A DOE Approach to Engine Deposit Testing used to Optimize the Design of a Gasoline Direct Injector Seat and Orifice

A series of designed experiments (DOE) was used to optimize the seat and orifice designs in a multi stream gasoline direct injector. The goal of the experiments was to minimize the effects of fuel deposits on the injector performance. Two different engines were used in the test campaign. One engine, a centrally injected turbocharged 1.6L four cylinder, was used to run a three factor full factorial DOE that tested the effects of SAC volume design, tip design and combustion seal position. Another, a centrally injected turbocharged 3.0L six cylinder, was used to run a three factor full factorial and a four factor half factorial DOE. The three factors in the full factorial were orifice hole divergence, orifice hole surface finish and the use of an inert amorphous silicon coating. A fourth factor, hydro erosive grinding of the orifice holes, was added to facilitate the calculation of a four factor half factorial DOE with only four additional engine tests. The engines were tested on engine dynamometers using specially formulated fuels to accelerate deposit formation. The specific test cycle for each engine was determined using injector tip temperature data and included multiple engine operating points run in succession. One cycle also included a soak phase. It was found that the optimized configuration included a protruded tip, the orifice hole entrances outside the SAC volume close to the seal band, smooth, straight stepped holes (mechanically drilled in this case) without the use of hydro erosive grinding. Combustion seal position and the presence of this particular type of coating proved to be statistically insignificant. SEM photographs of the seats and orifice holes and were also used to aid in the understanding of the results.