Investigations on the Influence of Fuel Oil Interaction on Preignition Events in highly boosted DI Gasoline Engines

Paper #:
  • 2018-01-1454

  • 2018-04-03
Premature and uncontrolled flame initiation, called preignition (PI), is a prominent issue in the development of spark-ignited engines. It is commonly assumed that this abnormal combustion mode hinders progress in engine downsizing, thus inhibiting development of more efficient engines. The phenomenon is primarily observed in highly turbocharged spark ignited (SI) engines in the full load regime at low engine speeds. Subsequent engine knock induces extremely high peak pressures, potentially causing severe engine damage. The mechanisms leading to this phenomenon are not completely understood. It seems to be common sense that a multiphase process is responsible for the preignition. One effect could be the interaction between injected fuel droplets and the oil film at the cylinder liner. Under certain conditions droplets of oil or oil fuel mixture can be detached leading to a preignition at the droplet surface towards the end of the compression phase. To gain further understanding of the possible mechanisms and to assess observed preignition events it is important to know, under which conditions splash effects can cause droplet detachment. In this paper pre-ignition events in a 2.0 liter 4-cylinder production engine are analyzed regarding the different boundary conditions of their occurrence. Variation parameters to support or prevent splash conditions are injection timing and pressure (fuel impingement on liner), liner temperature, boiling curves of the fuel, lubricating oil viscosity. The conditions leading to increased preignition rates are compared to a generic droplet wall film interaction experiment to evaluate whether splash affects are a likely cause or not. Therefore the impact of a single drop onto a wetted wall using different liquids is investigated. A large parameter range is obtained by varying drop diameter, impact velocity, film thickness and fluid combinations of drop and wall film. Finally, a dimensionless number K* is defined in order to describe the splashing threshold. Typical K* numbers for various operating conditions of the engine are then computed and compared with results from generic experiments.
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