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 pre-ignition in a probabilistic approach. The effect of residuals and in-cylinder 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).
Bibliographical noteKAUST Repository Item: Exported on 2020-10-01
Acknowledgements: The project is part of FUELCOM II. The authors would like to acknowledge the funding agency: Saudi Aramco for financial assistance. The authors would also like to thank Adrian Ichim, for his technical support in conducting experiments.