論文

© 2019 SAE Japan and SAE International. Ignition by Nanosecond Repetitively Pulsed Discharges (NRPD) at EXponential Increase of Minimum Ignition Energy (MIE-EXI) region under super lean SI engine conditions was studied. Fundamental experiments were conducted with a turbulent ignition test chamber with twin counter-rotating fans. The MIE-EXI region by arc discharge appeared over 6500 rpm of fan speed. In the MIE-EXI region (7000 rpm), successful ignition was achieved by establishing coupled ignition kernels with NRPD at 15 kHz although ignition was unsuccessful at 1 kHz. Results show that ignition by NRPD has potential advantages for lean burn applications. Preliminary engine test results with NRPD were also demonstrated.

© 2019 SAE Japan and SAE International. Lean burn is one method for improving thermal efficiency in spark ignition (SI) engines. Suppression of knocking provides higher thermal efficiency, and ethanol blending is considered an effective way to suppress knocking due to its high octane and high latent heat of evaporation. We investigate the effect of ethanol blending on knocking in an SI engine under lean operating conditions. The Livengood-Wu (LW) integral was performed based on ignition delay duration estimated from a zero-dimensional detailed chemical reaction calculation with pressure and temperature histories. Knocking was suppressed and thermal efficiency increased with ethanol-gasoline blending fuel, even at 0.5 equivalence ratio. Decrease in unburned gas temperature by latent heat of evaporation had a comparable influence on knocking suppression, which was supported by LW integral analysis.

© 2017 Elsevier B.V. Phosphor thermometry has been viewed as a promising method for non-contact temperature measurement. This article explores the temperature dependence of the Dy-doped phosphors: YAG: Dy, Al2O3:Dy, and Y2SiO5:Dy. The phosphors were energized at 355 nm from a 3rd-harmonic-generation Nd:YAG laser. The intensity of YAG:Dy was much stronger than the other phosphors, and the I458 nm/I497 nm intensity ratios had high temperature resolution over a wide range of temperatures. Although Al2O3:Dy and Y2SiO5:Dy had weaker intensity, the temperature resolution of intensity ratios was as substantial as YAG:Dy. The uncertainty in measured temperature of Dy-doped phosphors was evaluated using linearized error propagation theory, based on the noise-to-signal ratio and temperature sensitivity, revealing that the measurement uncertainty of YAG:Dy was less than the other two phosphors, and thus was the most reliable thermographic phosphor. In addition, the optimization of the bandpass filter combination was explored at each temperature range with YAG:Dy phosphor. Consequently, each temperature can be precisely measured by dividing the integrated intensity between 441 and 475 nm as a counter peak range and 476–500 nm as a main peak range. Thus, ideal bandpass filters are relatively constant throughout all temperatures.

The estimation of hydrogen permeation mole flux was performed by previously introduced theoretical equation which considers the decrease in H-2 concentration at the membrane surface, and was compared with experimental result, for various hydrogen mixtures with different inlet H-2 mole fractions and species. A flat sheet 77 wt.% Pd/23 wt.% Ag purification membrane with effective surface area of 3.14 x 10(-4) m(2) was used. For H-2:N-2 mixture, the comparison was performed under various feed flow rates (2.78 x 10(-5)-2.50 x 10(-4) mol/s), for inlet H-2 mole fraction of 0.70-0.80, at reference membrane temperature of 623 K and total upstream pressure of 0.25 MPa. Then, nitrogen was subsequently replaced with argon (Ar), helium (He) and carbon dioxide (CO2) to investigate the applicability of the estimation method for various H-2 mixtures with different species. The results demonstrate that the effect of H-2 permeation itself plays a significant role for permeation with H-2 mixture, which induces the phenomenon of concentration polarization. Overall, the introduced method is able to estimate H-2 permeation mole flux for H2 mixture, both qualitatively and quantitatively, regardless of inlet H-2 mole fraction and species. (C) 2015 Elsevier B.V. All rights reserved.

© 2019, Springer-Verlag GmbH Germany, part of Springer Nature. Abstract: The dynamics of a laminar premixed stagnating flame subjected to simultaneous velocity and equivalence ratio oscillations are investigated experimentally. A flame of premixed methane and air is formed in a wall-stagnating flow. The oscillations of velocity and equivalence ratio are imposed independently by two sets of dual cylinder–piston oscillators. To impose the equivalence ratio oscillation, methane/air mixtures with equivalence ratios of ϕ= 1.0 and 0.4 are supplied to different cylinder–piston units. The pistons move in anti-phase and create a sinusoidal equivalence ratio oscillation while keeping the volume flow rate constant. To impose the velocity oscillation, a mixture with ϕ= 0.7 is supplied to the single unit of another oscillator. The phase difference between the two oscillations is set by changing the relative position of the pulley teeth of the two oscillators. The oscillator frequency is varied between 2 and 20 Hz, meaning that the oscillation wavelengths are much longer than the flame thickness. When frequencies of the velocity and equivalence ratio fluctuations are in a quasi-steady state condition, the flame motion is affected independently by velocity oscillation and equivalence ratio oscillation and the linear-superposition assumption can be applied. When the velocity and equivalence ratio oscillation frequencies are increased and exceed the transition frequency from the quasi-steady state to unsteady state, the flame motion gradually deviates from linear-superposition assumption. This is due to the variations in the unsteady molecular diffusion and the back-support effect.

