Internal combustion engine heat transfer that occurs between hot combustion gases and the cooler walls is expected to be of primary importance in several advanced engine concepts aimed at increasing fuel economy and reducing pollutant formation. The heat transfer rates are ultimately determined by boundary layer-type flow along the in-cylinder walls. The underlying physical mechanisms within these boundary layers are still not well understood. A paucity of detailed measurements of gas velocity and temperature distributions adjacent to the walls has hindered the development of models that could be used for design purposes.
Intellectual Merit: This research will generate detailed experimental information regarding the velocity and temperature distributions within highly-transient boundary layers that occur within internal combustion engines. Velocity data will be acquired using a micro-particle image velocimetry technique with a high repetition rate laser source. Temperature distributions will be determined using laser induced fluorescence. Ensemble-averaged velocity and temperature data will be determined, as will two-point, auto-, and cross-correlation of parameters over a relatively wide range of operating parameters for transparent engine tests in both the motored and fired modes. Wall heat flux data will be simultaneously acquired. The data will be used by collaborators to develop and validate computational models of this complex and highly transient phenomena.
Broader Impacts: The ability to understand and predict wall heat transfer in internal combustion engines is a key element necessary to reduce pollutant formation and increase fuel efficiency in future engine designs. This research will involve graduate student training, including an international experience for the student. Undergraduate students will also work with collaborators at the Technical University of Darmstadt. The graduate student will participate in various initiatives aimed at preparing women for careers in engineering. Broad dissemination to non-traditional audiences is also planned.
The transfer of heat across the cylinder walls in an internal combustion engine is an energy loss mechanism that, if better understood, could be suppressed with targeted developments. In addition, in future engine concepts, undesired heat transfer might even affect the performance of the combustion process directly. A big impediment in making progress in this context is the poor fundamental understanding of how energy and mass are exchanged in the thin layer of gas – the boundary layer - that is in direct contact with the cylinder wall on one side and with the bulk gases (fuel and air that will burn) on the other side. Directly at the wall, the gases are at a complete rest; towards the inner volume of the cylinder the motion of the gases that will dictate how heat will move follows a complex evolution throughout the phases of an engine cycle. It is the reciprocating nature of internal combustion engine operation and the continuously changing pressure and temperature of the gases that make the flow motion near the surface in the cylinder so complicated. Models that have been successful in describing the structure of boundary layers in other systems fail in applications to internal combustion engines. Progress towards a better understanding of boundary layer flows in engines has been limited for quite some time now because the physical and chemical processes near the surfaces could not be studied accurately enough. This is because the experimental tools that are needed to measure images of velocities and temperatures with the required spatial and temporal resolution were not available. Therefore, improved models could not be developed based on sufficient and direct experimental evidence of the physical processes in the boundary layers. This project has contributed substantially towards overcoming these shortcomings by advancing several laser diagnostic measurement techniques for velocity and temperature imaging under the highly transient conditions in internal combustion engines. Not only do these techniques allow measurements at micrometer resolution they also provide several thousand images per second and therefore allow tracking of flow details with unprecedented resolution. The results of these experiments will now guide the development of new physics-based models that will eventually enable the reliable prediction of the heat losses in operating engines. Used as design tools they will help to design better engines with more reliable, cleaner, and more fuel-efficient operation. The project provided broad training opportunities for students from high school ages to postdoctoral fellows. Furthermore, research activities were integrated in a collaborative research partnership between the University of Michigan and TU Darmstadt, Germany. This allowed several graduate students to go overseas and conduct research in the partner institution's laboratories for up to six months at a time.
