Investigation Of Piston Geometry In Rapid Compression Machines And Sampling Methods For Internal Combustion Engines
2019-07
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Investigation Of Piston Geometry In Rapid Compression Machines And Sampling Methods For Internal Combustion Engines
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2019-07
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There is a growing effort to reduce carbon dioxide (CO2) emissions produced by internal combustion (IC) engines as an effort to curb anthropogenic climate change. The transportation sector accounts for 28% of anthropogenic CO2, motivating fundamental combustion research to understand and develop more efficient advanced combustion modes. Study of ignition delay time, autoignition pressure and temperature, the chemistry of fuel mixtures, and speciation of combustion products provide important insights into phenomena like pre-ignition (knock) and pollutants (CO2, oxides of nitrogen, soot, etc.) from modern-day IC engines. This body of work investigates novel speciation methods for studying combustion products from IC engines and unique piston geometries for rapid compression machines (RCMs). Quantifying combustion products is an important step in creating accurate numerical models for engine combustion. Many groups have used various instruments in conjunction to characterize a range of combustion generated hydrocarbons but few have used instruments in tandem to improve speciation methods during unconventional combustion modes and address the issues associated with off-line speciation. The first part of this thesis presents an investigation that quantified light unburned hydrocarbons (UHC) using a combination of Fourier transform infrared (FT-IR) spectroscopy and gas chromatography-mass spectroscopy (GC-MS). A light-duty diesel engine is used to generate hydrocarbons at various exhaust gas recirculation (EGR) levels and partially premixed low-temperature combustion (LTC) modes. Exhaust samples are extracted with a novel fixed-volume sampling system and sent into a gas chromatograph (GC) while minimizing unknown dilution, light unburned hydrocarbons (LHC) losses, and removing heavy unburned hydrocarbons (HHC). Along with the wide range of LHCs quantified in this study, focus is directed towards the problem of misidentification of propane by the FT-IR during LTC modes. In the region commonly identified as the absorption spectra of propane (2700 and 3100 cm-1), analysis of the FT-IR spectra indicates absorption band interference caused by components found in unburnt diesel fuel. One of the primary findings of this work is that GC-MS can aid in FT-IR spectral analysis to further refine FT-IR methods for real-time measurement of unconventional combustion mode exhaust species. Rapid compression machines (RCMs) and rapid compression and expansion machines (RCEMs) are apparatuses that have the ability to operate at engine-relevant conditions to study fuel autoignition and pollutant formation. These machines are currently limited for use in speciation studies due to thermal and mixture inhomogeneities caused by heat transfer and gas motion during compression. Studies have shown the disadvantages of using common flat and enlarged piston crevice designs for sampling reaction chamber gases during and after combustion. For instance, computer fluid dynamics (CFD) simulations performed by numerous groups, including collaborators on this work, have confirmed that unburnt fuel mixture emerges from the enlarged crevice after compression then subsequently mixes with reaction chamber gases during RCM and RCEM operation. This disadvantage renders whole-cylinder sampling techniques inaccurate for quantifying combustion products and reduces the relevance of RCMs and RCEMs for comparison with IC engines. Complex fast-sampling systems are implemented by a number of research groups to extract small quantities of gas from the center of the chamber before mixing occurs. Drawbacks with this approach include small sample volumes, local composition non-uniformities, and non-uniform progression of chemical kinetics during sampling. Experimental and computational studies emphasize the importance of piston design for the formation of a well-mixed, homogeneous core gas inside RCM and RCEM reaction chambers. In the second part of this thesis, a novel piston containing a bowl-like geometry similar to those used in diesel engines is implemented to overcome thermal and compositional non-uniformities within RCMs/RCEMs. By eliminating the enlarged crevice and introducing squish flow with the bowl piston, CFD studies show increased thermal uniformity for both RCM and RCEM trajectories. Experiments to characterize piston performance includes flat, enlarged crevice, and bowl piston profiles and four fuel mixtures using the University of Minnesota – Twin Cities controlled trajectory RCEM (CT-RCEM). Heat release analysis (HRA) indicates greater combustion efficiencies when using the bowl piston opposed to the standard flat and enlarged creviced pistons. This is indicative of smaller fractions of unburnt fuel left in the combustion chamber after combustion, ideal for dump sampling and the differentiation of unburnt fuel from combustion products during speciation. Ignition analysis for the bowl piston derived stronger ignition characteristics than the enlarged crevice and flat piston designs. As a result of stronger ignition and better uniform burning, the amount of fuel converted to products of combustion is increased.
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University of Minnesota Ph.D. dissertation. July 2019. Major: Mechanical Engineering. Advisor: William Northrop. 1 computer file (PDF); xvii, 114 pages.
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Dasrath, Dereck. (2019). Investigation Of Piston Geometry In Rapid Compression Machines And Sampling Methods For Internal Combustion Engines. Retrieved from the University Digital Conservancy, https://hdl.handle.net/11299/206676.
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