Analysis of cycle-to-cycle variations in IC engines using highspeed Tomographic Particle-Image Velocimetry

• Applicant: Dr.-Ing. Michael Klaas
• Subject Area: Fluid Mechanics
• Term: from 2017 to 2020
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 327875125

• Final Report Year: 2020

Final Report Abstract

To achieve the strict legislative restrictions for emissions from combustion engines, vast improvements in engine emissions and efficiency are required. Two major impacting factors for emissions and efficiency are the reliable generation of an effective mixture before ignition and a fast, stable combustion process. While the mixture of air and injected fuel is generated by highly three-dimensional, time-dependent flow phenomena during the intake and compression stroke, the turbulent flame propagation is directly affected by the turbulence level in the flow close to the advancing flame front. However, the flow field in the combustion chamber is highly turbulent and subject to cycle-to-cycle variations (CCV). To understand the fundamental mechanisms and interactions, high-speed tomographic particle-image velocimetry (HS-TPIV) measurements were conducted in an optical engine. The results have been validated against and supplemented by high-speed stereoscopic particle-image velocimetry measurements (HS-SPIV). An extensive error estimation and validation procedure proved the quality of the HS-TPIV data. The spatial and temporal resolution of the HS-TPIV setup were later improved by using graphite microspheres, which allowed to increase the sampling frequency due to the better light scattering behavior of the microspheres. With this improved setup it was possible to resolve the smallest integral length scale of the engine flow. The velocity fields from the PIV measurements were fed into the newly implemented triple-velocity decomposition algorithm, which allows to distinguish velocity fluctuations resulting from CCV from velocity fluctuations resulting from turbulence. The study focused on the investigation of the three-dimensional flow topology and development with special attention paid to the large-scale flow structure, i.e., the tumble vortex. Furthermore, the impact of increasing intake pressure was investigated. Therefore, the velocity fields were further processed to determine the tumble-vortex core and its path during one engine cycle, the vorticity of the flow field, and the kinetic fluctuation energies. With the triple-velocity decomposition it was possible to determine the contributions of CCV and turbulence to the total kinetic fluctuation energy and to detect an energy transfer from one to the other. The results showed that, although some engine cycles behave similar to the ensemble-averaged flow, a significant extent of fluctuations between instantaneous engine cycles was observed in the present internal combustion engine (ICE). Especially the break-up process of the tumble vortex into small-scale turbulences at the end of the compression stroke had extremely opposed tendencies. The results showed that flow fluctuations inside IC engines can have a significant three-dimensional extent and that flow topologies can change severely within several degrees in crank angle. Furthermore, fluctuations due to CCV can be significantly higher than pure turbulent fluctuations. The observed CCV were mainly caused by variabilities in the tumble vortex location, and in the magnitude and orientation of the intake jets. An increase in intake pressure, at least in the analyzed range, has an insignificant, almost negligible impact on the global scale of the in-cylinder flow, i.e., on the mean level of energy or vorticity. Nonetheless, distinct local effects of the intake pressure have been investigated. The velocity distribution in the combustion chamber was affected, and the vorticity in the cylinder was slightly redistributed. Furthermore, the tumble vortex core showed a well-rounded and more circular path through the combustion chamber and its low velocity core region increased in size, although no significant global intensity increase was found in the tumble vortex. The results further indicate a reduction of cyclic variations for higher intake pressure during the early intake stroke. The impact of boosting on the plane-averaged kinetic energy terms was investigated based on the velocity parts of the triple-velocity decomposition. The comparison of the two intake pressures, namely 100 kPa and 150 kPa, showed that despite an increase of the intake pressure by 50%, only a slight effect on the kinetic energy could be identified. However, in the first half of the intake stroke, different levels of CCV were investigated and a variation in the interaction dynamics of the intake jets and the tumble vortex was assumed. Overall, it was shown that the triple-velocity decomposition method yields useful results for the extensive discussion of cyclic variability of the in-cylinder engine flow. However, conclusions should be drawn with caution during the very early intake or late compression stroke.

Publications

• “Applying High-Speed Tomographic PIV in an IC Engine”. In: 19th International Symposium on Applications of Laser and Iage Techniques to Fluid Mechanics, 16.-19. July 2018, Lisbon, Portugal. 2018
  M. Braun, W. Schröder, and M. Klaas
  
• “High-speed 3D Measurements of the Influence of Intake Pressure on the In-Cylinder Flow in a DISI Engine”. In: SAE Technical Paper 2019-01-2251 (2019)
  M. Braun, M. Klaas, and W. Schröder
  
• “High-Speed Tomographic PIV Measurements in a DISI Engine”. In: Exp Fluids 60.9 (2019), p. 146
  M. Braun, W. Schröder, and M. Klaas
  
• “Inverstiagtion of teh 3D Flow in a Cumbustion Engine usig High-Speed Tomo-PIV”. in: 13th International Symposium on Particle-Image Velocimetry - ISPIV 2019, 22.-24. July 2019, Munich, Germany. 2019
  M. Braun, W. Schröder, and M. Klaas
  
• “Analysis of Cyclic Variation using Time-Resolved Tomographic Particle-Image Velocimetry”. In: SAE Technical Paper 2020-01-2021 (2020)
  M. Braun, M. Klaas, and W. Schröder