition

“…Increasing exhaust pressure increases the residual mass, which may carry pre-ignition precursors. Hence, increasing exhaust back pressure (more representative of a real engine) could yield more preignition events [49]. However, when comparing different strategies, a constant backpressure needs to be applied and fixed for all cases (zero in the current study).…”

“…Pre-ignition occurrence depends on the probability of activated precursors surviving the exhaust scavenging process (residuals for next cycle) and their ignition in the presence of an ignitable atmosphere [49]. Injecting water in the late exhaust stroke should reduce the probability of the former from occurring (should quench the precursors) while injecting water in the intake and compression strokes would ensure that the latter is reduced (charge cooling effect).…”