Increased levels of internal or external exhaust gas recirculation (EGR) are one concept to improve efficiency of spark-ignition (SI) engines in part-load operation. This sub-project investigates cycle-to-cycle variations and their potential causes within SI engines with homogeneous air/fuel mixtures and substantial EGR rates at part-load operation. The aim is to gain a better understanding of the underlying processes and a well-documented comprehensive data set for the validation of mathematical models and numerical simulations. To achieve this goal, flow velocities, flame fronts and mixture formation will be characterized using advanced laser diagnostics.
Figure 1. Darmstadt engine test bench
An optically accessible SI engine with port fuel injection will be used for this investigation. External EGR (without water vapor) will be provided by a gas mixing unit and internal EGR by a variation of valve timing. For this investigation an engine test bench is available at the institute of Reactive Flows and Diagnostics at TU Darmstadt which was built-up for model validation. It provides a reproducible engine operation and well-characterized boundary conditions. The turbulent flow field will be characterized by particle image velocimetry (PIV) prior to and during early flame development. Flame propagation will be captured simultaneously by laser induced fluorescence (LIF) or Mie scattering from evaporating oil droplets. Figure 2 shows an example flow and flame imaging measurement acquired simultaneously in the Darmstadt engine .
Figure 2. Phase-averaged normalized flow (PTV) and flame front contours (LIF) acquired simultaneously near the piston wall in the Darmstadt engine. A (800 rpm) and C (1500 rpm) represent full-load engine operation (intake pressure of 0.95 bar) while B (800 rpm) and D (1500 rpm) represent part-load operation (intake pressure of 0.4 bar) relevant to this project.
In the case of internal EGR the mixture field will be characterized using tracer-LIF prior to ignition. An appropriate tracer will be added to the air/fuel mixture within the intake pipe for tracer-LIF. Major challenges are the quantification of the LIF signals as well as the multi-parameter measurements, which give access to the interactions of relevant parameters. Whenever possible, useful high-speed techniques will be applied to enable an analysis of the data in time. Cause-and-effect chains will be analyzed in order to identify causal relationships between mixture- and turbulence-structures with ignition as well as early flame development. In cooperation with TP 2 (RWTH Aachen University ITV) experimental and numerical data will be used for a backwards analysis (in time) to identify flow and mixture structures impacting the combustion process and causing cycle-to-cycle fluctuations.
- Schmidt, M., Ding, C., Peterson, B. et al. Near-Wall Flame and Flow Measurements in an Optically Accessible SI Engine. Flow Turbulence Combust (2020).