Statistical study on engine knock oscillation and heat release using multiple spark plugs and pressure sensors

Knock research is an essential study because knock limits spark-ignition (SI) engine power output, durability, noise, fuel consumption, and emission performance. The thermal efficiency of an engine can be improved by increasing the compression ratio, but at the same time it leads to the possibility of end gas autoignition inside the chamber. The objective of this work was to show the effect of three different compression ratios (CRs) 8.5, 9.5, and 10.5 on the mechanism of knock. In order to achieve this four equispaced spark plugs were fitted around the circumference of a special metal liner to initiate flame fronts from those plugs by firing with different spark approaches (e.g. spark timing, number, and location). In addition, six pressure sensors were installed at various locations to precisely record the autoignition event by monitoring pressure oscillations, combustion characteristics, and knock intensity. In-cylinder pressure and heat release rate (HRR) analyses were applied to distinguish the knocking combustion for the three different CRs. The results showed that a greater number of spark plugs produced more stable combustion even for a delayed spark timing (ST) than a single spark plug, but also increased pressure oscillations, HRR and knock propensity with higher CRs due to an increase in pressure and temperature inside the cylinder. The maximum amplitude of pressure oscillation (MAPO) method was used to determine the knock intensity, and it was observed that at the lowest CR of 8.5 three spark plugs gave higher MAPO values compared to four spark plugs for the same operating conditions. However, four spark plugs instigated higher MAPO values than three spark plugs for the higher CRs of 9.5 and 10.5 because, with the increase of CR and spark plug numbers, the pressure and temperature were increased during the combustion event, which was coupled with the compression effect on the unburned end gas mixture. Additionally, Fast Fourier transform (FFT) and wavelet methods were implemented to observe the frequencies induced during knock. It was found that sparking four plugs provoked various vibration modes in different amplitude ranges with the different CRs.

Prediction of knock intensity and validation in an optical SI engine

An experimental study was performed to investigate the combustion instability characteristics of swirl-stabilized combustors. A premixed gas composed of ethylene and air was burned under various flow and geometric conditions. Experiments were conducted by changing the inlet mean velocity, equivalence ratio, swirler vane angle, and combustor length. Two dynamic pressure sensors, a hot-wire anemometer, and a photomultiplier tube were installed to detect the pressure oscillations, velocity perturbations, and heat release fluctuations in the inlet and combustion chambers, respectively. An ICCD camera was used to capture the time-averaged flame structure. The objective was to understand the relationship between combustion instability and the Rayleigh criterion/the flame structure. When combustion instability occurred, the pressure oscillations were in-phase with the heat release oscillations. Even if the Rayleigh criterion between the pressure and heat release oscillations was satisfied, stable combustion with low pressure fluctuations was possible. This was explained by analyzing the dynamic flow and combustion data. The root-mean-square value of the heat release fluctuations was observed to predict the combustion instability region better than that of the inlet velocity fluctuations. The bifurcation of the flame structure was a necessary condition for combustion instability in this combustor. The results shed new insight into combustion instability in swirl-stabilized combustors.

An investigation of the self-excitation of low frequency (100-600 Hz), longitudinal acoustic modes of a rearward-facing step combustor has been performed. As in combustion instabilities of air breathing propulsion systems, the pressure oscillations are excited by a fluctuating heat release from a flame that is stabilized in a recirculation zone. Flow visualization results and flame radiation intensity data reveal that large vortex structures are responsible for this fluctuating heat release. The vortices are shed at frequencies corresponding to longitudinal acoustic modes of the system or to the first subharmonic of one of the modes. A series of parametric studies were performed to determine the dependence of the vortex shedding frequency upon the step height, mean flow speed, fuel type, and equivalence ratio. It was discovered that the vortex shedding frequency can shift between modes as a result of changes in the chemical reaction time of the reactants or as a result of changes in the mixing process of the cold reactants with the hot products. Detailed investigations into the mechanism of sustenance of the oscillations during instability were performed for several operating conditions. The distribution of the combustion associated with vortex shedding was investigated by measuring the radiation intensity from the flame region. These results were used in Rayleigh's Criterion to determine regions of driving and damping of the oscillations. A one-dimensional linearized acoustic model is used to predict the natural modes of the system and a fluctuating volumetric source is used to model the oscillatory heat release. First, independent driving is applied to determine the system response to driving at different frequencies. One important result is that, the phase difference between the pressure oscillations and the velocity oscillations at the flameholder is very sensitive to the frequency of driving near natural modes of the system. Finally, a velocity-sensitive volumetric source with a time delay is included as feedback to determine the linear stability characteristics of the system. Various mechanisms leading to nonlinear limit cycle behavior are then discussed and compared to experimental data obtained during transition to instability.

