Development and application of tracer-based planar laser-induced fluorescence imaging diagnostics for HCCI engines [electronic resource]
Keywords: Mechanical Engineering

Homogeneous charge compression ignition (HCCI) is an emerging engine strategy that can provide both high efficiency and low emissions, particularly in terms of NOx and soot. An important challenge of HCCI is the inherent narrow load range, bounded by combustion instability and misfires at low-load, and high pressure rise- rate (PRR) at high-load. In response, researchers have devised a number of strategies to expand the limits of HCCI operation. Negative valve overlap (NVO) with pilot injection can extend the low-load gasoline HCCI operating limit by increasing sensible energy during main compression through hot residual gas retention. Chemical effects due to reformation of the pilot injection may further impact combustion. Similarly, the high-load limit can be extended by increasing naturally occurring thermal stratification (TS) of the in- cylinder charge. These non-uniformities result in sequential auto-ignition that can effectively lower the PRR and thus expand the high-load limit. While demonstrations of these strategies have been successful and multiple engine studies have been completed, further characterization of key processes such as residual gas mixing and TS development is needed. This motivates the development of quantitative imaging diagnostics to improve the understanding of these complicated processes. In this study, tracer-based planar laser-induced fluorescence (PLIF) diagnostics for temperature and composition have been refined and optimized for application in HCCI engines at both load extremes. Acetone and 3-pentanone (both ketones) have been selected as seeded PLIF tracers as they provide good overall sensitivity and performance. Single-line and two-line diagnostic variations have been investigated, with an emphasis on optimizing overall diagnostic performance through excitation wavelength selection. Based on a detailed uncertainty analysis excitation wavelengths of 277 nm and 308 nm were selected for subsequent studies. Resulting single-shot temperature precisions were typically on the order 4 K and 12 K for the single-line and two-line techniques respectively. The corresponding mole fraction precision for the two-line technique was typically 4-5%. These results are consistent with the uncertainty analysis and demonstrate the utility of the optimization. HCCI studies were performed in two optically accessible engines, each configured for a specific load extreme. Residual mixing for low-load HCCI operation with NVO was first studied using the two-line technique to provide the simultaneous temperature and composition distribution. These measurements indicated rapid mixing of retained residuals during gas exchange and early compression, reaching a steady-state value midway through compression. Temperature stratification gradually increased throughout the remainder of compression while compositional stratification effectively remained constant. Variation of operating parameters such as main and NVO injection timing exhibited minimal differences in thermal or compositional stratification during main compression. Measurement during NVO recompression and re-expansion were also acquired to assess the in-cylinder temperatures stratification prior to chemical reaction and gas exchange. Next the development of thermal stratification for high-load HCCI with conventional valve timing was investigated using the single-line technique. These studies indicated a progressive increase in TS during compression, reaching a maximum standard deviation of 10 K at top dead center. Comparison of results for motored and fired operation exhibited similar trends. This finding indicates that the mechanism producing the TS is the same for both cases, although some differences in magnitude can occur. A subsequent parametric study proved that these differences can be attributed to the impact of both incomplete fuel mixing and cylinder-wall temperature variation, depending on the type of engine operation (DI skipfiring or premixed continuous firing). All measurements demonstrate the feasibility of quantitative tracer-based PLIF diagnostics in harsh engine environments and provide useful information for future HCCI engine development.