Flame/flow dynamics at the piston surface of an IC engine measured by high-speed PLIF and PTV

TL;DR

This study uses high-speed PLIF and PTV to investigate the complex interaction between flame propagation and near-wall flow dynamics on the piston surface of an internal combustion engine, revealing transient boundary layer behaviors crucial for engine efficiency.

Contribution

It provides the first detailed high-speed measurements of simultaneous flame and flow dynamics at the piston surface in a running engine, highlighting the transient nature of boundary layer development during combustion.

Findings

Flow acceleration occurs ahead of the flame front.

Boundary layer thickness varies significantly between fired and motored cycles.

Transient flow behavior influences boundary layer development and combustion efficiency.

Abstract

Resolving fluid transport at engine surfaces is required to predict transient heat loss, which is becoming increasingly important for the development of high-efficiency internal combustion engines (ICE). The limited number of available investigations have focused on non-reacting flows near engine surfaces, while this work focuses on the near-wall flow field dynamics in response to a propagating flame front. Flow-field and flame distributions were measured simultaneously at kHz repetition rates using particle tracking velocimetry (PTV) and planar laser induced fluorescence (PLIF) of sulfur dioxide (SO2). Measurements were performed near the piston surface of an optically accessible engine operating at 800 rpm with homogeneous, stoichiometric isooctane-air mixtures. High-speed measurements reveal a strong interdependency between near-wall flow and flame development which also influences subsequent combustion. A conditional analysis is performed to analyze flame/flow dynamics at the piston surface for cycles with weak and strong flow velocities parallel to the surface. Faster flame propagation associated with higher velocities before ignition demonstrates a stronger flow acceleration ahead of the flame. Flow acceleration associated with an advancing flame front is a transient feature that strongly influences boundary layer development. The distance from the wall to 75% maximum velocity ({\delta}75) is analyzed to compare boundary layer development between fired and motored datasets. Decreases in {\delta}75 are strongly related to flow acceleration produced by an approaching flame front. Measurements reveal strong deviations of the boundary layer flow between fired and motored datasets, emphasizing the need to consider transient flow behavior when modelling boundary layer physics for reacting flows.