This paper describes an experimental study of the linear and nonlinear response of the gas turbine combustion process to imposed pressure oscillations. These data were obtained to improve current understanding of the nonlinear phenomenon in unstable combustors that play an important role in its limit cycle behavior. They were obtained by forcing oscillations in the combustor at discrete frequencies and measuring the resulting pressure and global CH* radical chemiluminescence oscillations. These data suggest that the nonlinear relationship between pressure and heat release oscillations, as opposed to nonlinear gas dynamic processes, play an important role in saturating the amplitude of self excited oscillations in gas turbine combustors. Specifically, it was found that the ratio of the magnitude of the heat release response to pressure perturbations decreased at large amplitudes of driving; i.e., that gain saturation plays a role in the combustors nonlinear dynamics. The role of amplitude dependence of the phase between the pressure and heat release fluctuations is less clear from the data, however, as it was found to be nearly independent and moderately dependent upon the drive amplitude at different driving frequencies. Also observed in these studies was the phenomenon of frequency locking, manifested by the decrease in amplitude of self excited oscillations at a frequency near the driving frequency with increasing amplitude of forcing. Introduction The occurrence of detrimental instabilities in lean, premixed combustors continues to hinder the development of industrial gas turbines [1-3]. These instabilities arise from interactions between oscillatory flow and heat release processes in the combustor, and often lead to large amplitude, organized oscillations of the combustor's flow fields. To prevent the onset of these instabilities or, at least, minimize their detrimental effects (e.g., through active control), an understanding of the processes responsible for initiating and sustaining them is needed. That is, an understanding of the mechanism(s) that are responsible for initiating instabilities is needed in order to provide engineers with insight on how to avoid them in the design stage. An understanding of the nonlinear processes that control the steady state or transient limit cycle oscillations of the unstable system is needed in order to develop capabilities for predicting the amplitude of the oscillations or in the development and optimization of active control methodologies. The objective of this paper is to elucidate the nonlinear processes controlling the limit cycle oscillations in these combustors. Some theoretical and experimental work on this topic has been reported previously which is briefly discussed below. Culick and co-workers have made extensive theoretical contributions to current understanding of nonlinear oscillations in unstable combustors (e.g., see Refs. [4-5]. This work has primarily focused on Copyright © 2001 by J. Harper, C. Johnson, Y. Neumeier, T. Lieuwen, and B.T. Zinn, Published by the American Institute of Aeronautics and Astronautics, Inc. with permission 1 American Institute of Aeronautics and Astronautics (c)2001 American Institute of Aeronautics & Astronautics or Published with Permission of Author(s) and/or Author(s)' Sponsoring Organization.

Flow–flame interactions causing acoustically coupled heat release fluctuations in a thermo-acoustically unstable gas turbine model combustor

A combined theoretical and experimental analysis of thermoacoustic interaction mechanisms of a lean pre-mixed swirl-stabilized gas turbine burner is presented. A full-scale gas turbine burner has been tested in an atmospheric test rig. The test facility was equipped with loudspeakers to excite the acoustic field and with arrays of microphones to measure the response of the acoustic field to the forcing signal. With this set-up transfer matrices relating the acoustic pressure and velocity on both sides of the flame front have been measured. A laser absorption measurement technique allowed for measurement of the fluctuations of fuel concentration in the mixture. Heat release fluctuations were monitored using a photo-multiplier. The measurement of the acoustic field, heat release and equivalence ratio fluctuations have been measured simultaneously. Special attention has been given to the role of fuel concentration fluctuations in the thermoacoustic interaction mechanism. In order to be able to clearly separate this mechanism from other possible mechanisms, all the experiments have been performed in pre-premixing mode as well. In pre-premixing mode the fuel is injected far upstream of the burner in order to avoid fuel concentration fluctuations at the burner location. This is in contrast with premixing mode where fuel and air are mixed in the burner. An acoustic flame model has been derived. The model includes the well-known interaction mechanism of equivalence ratio fluctuations but also includes a novel mechanism that is caused by fluctuations of vorticity. This latter mechanism relates the turbulent flame speed via turbulence intensity fluctuations to the acoustic field. The idea is that periodic acoustic fluctuations cause periodic changes of the turbulence intensity. The turbulence intensity strongly affects the turbulence flame speed. The fluctuations of the turbulent flame speed result in fluctuations of the heat release. This turbulence intensity fluctuation model has been compared with the measured pre-premix transfer functions and shows an excellent agreement. The measured transfer functions in premix mode have been compared with the model that includes fluctuations of fuel concentration and turbulence intensity. Also in this case a very good agreement is found. Moreover, it has been demonstrated that the phase relation between measured equivalence ratio fluctuation and heat release corresponds to the model.

