Cause-and-effect chain from flow and spray to heat release during lean gasoline combustion operation using conditional statistics

Abstract

This work comprises of experimental and numerical investigations of the volumetric flow in a direct injection spark-ignition engine to analyse the origin of cycle-to-cycle variations during stratified engine operation. High-speed two-dimensional two-component particle image velocimetry measurements are carried out simultaneously in the central tumble and mid-intake valve plane of an optically accessible engine, to capture the three-dimensional characteristic of the in-cylinder flow. Early investigation showed spray formation of stratified operation to be sensitive to cyclic fluctuations of the flow in a specific region below the spark within the central plane prior the first injection. Conditional statistics are used to track the origin of these variations back to the tumble flow in the valve plane during early compression underlining the three-dimensional structure of the flow. Moreover, conditional statistics reveals that the combustion performance is sensitive to the same specific flow region. According to this, computational fluid dynamics simulations are used to describe the in-cylinder flow with a map of four dominant flow structures. A new intake port geometry is derived from computational fluid dynamics simulations, optimized for high tumble generation. High-speed two-dimensional two-component particle image velocimetry in the central tumble plane is utilized to compare the optimized intake port with the original geometry. The new intake ports yield a considerable increased tumble movement with a favourable combustion performance and less cycle-to-cycle variation.