Investigations on the Influence of Fuel Oil Film Interaction on Pre-ignition Events in Highly Boosted DI Gasoline Engines 2018-01-1454

Premature and uncontrolled flame initiation, called pre-ignition (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; however, it is quite plausible that a multiphase process is responsible for the pre-ignition. One effect could be the interaction between injected fuel drops and the oil film on the cylinder liner. Under certain conditions, droplets of oil or oil/fuel mixture can detach or splash from the film, leading to pre-ignition at the droplet surface towards the end of the compression phase.

To gain further understanding of the possible mechanisms leading to pre-ignition events it is important to know under which conditions splash effects on the film can cause droplet detachment. In this paper pre-ignition events in a 2.0 liter 4-cylinder production engine are analyzed regarding the different operating conditions of their occurrence. Parameters effecting splash conditions are injection timing and pressure (fuel impingement on liner), liner temperature, boiling curves of the fuel and lubricating oil viscosity. Conditions leading to increased pre-ignition rates are compared to a generic drop/wall film interaction experiment to evaluate whether splash events are a likely cause or not. 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 drop impact experiments.