© 2020 by Takeshi Shoji, Yuki Iwasaki, Kato Kodai, Seiji Yoshida, Shigeru Tachibana, and Takeshi Yokomori. This experimental study explored the influence of global/local flame behaviors on direct combustion noise produced by a lean-premixed gaseous H2 ∕air low-swirl turbulent jet flame, with a focus on the mechanism resulting in characteristic peaks in combustion noise spectra. Ten kilohertz OH chemiluminescence and OH planar laser-induced fluorescence (OH-PLIF) imaging were used to study the spatiotemporal evolution of heat release and flame structure fluctuations, respectively, whereas 2-D particle image velocimetry (2-D PIV) measurements at 4 Hz were applied to study the mean-based relation between the velocity/vorticity fields and flame structures. Pressure and global heat release fluctuation measurements carried out alongside these optical diagnostics revealed pronounced double peaks in both combustion noise and global heat release fluctuation spectra at global equivalence ratios of ϕ ≥ 0.45. Spectral proper orthogonal decomposition of the OH chemiluminescence and OH-PLIF images revealed that the first peak in the noise spectra to be caused by flame oscillations over nearly the entire flame region, whereas the secondary peak was attributed to periodically generated vortical flame structures near the downstream side of the flame boundary. The 2-D PIV results suggest that vortical flame structures are likely generated by the interaction between the flame and the inner/outer shear layers.

In a direct-injection (DI) engine, the in-cylinder flow is complex. Fluctuations of DI and in-cylinder flow, and the relationships between those fluctuations, are considered to affect the cycle-to-cycle variation (CCV) of combustion. However, neither the relationship between the injection conditions and the CCV of the in-cylinder flow nor the effect of the in-cylinder tumble flow on the spray has been clarified in detail. In this study, a single-cylinder optical engine was used to perform high spatial resolution particle image velocimetry (PIV) measurements and to photograph spray behavior throughout the cylinder. The spray timing was changed to investigate the timing's effects on the in-cylinder flow, the CCV, and tumble vortex center behavior. The penetration length was calculated from the spray images and compared with the PIV measurement results. The PIV results showed that the behavior of the bulk flow in the cylinder, the position of the center of the tumble vortex, and the CCV of the velocity changed depending on the presence or absence of DI and the injection timing. The overall trend was that the shorter the time elapsed after injection, the stronger the effect on the in-cylinder flow remained. The results of spray imaging revealed that the ambient flow velocity affected the direct spray at the time of injection when the flow velocity was high in the spray direction and that the spray extended faster. In addition to the advantage of the longer diffusion time, the fast in-cylinder flow in the same direction as the direct jet did not cause much momentum exchange with the spray, while making it easier for the spray to diffuse throughout.

Improving the thermal efficiency of spark ignition (SI) engine is strongly required due to its widespread use but considerably less efficiency than that of compression ignition (CI) engine. Although lean SI engine operation can offer substantial improvements of the thermal efficiency relative to that of traditional stoichiometric SI operation, the cycle-to-cycle variations of combustion increases with the level of air dilution, and becomes unacceptable. To improve the stability of lean operation, this study examines the effects of spark discharge pattern and tumble level on cycle-to-cycle variations of combustion at lean limits. The spark discharge pattern was altered by a custom inductive ignition system using ten spark coils and the tumble level was increased by a custom adapter installed in the intake port (tumble adapter). The results show that increased discharge energy by ten spark coils (without discharge offset) extends the lean limits with an improvement of indicated thermal efficiency (th), relative to that of stoichiometric operation. Furthermore, a combination of increased discharge energy and higher tumble level by tumble adapter makes it possible to allow stable operation at more extended lean limit, which is mainly attributed to shortening the duration of ST-to-CA5 and CA10-to-CA90. To examine the effect of spark discharge pattern on cycle-to-cycle variations of combustion at lean limits, the spark discharge pattern was altered for optimized combination of discharge energy and duration by adjusting the time offset between five discharges. The discharge offset of 0.25ms enables most stable ultra-lean operation, with COV of IMEPn less than 5% up to =2.0.

© IMechE 2020. Reducing the cycle-to-cycle variation in an engine is a key to expanding the operational limits of high-efficiency engines such as lean-burn engines. The cycle-to-cycle variation of the engine output is caused by unstable combustion and is especially induced by the cycle-to-cycle variation of the in-cylinder flow. This investigation aims to clarify how the cycle-to-cycle variation of in-cylinder flow develops during intake and compression strokes. To achieve this aim, 12 kHz time-resolved particle image velocimetry measurements were conducted on the symmetrical vertical plane in a four-valve, four-stroke optically accessible engine. This study focuses on the relationship between the flow at the plug position at the ignition timing and the flow distribution in the intake and compression stroke. Correlation map analysis reveals this relationship, and some flow distributions are extracted. The fluctuations in the flow distribution induce the cycle-to-cycle variation of the flow direction at the plug position at the ignition timing, the position of the stagnation plane between the intake flow from the intake valves and the upper side of the tumble vortex, the strength of the blowback flow into the intake port generated by the Miller cycle, and the position of the tumble vortex in the late compression stroke. The flow distributions extracted by the correlation map analysis clearly change between cycles. These results also suggest that correlation map analysis is an effective way to elucidate the factors that induce the cycle-to-cycle variation of the in-cylinder flow.