title: Experimental Investigations of Near-Wall Processes in an Optically Accessible Spark-Ignition Engine
Creators: name: Schmidt, Marius
Publisher: name: TUprints
Year: 2024

description: Within this dissertation, near-wall processes in an optically accessible direct-injection spark-ignition engine are investigated experimentally. These processes affect the thermodynamic efficiency and pollutant emission, specifically through heat transfer, fuel wall film formation, turbulent transport and mixing, flame-wall interaction, and pollutant formation. Because reducing greenhouse gas and pollutant emissions is imperative in the context of climate change mitigation and compliance with future regulations, a comprehensive understanding of these interacting processes is required. 

In this work, experimental investigations are performed using a combination of minimally invasive optical diagnostics in a well-characterized engine test bench. Thermographic phosphors are applied at several locations in the combustion chamber to measure wall temperatures for motored and fired operation. During fired operation, temperatures increase steadily, with the greatest rise occurring in the first 50 cycles. The flame-wall interaction in the crevice, into which the flame is pushed during combustion, is investigated for the first time. Furthermore, heat fluxes are estimated from the time-resolved surface temperatures. Peak heat fluxes during fired operation are up to 15 MWm−2 on the metallic piston side and approximately 1 MWm−2 on the piston and cylinder glass. 

The velocity boundary layer above the piston is resolved down to the viscous sublayer with high-resolution particle tracking velocimetry. At an engine speed of 2500 rpm, the viscous sublayer is as thin as 30 µm. Furthermore, there is a strong overlap of the inner and outer layer and velocity profiles do not adhere to the logarithmic law. However, in both velocity magnitude and fluctuations, similarities to impinging wall jets are observed. Conditional averaging reveals that the degree of boundary layer development depends on the horizontal location and instantaneous flow mode. 

Emission formation in the context of wall films is studied with simultaneous diagnostics combining velocimetry, laser-induced fluorescence of acetone, and flame as well as soot visualizations. Therefore, a fuel wall film is deliberately created by a late single-hole injection of acetone. The resulting soot luminosity above the piston (commonly called pool fire) is characterized and attributed to flame-free pyrolysis of fuel-rich zones in the hot burnt gas. A strong sensitivity of vaporization and consequently soot formation processes to the wall temperature is observed. Measured acetone mole fractions in the bulk flow are on average below 2 %, which indicates that soot formation is limited to sufficiently high fuel-air ratios in the direct wall vicinity. Thereby, cycle-to-cycle variations of soot luminosity are high. Strong correlations between the velocity and mixing field, and soot luminosity are found. Conditional statistics reveal the influence of the flow on the flame propagation. It is hypothesized that combustion-induced convection of soot nests along the colder temperature boundary layer is responsible for reduced soot luminosity in some cycles. 

The presented results provide novel insights into relevant near-wall phenomena in an engine and extend the data available for validation and the boundary conditions for numerical simulations.