본 실험에서는 실제 태양광 발전용 인버터의 냉각에 사용할 2개의 압출형과 2개의 압입형 히트싱크의 방열 성능을 평가하였다. 두 압입형 히트싱크의 핀의 개수는 62개와 98개, 전열면적은 2.8 ㎡, 5.3 ㎡이고, 두 압출형 히트싱크의 핀의 개수는 38개와 47개, 전열면적은 1.8 ㎡, 1.9 ㎡이다. 압입형 히트싱크의 방열율은 각각 82.7 %, 86.3 %, 압출형 히트싱크의 방열율은 각각 79.6 %, 81.6 %로 측정되었다. 각 히트싱크의 방열성능 평가결과에서 히트싱크의 전열면적이 증가할수록 방열율이 증가하는 경향을 보였다. 압입형인 S-62 히트싱크는 압출형인 E-47 히트싱크 보다 전열면적이 47.4 % 증가하였음에도 불구하고 방열량은 1.3 % 증가하는데 그쳤다. 이는 압출형의 우수한 전열성능 때문인 것으로 판단된다. 또한 압입형인 S-98 히트싱크는 동일한 압입형인 S-62 히트싱크에 비해 전열면적이 89.3 % 증대되었음에도 방열량 증가는 4.4 %에 불과하여 전열면적에 대한 최적화가 필요함을 알 수 있었다.

Combustion dynamic phenomena are critical to the performance and stability of propulsion and energy systems. Fluctuations in heat release (inherent to the combustion process) cause pressure oscillations within the combustor. Thermoacoustic instability occurs when the frequency of heat release oscillations aligns with one of the combustor's acoustic modes, creating a positive feedback loop that amplifies these oscillations. This can lead to high-amplitude pressure fluctuations, potentially causing limit cycle oscillations. The present study examines the visual flame morphology of a flame stabilized over a swirl coaxial injector using methane and oxygen as propellants. Experiments were conducted at atmospheric pressure and standard injection temperatures, with the flame confined within a quartz combustor. High-speed imaging captured the flame morphology, while a microphone recorded sound pressure levels to correlate acoustic events. The study investigated flame morphology across equivalence ratios from φ = 1.29 To φ = 0.2. Three flame characteristics were observed: stable flames in fuel-rich conditions while oscillating and near-blowout flames in lean conditions. A consistent combustion instability mode at around 416 Hz, the first longitudinal mode of the combustor, was noted, with additional modes up to 7800 Hz and 13600 Hz (due to excitation of injector acoustics) as fuel mass flow rates varied. Observed changes in flame morphology included variations in flame length and cyclic behavior, characterized by flame growth and blow-off. The study also mapped the stable and unstable combustion regimes.

<div class="section abstract"><div class="htmlview paragraph">The article presents the simulation results of the combustion process made based on a 3D-CFD mathematical model of the aircraft radial engine. The object of research was the 9-cylinder aircraft engine ASZ-62IR. The model comprised of the internal geometry of the cylinder and pipes in the cylinder head together with the inlet pipe. The model has been verified on the base of the pressure characteristics in the inlet pipe obtained from the experiment. In order to reproduce the same process of mixture formation as in the real engine, the model involved the process of induction, compression and combustion. The conducted research was aimed at verifying the influence of switching off one of the spark plugs on the combustion process. The simulation was done for three versions of ignition, for two spark plugs at the same time, only the front spark plug, only the rear spark plug. The research has been conducted in the conditions of maximum constant power of engine that is 686 kW.</div><div class="htmlview paragraph">As a result of the conducted research, the reaction progress variables in the combustion chamber representing the spread of flame and rate of heat release have been obtained. A distinct influence of the number of operating spark plugs on the combustion process has been revealed. In fact, in the case of lack of ignition in one of the spark plugs the, combustion occurs, but the process prolongs in time. In order to optimize the process of combustion, research for various angles of ignition will be conducted in the future.</div></div>

