Subproject C01 Böhm

Subproject C01 Böhm

Experimental characterization of in-cylinder near-wall flow and combustion processes

Motivation

Flame-wall interactions play a crucial role in technically relevant enclosed combustion systems as in internal combustion engines. Heat losses over the engine walls substantially impact efficiencies and emissions. For safe engine operation the resulting thermal loads on the engine components need to be limited, this requires reliable thermal management. The current trend towards smaller engine displacement volumes together with increasing energy densities leads additionally to increasing surface-to-volume-ratios. Hence, near-wall phenomena are of increasing interest, but not comprehensively understood or described.

Objectives

Measurements of the in-cylinder core flow are commonly performed within IC engines while measurements in the vicinity of walls are rare. This subproject aims for an experimental characterization of these flame-wall interactions within a spark-ignition engine. This includes the near-wall flow and combustion processes as well as their coupling with the in-cylinder core flow. The aim is to gain an understanding of the relevant physical processes and to provide data for the validation of engine simulations performed within the numerical subprojects of this initiative.

While in the recent years one-phase-flow and homogeneous operation were in focus, the complexity is increased in the second funding period with the addition of a fluid phase. Direct injection is used to investigate wall wetting and mixture inhomogeneities near the walls.

SpaciPro technology _Model monolith and operating principal of the measurement
Flame-wall interaction in internal combustion engines.

Previous Findings

High resolution optical techniques have been used to resolve the flow, including the small-scale boundary layer, and to analyze the interaction between the flow, flame propagation, and piston wall surface. These measurements reveal that strong deviations between the flows of the motored and fired operation cases as well as between common models exist. In addition it was proven that the flame influences the near-wall flow development.

Flame-wall interaction in internal combustion engines
PTV-visualization of the near-wall boundary flow in high resolution (coin for comparison).

Approach

The experiments are performed within an experimental spray-guided, direct-injection, spark-ignition engine with a centrally mounted injector. The engine is optically accessible through a quartz glass window in the piston and a quartz glass liner. The entire engine test bench was especially designed for validation purposes. This includes a simplified geometry, comprehensively characterized boundary conditions and a reproducible engine operation. The test bench is equipped with various temperature and pressure measurement systems to control the boundary conditions and for high-resolution in-cylinder pressure measurements. These quantities are recorded and available for all experiments.

The engine characterization builds upon the acquired data from motored engine operation without fuel injection and fired operation with a homogeneous mixture. The complexity is now successively increased by adding inhomogeneous mixtures. Investigating these mixing processes and the following heat release is facilitated by implementing a Spray-G injector for direct injection and furthermore a single-hole injector for targeted wall wetting. A comprehensive database already exists for the in-cylinder flow which is used by several groups worldwide for model validation within the context of the “Darmstadt Engine Workshop.” This database is successively extended by engine operation conditions including combustion and mixture preparation.

Current Work

Current objectives include the continued characterization of the in-cylinder flow with high-speed and high-resolution diagnostics. Furthermore the spray formation, spray-wall as well as spray-flow interaction will be captured. Next to wall temperature measurements additional gas phase temperature measurements will be applied and complemented by emission measurements. Combining these methods in simultaneous measurements enables correlated analysis of these complex processes.

Cooperations

The main goal of this subproject is to improve the general knowledge of these near wall processes. In addition, these measurements also aid the modelling of internal combustion engine flows (B03, C03) by providing indispensable validation data. Laser absorption spectroscopy for emission diagnostics, which are developed in A05 in close collaboration with this subproject, and are applied to the engine. Synergy effects related to the usage of experimental equipment and improvement or adaption of optical measurement methods arise with other subprojects (A04, A06, C02).

Visualization of the injection-spray; pool-fire on the piston wall surface.
Visualization of the injection-spray; pool-fire on the piston wall surface.

Selected Publications