Browsing by Subject "Rapid Compression Machine"

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    Design, Control, and Characterization of a Controlled Trajectory Rapid Compression and Expansion Machine (CT-RCEM)
    (2019-08) Tripathi, Abhinav
    This thesis presents the design, control and characterization of a novel experimental facility for fundamental and applied combustion investigations – a controlled trajectory rapid compression and expansion machine (CT-RCEM). Rapid compression machine (RCM) has been a popular experimental facility used for the investigation of combustion characteristics of fuels in low to intermediate temperature ranges. The CT-RCEM, developed in this research, addresses a key limitation of the conventional RCM, i.e. an open-loop and calibration-based actuation philosophy. The CT-RCEM uses an electrohydraulic actuator driven by a precise motion controller to drive the piston in the combustion chamber. Any changes in the operating parameters can thus be made by electronically changing the piston trajectory sent to the controller, unlike the conventional RCM which requires hardware intervention. This allows the CT-RCEM to provide ultimate flexibility in the choice of operating parameters, a wider operating range with higher resolution, lower turnaround time, and exceptional run-to-run repeatability. The key novelty of CT-RCEM, however, lies in the new paradigm of experimental investigation enabled by the ability to tailor the thermodynamic path inside the combustion chamber by suitable choice of piston trajectory. Specific examples include, the ability to investigate the effect of changing the thermodynamic path of compression on ignition delay, the ability to quench the chemical kinetics in the combustion chamber by extremely rapid expansion, and, the ability to produce isobaric conditions inside combustion chamber by slow creeping of the piston at a rate which offsets the rate of wall heat loss. In this research, first, a control oriented dynamic model of the CT-RCEM is developed. The model serves three purposes for the development of the CT-RCEM – (i) to understand the impact of various design parameters of the CT-RCEM on its performance and tuning them to obtain a suitable mechanical design; (ii) to design a model based high bandwidth controller that can provide precise tracking performance for the piston motion (iii) to guide the design of the various subsystems of the CT-RCEM. Next, a model based, iterative learning control (ILC) scheme is implemented for the control of the actuation system of the CT-RCEM. Since the choice of the initial control signal for the first iteration has a significant impact on the number of iterations required for ILC convergence, the initial signal for the ILC is generated through simulation, from the dynamic model of the CT-RCEM, which uses a repetitive controller. This is followed by the characterization of the CT-RCEM which essentially involves demonstrating that the facility can provide the desired functionality – fast compression and repeatable pressure history – over the designed operating range for both non-reactive and reactive mixtures. Also, the new capabilities of CT-RCEM enabled by the ability to tailor the thermodynamic path are demonstrated. Next, the utility of the CT-RCEM is demonstrated for applied engine research where a single combustion event of an engine can be recreated with well controlled initial and boundary conditions. It is demonstrated that the CT-RCEM can be used to perform a benchmarking study of the combustion characteristics of a realistic free piston engine operation and its effect on the piston trajectory by presenting a case study. Finally, a multi-zone thermo-kinetic model is developed for computationally efficient analysis of the experimental data obtained from the CT-RCEM. A reaction path analysis performed using this model is used to explain a highly counter-intuitive experimental observation made using the CT-RCEM – a faster compression does not necessarily lead to smaller pre-ignition reaction progress during compression and (consequently) a longer ignition delay. It is shown that the degree of pre-ignition reaction progress and consequently the radical pool at the end of compression can be quite sensitive to the thermodynamic path of compression, not just the end of compression thermodynamic state.
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    Investigation Of Piston Geometry In Rapid Compression Machines And Sampling Methods For Internal Combustion Engines
    (2019-07) Dasrath, Dereck
    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.