Self‐excited thermoacoustic instability (SETAI) is a dangerous phenomenon in combustion equipment. While it is widely acknowledged that SETAI behavior is determined by the couple between pressure and heat release oscillation, their phase difference is difficult to predict, which impedes the development of SETAI control technology. With the aim of passive control technology development, this paper conducted experiment on a premixed hedge combustor to explore the mechanism whereby premixed chamber length (LP) and equivalence ratio (φ) collaboratively influence SETAI behavior. Results showed LP mainly affects the pressure mode shape within premixed chamber and consequently alters the phase difference between pressure and flowrate oscillation at combustion chamber inlet. Changing φ gives rise to different reaction time‐lag (τ), thus altering the phase difference between flowrate and reaction heat release oscillation. By introducing this flowrate oscillation, how LP and φ collaboratively determine phase difference between pressure oscillation and heat release oscillation was clarified. The mechanisms identified in this study are consistent with the emerging rationalization of the factors contributing to SETAI, and also provides better understanding on Rayleigh criterion and guidance for SETAI control. With further work on heat release and flow rate measurement, as well as the development on τ description, SETAI can be better predicted and controlled.

In this study, thetime series data for sound pressure and heatrelease fluctuations were analyzed to establish the stability windowof the propane-oxyflames under different operating conditions. TheCO2 dilution level and the combustor power density werevaried at a fixed global equivalence ratio to investigate the thermoacousticinstability of the flames. The phase difference between the fluctuationsand the instantaneous Rayleigh index reveals whether the heat releaseand pressure fluctuations are coupled, which can result in the amplificationof the sound pressure fluctuations. Results showed a negative Rayleighindex and uncoupled fluctuations at low (<40%) and high (>60%)CO2 dilution with coupled fluctuations and sound pressureamplification at intermediate CO2 dilution levels. A peakof varying magnitude appeared in the frequency domain at 465 Hz forboth the heat release and pressure fluctuations in the coupled mode.The Strouhal number at different CO2 concentrations revealeda range of vortex-shedding frequencies (300–1000 Hz), suggestingthat the coupled mode is vortex-induced. Phase space reconstructionfor the sound pressure fluctuations was carried out and it is observedthat although the pressure fluctuations are amplified in the coupledmode, limit cycle amplitudes have not been reached. The recorded couplingand uncoupling of the oscillations associated with flame-vortex interactionsat certain CO2 concentrations provides a valuable insighton the combustor’s dynamics and toward the development of nonpremixed-oxy-flamescombustors.

Development of a quasi-dimensional, fractal-base combustion model for SI engines by simulating flame-wall interaction phenomenon

Acoustic and heat release signatures for swirl assisted distributed combustion

&lt;div class="section abstract"&gt;&lt;div class="htmlview paragraph"&gt;The present investigation pertains to the development of fast idle catalyst light-off strategy for a light duty gasoline compression ignition (GCI) engine. The engine cold start fast idle operation poses a problem of increased criteria emissions if the catalyst is not activated during the warm up period. Therefore, a control strategy is proposed here to minimize the criteria pollutants during the fast idle phase via enabling fast catalyst light off in a GCI engine and relying on the spark ignition of a globally stoichiometric fuel air mixture. The engine has unique design features such as certain geometry configuration between spark plug and fuel injector arrangement, and the location of spark plug in a high compression ratio (CR) diesel-like combustion chamber. The experiments were performed in a single cylinder GCI engine at cold start fast idle conditions using certification gasoline fuel (RON 91). Split fuel injection and extremely retarded spark timing strategy are applied to generate high enthalpy flow in the exhaust pipe with reduced hydrocarbon and other emissions, improved combustion stability, and fuel economy. Based on the experimentation, split injection cases namely 1) tailed injection case - SOI&lt;sup&gt;1st&lt;/sup&gt; = -350 CAD aTDC and SOI2&lt;sup&gt;nd&lt;/sup&gt; = 15 CAD aTDC, 2) intake injection case - SOI1&lt;sup&gt;st&lt;/sup&gt; = -350 CAD aTDC and SOI2&lt;sup&gt;nd&lt;/sup&gt; = -270 CAD aTDC, showed better results in terms of COV and criteria emissions when compared to other fuel injection cases. The steady state experimental results also indicate that, regardless of fuel injection strategy, retarded spark timing of 15 CAD aTDC is required to attain a higher exhaust temperature of around 700°C to 800 °C. In summary, evaluation of cold start performance index identifies tailed injection strategy as optimum to generate higher exhaust energy without compromising the combustion stability to effectively light off the catalyst and reduce the criteria emissions.&lt;/div&gt;&lt;/div&gt;